FWB`s Guide to Storage
GtoS 2nd Ed. Book Page i Friday, March 27, 1998 12:05 PM
FWB’s Guide to Storage
Second Edition
GtoS 2nd Ed. Book Page ii Friday, March 27, 1998 12:05 PM
Copyright Notice
This manual is copyrighted by FWB Software, LLC (FWB) with all rights
reserved. Your rights with regard to this manual are subject to the restrictions
and limitations imposed by the copyright laws of the United States of America. Under the copyright laws, this manual may not be copied, reproduced,
translated, transmitted or reduced to any printed or electronic medium or to
any machine-readable form, in whole or in part, without the written consent
of FWB.
© 1991, 1996 By FWB Software, LLC
December 1996
Part No.: 07-00841-201
GtoS 2nd Ed. Book Page iii Friday, March 27, 1998 12:05 PM
Trademarks
Hard Disk ToolKit, RAID ToolKit, SpaceMaker ToolKit, and CD-ROM ToolKit are trademarks of FWB Software, LLC. FWB is a registered trademark of
FWB Software, LLC.
All brand and product names are trademarks or registered trademarks of their
respective holders.
Credits
This guide was written by Norman Fong with help from Joan Carter, Steve
Dalton, Bruce Dundas, Eric Herzog, Al Pierce, Stuart Saraquse, and Fred Swan.
It was designed, edited, and composed by Joan Carter with help from Allan
Levite. Illustrations were produced by Deane Morris. The original version of
this manual was written in 1991 by Leslie Feldman, Norman Fong, Kevin
Kachadourian, Neil Strudwick, and Paul Worthington as part of FWB’s Hard
Disk ToolKit 1.x manual.
Dedication
This book is dedicated to my family and friends who put up with the long
hours put into this effort. Kudos to those who enjoyed the original and
inspired us to push the envelope once again five years later!
GtoS 2nd Ed. Book Page iv Friday, March 27, 1998 12:05 PM
GtoS 2nd Ed. Book Page v Friday, March 27, 1998 12:05 PM
Preface
Thank you for purchasing the second edition of FWB’s Guide to Storage. We
hope you will find this an informative guide to one of the most important
components of your computer: your storage peripherals.
Most users never think twice about their drives until they cannot install a
new program or save a file. Yet behind the drive letter or icon on your screen
is a vast amount of technology representing millions of hours of research and
development. Storage peripherals are another way technology has enabled
users to dramatically increase their productivity.
This guide is not meant to document each subject exhaustively, but to serve
as a bridge between non-technical documentation on storage and the technical specifications that define the major standards and technologies. We will
cover many of the most important storage-related technologies in detail.
Once you have read this guide, if you are interested in deepening your understanding of storage technology, there are many fine books available that focus
on specific subjects, such as drive interfaces, jumper settings, and other storage technologies. Look in the Bibliography for additional relevant titles.
Please contact us if you have suggestions for additions to this Guide.
NORMAN FONG
President, FWB Software, LLC
Preface
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FWB’s Guide to Storage
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Ta b l e o f C o n t e n t s
List of Figures ............................................................................................. xiii
List of Tables ............................................................................................... xvi
Chapter 1. Introduction .............................................................................. 19
About This Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Overview of Chapters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Chapter 2. All About Drives ......................................................................
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Storage Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Capacity and Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Short History of Hard Drives . . . . . . . . . . . . . . . . . . . . . . . . . . .
BH (before hard drives) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The first true hard drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Anatomy of a hard drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Read/Write channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Medium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spinning ‘round . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Head parking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Form Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Formatting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Error correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interleaving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sector skew . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data transfer rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Caching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Thermal recalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
When Things Go Wrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Software errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hardware errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Drives Don’t Have to Die Quickly! . . . . . . . . . . . . . . . . . . . . . . .
Saving data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Disk duplexing and disk mirroring . . . . . . . . . . . . . . . . . . . . .
RAID, or Redundant Array of Independent Disks . . . . . . . . .
Table of Contents
21
21
21
22
25
25
26
27
27
37
38
39
39
40
43
43
44
47
47
48
49
50
52
58
59
59
60
60
62
62
63
64
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But When You Gotta Go … . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preventative measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Defragmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Need a New Drive? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Indicators of durability and reliability . . . . . . . . . . . . . . . . . .
Other statistics to consider . . . . . . . . . . . . . . . . . . . . . . . . . . .
Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Trends in Hard Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Smaller platters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Smaller drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Faster, smaller controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Connector size is slow to change . . . . . . . . . . . . . . . . . . . . . .
Faster, lighter heads and actuators . . . . . . . . . . . . . . . . . . . . .
Green drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
All digital drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Holographic memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Large volumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
66
67
68
69
69
72
72
73
73
73
74
74
75
75
76
76
77
77
Chapter 3. RAID Technology ....................................................................
What is RAID? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
What to look for . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Specific features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID Levels—Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID Level 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID Level 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID Level 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID Level 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID Level 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID Level 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID 5 and Fault Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Up to two disks can fail . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Increasing fault tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other RAID Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hardware-Based Disk Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bus-Based Hardware Disk Arrays . . . . . . . . . . . . . . . . . . . . . . . . .
Software-Based Disk Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SAF-TE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID Advisory Board (RAB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
79
80
80
82
84
86
86
87
88
88
89
93
93
94
94
95
95
95
96
Chapter 4. Other Types of Storage Devices .............................................. 97
Removable drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Removable cartridge drives . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Zip drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Bernoulli drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
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Table of Contents
Magneto-optical drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Phase change optical drives . . . . . . . . . . . . . . . . . . . . . . . . . .
Floptical drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CD-ROM drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compact Disc Recordable drives (CD-R) . . . . . . . . . . . . . . .
WORM drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CD-ROM Rewritable drives (CD-RW) . . . . . . . . . . . . . . . . .
Jukeboxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Digital Versatile Disc drives . . . . . . . . . . . . . . . . . . . . . . . . .
Tape drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
102
104
105
106
109
118
118
119
120
122
Chapter 5. All About SCSI .......................................................................
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI Is Simple … . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
… and Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Essential Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The Origin of SCSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Apple’s SCSI Influence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Good Start... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Interoperability or connectivity . . . . . . . . . . . . . . . . . . . . . .
Boosting I/O performance . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hardware economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
… But It Needs Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Who Uses SCSI? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI Integrated Circuits (Chips) . . . . . . . . . . . . . . . . . . . . . . . . .
Chip overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI chip families . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Transfer Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Essential terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Asynchronous data transfer . . . . . . . . . . . . . . . . . . . . . . . . .
Synchronous data transfer . . . . . . . . . . . . . . . . . . . . . . . . . . .
Signal lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI ID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Physical connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electrical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pin assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bus Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Attention condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reset condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Executing Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Command Descriptor Block . . . . . . . . . . . . . . . . . . . . . . . . .
Pointers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
125
125
125
126
127
127
128
129
130
130
132
133
133
134
135
135
136
138
139
139
140
141
143
146
147
147
151
152
153
158
158
158
159
159
160
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x
Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mode Pages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bus Phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI, Apple-Style . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ANSI irregularities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Apple exceptions to the SCSI specification . . . . . . . . . . . . .
PowerBook SCSI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI Manager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Device drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Call Chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inside the Mac HFS partition . . . . . . . . . . . . . . . . . . . . . . . .
Macintosh volume limits . . . . . . . . . . . . . . . . . . . . . . . . . . .
Macintosh SCSI performance . . . . . . . . . . . . . . . . . . . . . . . .
PC SCSI Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Windows® . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Windows 95 and NT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PC Plug-and-Play . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PC SCSI performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI-2: A Transition From SCSI-1 . . . . . . . . . . . . . . . . . . . . . . .
SCSI-2 overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI-2 commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI-2 highlights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The SCSI-2 Common Access Method (CAM) . . . . . . . . . . .
SCSI-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
161
164
164
169
173
175
176
177
178
181
183
185
186
187
188
189
190
190
191
191
192
193
193
195
195
200
202
Chapter 6. All About Serial SCSI ............................................................
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
USB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1394 (FireWire) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fibre Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Why use Fibre Channel? . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Serial Storage Architecture (SSA) . . . . . . . . . . . . . . . . . . . . . . . .
Fibre Channel Enhanced Loop . . . . . . . . . . . . . . . . . . . . . . . . . . .
209
209
211
211
213
213
216
218
221
Chapter 7. Other Storage Interfaces ........................................................
ST-506/ST-412 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ESDI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Origins of IDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDE details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDE data transfers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDE limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
222
222
224
225
225
227
227
229
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Table of Contents
PC IDE implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Macintosh IDE implementation . . . . . . . . . . . . . . . . . . . . . .
Special considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Enhanced IDE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fast ATA-2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ATAPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Basic ATAPI operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ATAPI CD-ROM Mode Pages . . . . . . . . . . . . . . . . . . . . . . . .
ATAPI tape Mode Pages . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ATA-3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ultra-ATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Other Drive Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
FIPS-60 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SMD-E and SMD-H . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IPI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
230
234
235
235
238
238
239
239
239
240
241
241
241
241
241
Chapter 8. All About Expansion Buses ...................................................
NuBus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ISA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MicroChannel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
EISA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VESA Local Bus (VLB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PCI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PCI features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Macintosh PCI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Open Firmware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PCI Mezzanine Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PCMCIA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
243
243
246
247
248
248
249
251
253
255
261
261
Chapter 9. Storage Software .....................................................................
Formatter Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
RAID Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CD-ROM Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tape Backup Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Compression Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hierarchical Storage Management Software . . . . . . . . . . . . . . . .
Managing data storage: the hidden costs . . . . . . . . . . . . . . .
On-line, off-line, and near-line storage . . . . . . . . . . . . . . . . .
Archiving, backup, and HSM . . . . . . . . . . . . . . . . . . . . . . . .
CD-R Mastering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Disk Benchmarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
263
263
264
265
266
266
267
268
268
269
272
272
Chapter 10. Troubleshooting ...................................................................
General Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Macintosh Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mac boot process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
274
274
277
277
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Macintosh problems and recommended actions . . . . . . . . .
Happy Mac problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sad Mac Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PC Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PC boot process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PC problems and recommended actions . . . . . . . . . . . . . . .
278
278
279
285
285
286
Appendix A. Additional Information Sources ........................................ 288
Appendix B. Suggested Additional Reading ............................................ 292
Appendix C. The Deep End .....................................................................
SCSI Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI signals and bus phases . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI Command Descriptor Block . . . . . . . . . . . . . . . . . . . .
SCSI-1 commands and opcodes . . . . . . . . . . . . . . . . . . . . . .
SCSI-1 Sense Keys and Sense Data . . . . . . . . . . . . . . . . . . . .
SCSI-1 messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI control signals and Information Transfer phases . . . .
PC SCSI: ASPI for Win32 calls . . . . . . . . . . . . . . . . . . . . . . .
SCSI-2: new commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SCSI-2 mode pages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ST-506 Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ST-506 commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ST-506 data signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDE Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDE data signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDE registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDE commands and opcodes . . . . . . . . . . . . . . . . . . . . . . . . .
PC IDE: INT13 call codes . . . . . . . . . . . . . . . . . . . . . . . . . . .
IDE pin assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Apple ATA Manager calls . . . . . . . . . . . . . . . . . . . . . . . . . . .
Enhanced IDE commands and opcodes . . . . . . . . . . . . . . . .
ATAPI Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ATAPI commands and opcodes . . . . . . . . . . . . . . . . . . . . . .
ATAPI registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ATA-3 Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PCI Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
294
294
294
295
298
301
304
308
309
309
314
317
317
318
319
319
321
322
323
324
325
327
327
327
328
328
329
Glossary ...................................................................................................... 330
Index ........................................................................................................... 381
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List of Figures
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
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Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
Figure
List of Figures
1. Comparative height of disk flight ................................................
2. Exploded internal view of a hard drive ........................................
3. Cross section of an external (enclosed) hard drive ......................
4. Stepper motor head actuator ........................................................
5. Formula for calculating tracks per inch .......................................
6. Formula for determining access time ..........................................
7. Stepper motor actuator .................................................................
8. Voice coil actuator ........................................................................
9. Safe landing zone for head parking ..............................................
10. A 2.5-inch drive ...........................................................................
11. Examples of 1:1 and 6:1 interleaving .........................................
12. Sector skew offset .......................................................................
13. Data flow in memory caches ......................................................
14. Data transfer rates increase the closer cache is to the CPU .....
15. Time it takes to complete a 4 KB random write .......................
16. Most frequent causes of system failure .....................................
17. Fragmented and defragmented disks ..........................................
18. Interpretation of the MTBF rating .............................................
19. Formula for determining MTBF by Field Rate of Return .........
20. Interpretation of the MTBF rating based on FRR ......................
21. RAID subsystem with a hot-swap drive ....................................
22. Hot-swappable power supply .....................................................
23. Six-drive, level 0 RAID ...............................................................
24. RAID 0 array ...............................................................................
25. Six-drive, level 1 RAID ...............................................................
26. Level 3 RAID ...............................................................................
27. Level 5 RAID ...............................................................................
28. Creating parity data on RAID 5 using XOR ..............................
29. Level 5 RAID ...............................................................................
30. Data organization in RAID level 5 .............................................
31. RAID 5 reconstruction ...............................................................
32. Removable canister drive ...........................................................
33. Removable hard cartridge system ..............................................
24
28
29
30
31
33
35
36
39
41
48
49
52
53
57
59
68
69
71
71
81
81
84
84
86
87
88
90
90
91
92
98
99
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Figure
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Figure
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Figure
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Figure
xiv
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
Bernoulli cartridge ....................................................................
Magneto-optical drive and optical cartridge ............................
6Plex CD-ROM drive ...............................................................
CD-Recorder .............................................................................
Comparison of CD-ROM and CD-R media .............................
Order of information on a DAO CD ........................................
Order of information on an incremental CD ..........................
Order of information on a packet CD ......................................
Order of information on a multisession CD ...........................
Order of information on a Kodak multisession CD ................
Comparison of single- and dual-layer DVDs ...........................
DAT and DAT autoloader tape drives .....................................
DLT tape drive ..........................................................................
SCSI time line ...........................................................................
Single host/multiple controllers ..............................................
Asynchronous data transfer mode ...........................................
Signal lines ................................................................................
25-50 Macintosh SCSI system cable ........................................
50-high-density 50 PC SCSI system cable ...............................
50-50 peripheral interface cable ...............................................
Daisy chaining external devices ..............................................
Cable extender ..........................................................................
Internal fast/wide drive with a 68-pin connector ....................
Termination circuits: single-ended and differential ...............
68-pin SCSI terminator .............................................................
Termination power circuit .......................................................
Phase sequence with arbitration ..............................................
Phase sequence without arbitration ........................................
SCSI Manager software hierarchy ............................................
Typical parameter block construct ..........................................
Partition map ............................................................................
1394 (FireWire) theoretical physical topology .........................
Fibre channel arbitrated loop ...................................................
SCSI parallel implementation ..................................................
SSA single-domain implementation ........................................
SSA dual-domain implementation ..........................................
PC PCI IDE implementation ....................................................
IDE application call chain ........................................................
DOS disk layout (3.0 and newer) .............................................
101
102
106
109
111
114
115
116
116
117
120
122
124
128
135
140
143
148
148
149
149
150
150
151
153
156
169
170
178
180
185
211
213
215
219
220
230
231
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Figure
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Figure
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Figure
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List of Figures
73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
83.
84.
85.
86.
Mac OS IDE implementation ...................................................
Formula for the Logical Block Address ....................................
NuBus SCSI expansion card .....................................................
16-bit ISA SCSI card ..................................................................
PCI SCSI card ............................................................................
Certified PCI local bus trademark ...........................................
PCI and NuBus transfer rates using Benchtest ........................
PCI and NuBus transfer rates using Adobe Photoshop™ ........
Apple’s certified PCI trademark ...............................................
FWB’s Hard Disk ToolKit™ ......................................................
FWB’s RAID ToolKit™ .............................................................
FWB’s CD-ROM ToolKit™ .......................................................
FWB’s SpaceMaker ToolKit™ ...................................................
Opcodes reserved for vendor-unique commands ....................
234
236
243
246
249
250
257
258
259
263
264
265
267
311
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L i s t o f Ta b l e s
Table
Table
Table
Table
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Table
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Table
Table
List of Tables
1. Typical operational speeds and data density in 1996 ................... 50
2. Comparison of transfer rates and CPU speeds ............................. 51
3. Prolonging disk life and maintaining data integrity .................... 66
4. Worldwide volume of drives ......................................................... 77
5. Current and future capacities of 3.5-inch drives .......................... 77
6. Amount of storage on an average file server ................................ 78
7. Historical and predicted cost per MB ............................................ 78
8. Escalation of typical drive speeds in RPM .................................... 78
9. Features of RAID subsystems ....................................................... 80
10. Comparison of common RAID levels ......................................... 83
11. Other RAID levels ....................................................................... 94
12. Overview of other types of storage devices ................................ 97
13. Advantages/disadvantages of removable drives ......................... 99
14. Advantages/disadvantages of removable cartridge drives ........ 100
15. Advantages/disadvantages of Zip drives ................................... 101
16. Advantages/disadvantages of Bernoulli drives ......................... 102
17. Advantages/disadvantages of magneto-optical drives .............. 104
18. Advantages/disadvantages of phase change optical drives ...... 104
19. Advantages/disadvantages of floptical drives ........................... 105
20. Pros and cons of CD-ROM file system formats ....................... 107
21. Features to look for in CD-ROM drives ................................... 108
22. Advantages/disadvantages of CD-ROM drives ........................ 109
23. Overview of CD formats and standards .................................... 113
24. Advantages/disadvantages of WORM drives ............................ 118
25. Advantages/disadvantages of jukeboxes ................................... 119
26. Comparison of features of DDS-1 through 4 ............................ 123
27. Advantages/disadvantages of tape drives .................................. 124
28. ANSI X3 categories for current technologies ........................... 126
29. The nine control signals ............................................................ 145
30. Data buses and related SCSI IDs ............................................... 146
31. Types of termination ................................................................. 154
32. Examples of termination ........................................................... 157
33. General device commands ........................................................ 161
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List of Tables
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
SCSI messages ............................................................................
The eight SCSI bus phases .........................................................
Original SCSI Manager code ......................................................
SCSI Manager 4.3 calls ..............................................................
Driver requests ...........................................................................
Data structures in the Macintosh HFS partition .....................
Progress of data transfer rates in the Macintosh ......................
Overview of improved transfer rates in PCs ............................
Mandated SCSI-2 supported features ........................................
Optional features allowed in SCSI-2 .........................................
SCSI-3 files and topics ...............................................................
Features of a fibre channel arbitrated loop ...............................
IDE data transfer rates ...............................................................
IDE data transfer limitations on a PC .......................................
Operating system drive capacity access limitations ................
FAT cluster size limitations ......................................................
Evolution of Macintosh data transfer rates .............................
Maximum data transfer rates of Intel chipsets ........................
Macintosh PowerPC to PCI data transfer rate .........................
Macintosh PCI master to memory data transfer rate ..............
Three common techniques for storage management ...............
On-line, off-line, and near-line storage ....................................
I/O interface reflectors ...............................................................
Signals asserted during different bus phases ............................
Group 0 Command Descriptor Block .......................................
Group 1 Command Descriptor Block .......................................
Group 5 Command Descriptor Block .......................................
General device commands and opcodes ...................................
Sense Keys ..................................................................................
Common Sense Keys with ASC and ASCQ decoding .............
SCSI messages ............................................................................
Control signals and Information Transfer phases ....................
ASPI for Win32 calls ..................................................................
General device commands introduced in SCSI-2 .....................
Commands and opcodes for CD-ROM devices ........................
Commands and opcodes for tape devices .................................
Commands and opcodes for communication devices ..............
SCSI-2 defined mode pages ........................................................
SCSI-2 defined CD-ROM mode pages .......................................
165
171
179
181
182
186
188
192
196
198
202
214
228
229
233
233
245
252
255
255
268
269
289
294
295
296
297
298
301
302
304
308
309
309
312
313
313
314
316
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73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
83.
84.
85.
86.
87.
88.
SCSI-2 defined tape mode pages ................................................
ST-506 commands and opcodes ................................................
34-Pin ST-506 data signals ........................................................
20-Pin ST-506 data signals ........................................................
40-Pin IDE data signals ..............................................................
IDE registers for accessing drives ..............................................
IDE commands and opcodes .....................................................
Call codes available through INT13 .........................................
IDE hard disk connector pin assignments ................................
ATA Manager parameter block header structure ....................
Functions supported by the ATA Manager ..............................
Enhanced IDE commands and opcodes ....................................
ATAPI-inspired enhanced IDE commands and opcodes ..........
ATAPI registers ..........................................................................
ATA-3 new commands and opcodes ........................................
PCI commands and initiators ...................................................
316
317
318
319
319
321
322
323
324
325
326
327
327
328
328
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1
Introduction
About This Guide
FWB’s Guide to Storage is presented in ten sections:
•
•
•
•
•
•
•
•
•
•
Introduction
All About Drives
RAID Technology
Other Types of Storage Devices
All About SCSI (Small Computer System Interface)
All About Serial SCSI
Other Storage Interfaces
All About Expansion Buses
Storage Software
Troubleshooting
Appendix, Glossary, Bibliography, and Index information are included at the
back of the book.
Overview of Chapters
This manual offers a foundation of information to help you take full advantage of the power of your storage peripherals and utility software. A brief overview of chapters is provided below.
Chapter 2, All About Drives
Chapter 2 provides detailed information on the functioning of hard disk drives
and other storage devices. If you are not knowledgeable about hard drives, read
this chapter before changing any formatting options in your disk formatting
software.
Chapter 3, RAID Technology
Chapter 3 explains RAID technology, lists and describes common RAID levels, and walks you through the ways some RAID levels store data to disks.
Chapter 1: Introduction
19
GtoS 2nd Ed. Book Page 20 Friday, March 27, 1998 12:05 PM
Chapter 4, Other Types of Storage Devices
Chapter 4 discusses types of storage devices other than internal and external
hard disk drives. It includes device descriptions and detailed explanations of
different media and ways data is stored it.
Chapter 5, All About SCSI
Chapter 5 describes the Small Computer System Interface (SCSI). Read; this
chapter to discover how SCSI enables your computer to communicate with
various peripherals, including drives.
Chapter 6, All About Serial SCSI
Chapter 6 describes the serial version of the Small Computer System Interface. Read this chapter to discover how Serial SCSI enables your computer to
transfer data at rates not possible with normal parallel SCSI.
Chapter 7, Other Storage Interfaces
Chapter 7 describes the variety of storage interface technology available
today, including IDE, Intelligent Drive Electronics interface. Read this chapter
to boost your understanding of other storage interfaces.
Chapter 8, All About Expansion Buses
Chapter 8 describes the internal expansion buses of microcomputers. Read
this chapter to develop your understanding of how buses extend your computer’s speed and expandability.
Chapter 9, Storage Software
Chapter 9 describes the various categories of software designed to enhance the
flexibility of your storage peripherals. Read this chapter to learn about the different types of utility software currently on the market.
Chapter 10, Troubleshooting
Chapter 10 describes some typical problems with using storage peripherals
and suggests solutions. Read this chapter to develop your knowledge of steps
to take when things go wrong.
20
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2
All About Drives
Overview
Hard disk drives, like most other storage devices, are named for their storage
medium. There is an actual hard disk platter, or multiple platters, inside the
mechanism. This is where data is stored and retrieved. A hard disk platter is
about one-eighth of an inch thick. Its diameter varies.
NOTE
See “Form Factor” on page 40 for more information on disk diameters.
Hard disk drives, or hard drives, are crucial parts of any computer system.
They provide nonvolatile, online access to vast amounts of information, such
as the software you purchase and the work you perform.
STOP
Nonvolatile means that data is not lost when the computer is turned off. This is in contrast to
volatile, where data is lost once the computer is turned off. An example of nonvolatile is the data
you store on your hard disk. An example of volatile is the data held in RAM.
The central processing unit (CPU) executes programs that reside in volatile
RAM, but those programs and corresponding data need to be loaded from disk.
All data is lost when power is withdrawn from volatile RAM, but nonvolatile
disk storage maintains its data after power loss. Some of the most expensive
pieces of hardware in your computer include:
• the central processing unit (CPU)
• the random-access memory (RAM)
• the hard disk
Storage Life
Compared to the computer’s RAM, which retains information only as long as
the machine is running, hard drives do a much better job of preserving information:
• They have far greater capacity.
• They’re cheaper (though not cheap).
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The cost of magnetic media (i.e., a hard drive) is much lower than the cost
of silicon chips (i.e., RAM). This cost differential has been growing,
making drive storage over 100 times cheaper per megabyte than RAM.
• They’re far less subject to adverse environmental conditions.
The disk platter that data is stored on is permanently encased in the drive
and never contacts the drive’s heads during normal use. The fact that the
disk is neither exposed to air nor directly touched by the drive’s heads
minimizes the amount of degradation that naturally occurs over time.
If used and stored in accordance with the manufacturer’s recommendations, a
hard disk drive could store data without loss for ten or more years. But no
storage medium is truly permanent. All will eventually degrade and lose
information.
For a longer storage life, CD-ROM and other optical drives are the only storage media that may approach permanence. Current assessments anticipate
that CDs will last for 100 years or more (although, with the way technology
has been advancing, it’s open to question whether this technology will be in
use a hundred years from now).
NOTE
See Chapter 4, “Other Types of Storage Devices,” for more information on other types of media.
Capacity and Access
Most computer owners want greater storage capacity and faster data access
than is offered by floppy disk drives. A computer can access data from a hard
disk in 10 milliseconds (ms). A computer can access data from a floppy disk in
200 ms (average access time). Access from a hard disk is twenty times faster
than access from a floppy. (They’re both slowpokes compared to RAM, which
can be accessed in 100 nanoseconds—200 times faster than a hard drive.)
A hard drive’s storage media spins faster than a floppy’s—5,400 revolutions
per minute (RPM) or faster, compared to 360 RPM for floppy disks—and thus
can access data more quickly. The disk platter in a hard disk drive spins constantly. It’s always ready to do its job. A floppy motor turns on and off, which
slows disk access and adds latency.
22
STOP
Latency is the time it takes for the requested magnetic or optical disk track or sector to rotate to
the correct position under the read/write heads of a disk drive. This is one of the factors that
govern access speed.
NOTE
For more information on the mechanics of a hard drive, see “Anatomy of a hard drive” on page 27.
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Most hard drives have data transfer rates that average from five to hundreds of
megabits per second (Mb/s or millions of bits per second). Floppy drives can
transfer data only as fast as 0.2 to 0.4 Mb/s.
As of 1996, the storage limit on 5.25-inch hard disk drives is as much as 23
gigabytes (GB; one GB equals 1,024 MB). The 1996 storage limit on standard
floppies is about 1.4 MB.
STOP
The platter in a hard disk drive really is a hard disk; the medium is rigid, not flexible, and because
its surface is dense, a large amount of data can be stored on it.
Before getting more deeply into the technology, let’s explore some basic facts
about hard drives.
In several respects the operation of a hard drive is like a record player. A
mechanism on the hard drive travels across the surface of the hard disk to
read and transmit information similar to the way a tone arm on a record
player travels across the surface of a record to provide audio.
Instead of a needle that detects bumps and pits in the record surface, disk
drives have electromagnetic read/write heads that create and detect magnetic
information on the surface of the disk platters. The read/write head is a tiny
electromagnet at the end of an armature. The head is analogous to a record
player’s needle or stylus.
STOP
An armature is the drive head’s supporting framework. (The tone arm on a record player is an
armature.)
High-capacity drives have more than one platter. This is similar to having
many records stacked on top of a turntable, with the following exceptions:
• Unlike stacked records, stacked disk platters all rotate together.
• Most high-capacity drives have a separate armature for each platter.
• Often, both sides of a platter are used simultaneously.
Unlike a record player, the hard drive’s “needle” does not come into contact
with the disk. In fact, nothing should touch the platter. Most drives are virtually air tight so that not even dust particles can affect their operation. As the
disk spins, it creates an air cushion upon which the read/write head floats.
Chapter 2: All About Drives
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À
@
€
@
€
À
This air-cushion gap is much narrower than a speck of dust: only a couple
microns tall (millionths of a meter) (Figure 1).
Flight
Height
Drive Head
Smoke Particle
Finger Print
Dust Particle
Human Hair
Figure 1. Comparative height of disk flight
STOP
The flight height of a modern drive head is equivalent to a jumbo jet flying at less than one inch
above the ground. Luckily, the surface of a disk is much flatter than the surface of the earth.
Instead of plowing along physical grooves on a vinyl record and moving
steadily inward, the read/write head follows tracks and moves in and out.
Tracks are the invisible magnetic “grooves” upon which data is physically
stored. Instead of one continuous track, as you’d find on a vinyl record, tracks
form distinct concentric circles that emanate from the center, or hub, of the
platter out to the perimeter.
hiddne
Information is sent as an electric
current by the hard drive’s controller to the
controlling chips. This current passes through the magnetic read/write head,
which magnetizes particles on the platter’s tracks in binary on/off patterns, or
bits—the standard building blocks of the digital language used by computers.
The media stays magnetized when the power is turned off, just as a record
holds a lasting impression after it is cast.
24
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A Short History of Hard Drives
BH (before hard drives)
Storage for computers has existed since the early 1950s, when companies such
as IBM stored data on what was known as “Drum Storage.” These drives were
shaped like drums, had heads that touched the platters, and held very little
data.
STOP
In September 1956, a small group of IBM engineers developed the RAMAC 305. It stored 5 MB on
50 24-inch disks and had a 600 ms access time. It cost about $35,000 a year to lease. RAMAC is
an acronym for Random Access Method of Accounting and Control. The RAMAC was the first
storage device with random access to data. Random access meant the drive head could access
requested data without having to travel through all the data stored before it in sequence.
Sequential access devices, such as tape drives, require that all data stored in front of the requested
data must travel across the read/write head before you can access it.
The punch card was an early form of data storage for computers. Some mainframe computers still use 80-column punch cards. Binary data is encoded in
the card using holes (or lack of holes) to represent ones and zeros (data bits).
These principles were applied to punched paper tape, and later to magnetic
tape, which held more data than could its paper counterpart. Instead of holes,
magnetic tape used recorded magnetic signals in a fashion similar to audio
tape recorders.
STOP
The original Apple II and IBM PCs had a port to which you could attach an audio tape recorder for
data storage.
One disadvantage of all tape drives is that they access data relatively slowly.
This is a limitation of their design. Most tape drive’s read/write heads stand
still while the tape is moved over it. To get information from two sectors at
opposite ends of the tape, the drive must travel across the entire distance
between them (this is known as sequential access). No matter how good the
equipment, or how fast the motor, tape can wind only so fast before it snaps.
Disk drives are a great improvement. By using a spinning platter instead of
winding tape, the media can move at far greater speeds. But the greatest
advantage comes from putting the read/write head on an armature that can
move across the platter to access the information “randomly.” The head no
longer has to travel down the length of the media but can cut across tracks of
information to access what it needs directly.
STOP
The first disks used for data storage were 8- and 14-inch floppy disks. Despite their large size,
these early floppies had very limited storage capacities, typically 100 to 200 Kilobytes (KB).
Chapter 2: All About Drives
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The first true hard drive
IBM developed the first true hard drive technology in 1973. It was named the
“Winchester 30/30,” after the Winchester 30/30 rifle. The hard drive used two
30 MB disks, one fixed and one removable. The unit eventually got marketed
as the model 3340.
STOP
The first widely available floppy drive was a 143 KB, 5.25-inch disk developed for the Apple II in
1979. To date, most hard drives are still manufactured by American firms, although companies
from other countries are always trying to break into the market.
It wasn’t until 10 years later, in 1983, that hard drives hit the mainstream
with IBM’s introduction of the PC XT. As technology improved, hard drives
grew. Capacity doubled almost every 18 months. Price per megabyte dropped
rapidly, from around $50 per megabyte in 1983 to under $0.25 per megabyte in
1995.
STOP
Price per megabyte continues to drop at a rate of 10 percent or more per quarter.
The density of the magnetic material on the surface of the hard drive’s disk
platter increased, allowing storage of more bits of data on increasingly smaller
areas. The read/write heads were made smaller and more sensitive so they
could read a smaller magnetic domain. This made it possible to store even
more bits per inch on the disk surface.
STOP
A magnetic domain is the amount of space on a magnetic disk needed to store one bit of data.
The number of platters and heads have also increased. Some hard disk drives
today have as many as ten platters, with two heads per platter—one reading/
writing the top of the platter, the other the bottom.
Improvements in data encoding reduced the amount of platter space needed
for mapping out the location of data on the disks. Originally, encoding used
half the space on each disk. Now, a multiple-platter hard drive using a dedicated servo head actuator can put all the encoding data on one side of one
disk, freeing the remainder of the disks for storage of the user’s information.
STOP
NOTE
26
A head actuator is the physical device that moves the armature and thus the read/write head(s)
across the surface of the platter. Servo information is used for guiding the head’s positioning over
data.
For more information on data encoding, see “Encoding” on page 44.
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Mechanics
To understand a hard drive, you need to look at it from both a hardware and
software perspective. Hardware is discussed below. Software is discussed in
“Operations” on page 43.
Anatomy of a hard drive
Overview
A hard drive is a small box of magic. It is the result of years of cooperative
development in many areas:
•
•
•
•
computer software
electrical engineering
mechanical engineering
metallurgy
All these disciplines working together to create such a successful product is a
great example of teamwork.
The hard drive’s mechanics can be divided into two main functions:
• head movement (positioning it over the correct track)
• disk movement (spinning it so the desired sectors pass under the head)
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Magnetic Spinning Platters
Housing Unit
Magnetic Heads and Armature
DC Spindle Motor
Figure 2. Exploded internal view of a hard drive
Internal hard drive components include:
•
•
•
•
28
magnetic read/write head
magnetic spinning platter
power supply
specially designed electric DC motor for spinning the spindle at a
constant rate to which the platters are attached
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•
•
•
•
armature to which the read/write head is attached
atmospheric controls
vibration isolators
head actuator
Some of these have been touched upon in the preceding text and are illustrated in Figure 2, above.
On most drive mechanisms, jumper pins or DIP switches allow you to adjust
the following settings:
•
•
•
•
•
SCSI ID
terminator power
parity
wait spin
various diagnostics
These jumpers and DIP switches are located on the drive’s controller. A drive
controller is circuitry located on the housing unit of the drive that provides
the hardware interface between the disk drive and the computer, sending and
receiving signals. It interprets the computer’s signals and controls the operations of the disk drive.
STOP
The “DIP” in DIP switches, stands for dual in-line package.
Chassis
Drive Housing Unit
Fuse
Power Switch
Vent
Fan
AC Power Port
SCSI ID Selector
SCSI Ports
Figure 3. Cross section of an external (enclosed) hard drive
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External components of a typical external hard drive include:
• chassis (also called casing box or enclosure)
• LED indicating that power is on
• LED indicating when the drive is actively transferring data (on most, but
not all drives)
• power switch
• AC power jack
• SCSI ports (usually two)
• SCSI ID switch
STOP
Some drives require jumper pins or software to change the SCSI ID.
Head actuators
The read/write head is attached to an armature that is moved across the platter in precise incremental distances by mechanical devices called head actuators. The head actuator moves the head in a slight arcing motion across the
surface of the platter. It uses the spinning of the platter in conjunction with
the head’s arcing movement to access off-center areas on a data track.
Figure 4. Stepper motor head actuator
Read/write heads are designed to detect and create magnetic domains on the
drive’s platters. A magnetic domain is the area on a platter that contains one
bit of data. The most common type of read/write head is the inductive head. It
consists of an iron core with 8 to 30 coil turns of wire banded around it. Current flowing through the coils of wire causes a magnetic field to flow through
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a gap in the iron core. The nature of the magnetic field depends on the direction current is flowing in the coil:
• During a read operation, the head passes over a magnetic domain on the
media. This causes the field to be inducted into the core and current to
flow into the coil. Information is gathered from the media into the core
and passed on to the read channel electronics.
• During a write operation, the drive supplies the current to the coil causing
a field to be induced onto the media.
The width of the head gap in the iron core determines to a great extent how
dense data is on the disk. The wider the gap, the wider the tracks will be and
the less dense the data will be on the disk. A smaller gap allows the head to fly
closer to the platter, increasing reliability and data pickup. Gap width determines track density in tracks per inch (TPI).
Most modern hard drives have TPI in the 3000-plus area. You can calculate
the number of tracks per inch using the following formula (Figure 5):
Tracks per inch (TPI) =
Outer disk radius – Inner disk radius
Track Width
Figure 5. Formula for calculating tracks per inch
Head disk assembly
The head disk assembly (HDA) must be sealed with very clean air inside. The
HDA is not hermetically sealed; it needs air circulation to operate. But since
the smallest particle would interfere with operation of the drive, most HDAs
have absolute filters that prevent particles bigger than a certain size from contaminating the disk. These filters usually have a filtering ability of several
tenths of a micron. At the same time, they allow outside air into the sealed
area of the drive to equalize pressures and temperatures in accordance with
changes in the computer’s external environment.
STOP
Air is pulled into the drive by the centrifugal action of the platter and the spindle.
The head is mounted on a slider. The head/slider assembly is designed aerodynamically to enable it to fly across the disk surface on an air cushion created
by the spinning disk.
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STOP
A slider is a rigid block of material upon which a disk head is mounted. It provides lift, stability
and mechanical strength to the head assembly. There are many types of sliders: A composite
slider has a ceramic body and a thin core bonded with glass; a mini-composite slider is like a
composite only 2/3 smaller; a monolithic slider is the oldest type of Winchester slider, made
entirely of magnetic ferrite; a mini-monolithic slider is like a monolithic slider only 2/3 smaller;
and a thin film slider, which is a silicon-based slider. In a thin film slider, the head, head core, and
coil are all built into the slider.
Heads are primarily made by a thin film process. The process is similar to
making chips: a silicon substrate is doped and etched to form the slider. Hundreds of heads are created on a single wafer.
Some heads can record data longitudinally. This means that the magnetic
field lines that move from the gap to the platter travel in a direction that is
parallel to the platter surface and the track’s length.
Other heads record data in a perpendicular or vertical method. In this case,
regions of magnetic domains have their magnetism pointing up or down, into
or out of the surface. This type of recording allows for greater data densities.
With inductive heads, the higher the bit density, the smaller the signals
induced by these bits. The smaller the signal, the harder it is to measure reliably. This increases the need for a highly sensitive head, capable of accurately
locating and reading the weaker signal.
To address this problem, IBM developed the magneto-resistive (MR) head in
the late 1980’s. MR heads are thin conductive strips that detect magnetic
domain transitions by measuring magnetic effects on the resistive element
within the head. This element changes its resistance as its angle of magnetization changes. IBM first brought this technology to market in 1991 in a 1 GB
3.5-inch drive. It made possible data densities of over 1000 megabits per
square inch, a 10 to 25 percent improvement over previous technology. Second generation MR heads doubled densities to over 2000 megabits per square
inch.
MR technology has two major limitations that have prevented it from being
utilized by more than a few of the latest high-end drives:
• MR heads only read data; drives still need an inductive write head.
• They’re expensive; MR heads are double the price of inductive heads.
The next head technology to supersede MR is giant magnetoresistance (GMR).
GMR allows for large changes in resistance even when its exposed to a very
small magnetic field of bit transitions. It may permit magnetic recording in
the tens of gigabits per square inch and data rates in the tens of megabytes per
second. This new technology has sensitivity in the greater-than 3000 microvolt per micron of track width range. This should continue the 60 percent per
year growth rate of data density.
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Access time and seek time
Seek Time + Latency + Time to Read a Sector = Access Time
9 ms + 6.7 ms +
1 ms
= 16.7 ms
Figure 6. Formula for determining access time
Access time refers to the time it takes the read/write head to move back and
forth in search of the appropriate platter and sector. Seek time is similar but
involves finding a track rather than accessing data. Seek time is the time it
takes for the read/write head to move back and forth before it locates the
appropriate sector of information. Average seek time is the amount of time it
takes to position the drive’s heads on a randomly located place on the disk.
Seek time usually runs in the 2 ms range. Seek time is a factor in the formula
that determines access time. Other factors include:
• latency
Latency is the time it takes for a desired sector on the platter to pass
underneath the head after head positioning is complete. For a platter
spinning at the 7,200 RPM rate, average latency is 2.99 milliseconds. The
faster the platter rotates, and the more read/write heads per platter, the
smaller the latency and the better the performance.
• head switch time
Head switch time is the time it takes to switch between any two read/
write heads. Head switch time runs in the 1 ms range.
• cylinder switch time
Cylinder switch time is the amount of time it takes to move from one
track to another.
• command overhead
Command overhead is the time a controller needs to interpret and act
upon a computer’s command.
Access time equals latency plus seek time plus command overhead. Some
operations do not include command overhead time in the equation, while others add data transfer rates into the equation.
Heads and platters
In drives with more than one platter, the multiple armatures make up a comblike structure whose “teeth”—the arms and heads—are interspersed with the
platters. Most drives have two heads per armature (and thus per platter). In
this scheme, one head is on top of the platter, the other below. Data is written
Chapter 2: All About Drives
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to and read from both sides. This allows the entire platter to fill with useful
data. On drives with a dedicated servo positioning platter, the head for the
servo platter is used to position the other heads.
NOTE
For more information on servo positioning, see “Servo positioning information” on page 37.
A drive with five platters would have 10 heads, with only one being used at
any one time. There are special drives, such as the Seagate ST12450W, that
utilize parallel transfers from two heads at the same time. This effectively
doubles transfer rates but decreases data storage. A normal, transferable data
sector is 512 bytes. In order for both heads to participate simultaneously in
data transfer, each head handles half that load. Consequently, sectors need to
be half their size on each platter to form a normal 512 byte data sector—storage capacity is subsequently reduced due to the extra error correction code
and sector overhead information.
NOTE
For more information on error correction code, see “Error correction” on page 47.
Double actuators
A few drives feature two actuators, positioned at opposite sides of a disk. This
design was first seen on Conner’s Chinook in 1991. This scheme cuts the
drive’s latency time in half. Drive latency is the time it takes for the spinning
platter to bring around the desired platter location to where the read/write
head can access it. You could write with one head and read with another. But
because reading and writing are not simultaneous in non-multitasking operations, this design—although costly—offers no performance improvement in
those applications. Typically, it’s cheaper to buy multiple drives.
NOTE
For more information on latency, see “Spinning ‘round” on page 39.
Actuator movement
There are two methods for head actuator movement:
• stepper motor, a type of rotational pivot used in a floppy drive
• servo voice-coil actuator, which is a more efficient and accurate method
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Stepper motors
Figure 7. Stepper motor actuator
Stepper motors use a system somewhat like a car’s transmission to convert an
incremental rotary movement into linear travel. The motor rotates either
direction a few degrees at a time. Connected to it is the stepper band, which
converts that incremental rotary motion into linear movement, repositioning
the armature and thus changing the read/write head’s position over the platter. As the assembly ages, stepper motors will usually cause problems with
precision, known as head drift. This is due to the unadjustable mechanical
nature of the motor. Stepper motors went out of use in the early 90’s.
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Servo voice coil actuators
Figure 8. Voice coil actuator
Voice coils, or servos, work like common audio speakers. The sound of a loudspeaker is determined by the strength of the current pumped through its electromagnet, which pulls on a diaphragm connected to the speaker’s cone. In a
hard disk drive, the current passes through a voice-coil electromagnet that
pulls the armature toward it. The armature is held back by a spring that provides a counter-force to the magnet and automatically moves the head back
when the current is decreased. The strength of the current going to the voice
coil determines the position of the head over the platter, making it infinitely
adjustable.
STOP
Servo control is the process used in disk drives to regulate head positioning and seeking. It relies
on “positioning data” that is either contained on a dedicated servo disk or embedded among data
blocks on all disk surfaces.
Voice coils provide an infinite degree of positioning control. This is superior
to the stepper, whose accuracy is limited by its incremental rotational step.
By moving the head in smaller increments, the voice coil actuator can take
advantage of higher-density platters, which squeeze more tracks onto the
same platter. They are also less susceptible to head drift. As the head assembly ages, the servo system can automatically compensate for the wear.
The voice coil system performs much faster than the old stepper motor. A
large amount of current can be sent into the coils to push the head more
quickly. With the stepper motor, the current is a fixed quantity.
Another characteristic of the voice-coil method—and another advantage it has
over the stepper method—is that the head is usually “autoparking.” Autoparking helps to prevent head crashes. Upon power loss, the heads are automatically pulled by the spring to a safe position on the disk—the landing or
36
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parking zone. Usually a capacitor is used to store extra charge, forcing the
voice-coil to autopark onto the mechanical stop even if power is lost. Some
stepper-motor drives come with an autoparking feature. This can be a function of the controller or part of the installation software.
Servo positioning information
The slightest fluctuation in the electric current can cause the head to wander
away from the center of the track. To prevent this, servo data is embedded on
the platter between the tracks in the form of magnetic bursts known as
embedded servo or wedged servo. Wedged servo was the first attempt at
embedding servo information on the platter. It placed a single burst of servo
position information on each disk. Because there is only one burst, the drive
would have to wait a full rotation to get the information, slowing performance. Others employed doubled embedded servos, placing servo information
at the beginning and middle of the sector.
Later designed drives employed an embedded servo where multiple servo
bursts are present on each track, each in front of a sector. This allowed for
even greater accuracy. When sensors on the head sense that the bursts are too
strong, the controller knows that the head is wandering from the center of the
track and adjusts the current accordingly.
Embedded servo tracks take up space on the platter and reduce its data storage
capacity. Some high-capacity, multiple-platter drives utilize a dedicated servo
surface, in which one side of one of the platters contains only servo data,
which is used to guide the head. The head for that surface is used solely for
positioning. Others use embedded servo information encoded within a normal
data platter. Because all heads are attached to the same actuator, all of the
heads will be aligned and all other surfaces can be used for data.
Some drives employ a hybrid servo system where there are dedicated servo
platters as well as embedded servo information on each of the other drive platters. This ensures that the drive can make precise positioning changes during
read and write operations. The disadvantage is that a great deal of capacity is
wasted.
Read/Write channel
After the read/write head is positioned over the data, a group of circuits
known as the read/write channel actually transfers the data off the platter.
These circuits read the sector header information to ensure that the data
being read or written is the data that is desired. They also amplify signals to
ensure they are strong enough to record information. These circuits must be
extremely fast to handle the transitions between looking at sector information and reading or writing the data into the sector in the same rotation.
These circuits are often the limiting factor in drive performance.
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The Medium
Disk platters are made in a variety of materials:
• aluminum
• ceramic
• glass
Glass platters have been used in 1.8-inch drives. They allow 20 to 30 percent
greater density than metal platters and offer a smoother, more rigid surface so
heads can fly lower. Most platters are made of finely machined aluminum.
Aluminum is preferred because it is strong and highly pliable. This is ideal for
creating thin platters. Aluminum does not expand much when it is heated.
A platter’s structural integrity and weight are important, but what matters
most is the coating applied to its surface. For forty years, ferric oxide (FE2O3)
coatings have been used as the primary medium for data storage. In recent
years, a variant using cobalt-nickel alloys has been used to improve densities
and provide better reliability. If a floppy’s surface looks rusty to you, it’s
because it’s covered with rust-oxidized iron particles held in place with a
binding agent (typically plastic). These are the particles that are magnetized
by the magnetic head to represent the on (1) or off (0) patterns of data.
Magnetic recording is based on the fact that metals can be permanently magnetized by external magnetic fields. Areas saturated with magnetic charge are
known as “domains.” The magnetic head of the disk drive senses data by the
amount of current induced in the head as it passes over the boundaries
between domains and passes this on to read-channel electronics.
STOP
A magnet is a metal that has been exposed to a magnetic field, causing the electrons in the metal
to spin in a consistent direction. Electrons are theorized to spin in either a North-South or SouthNorth orientation. Groups of atoms can be magnetized to align themselves in a particular
direction.
Much more difficult, but more rewarding, is the application of a chrome-like
covering found on most late-model, high-speed, high-capacity hard drives. A
pure metal film is plated onto an aluminum platter, either by vapor deposition
or sputtering. The thin-film media of a plated surface, as it is called, is onetenth as thick as the standard oxide coating. Because this surface does not
have a binding agent mixed in, the magnetic particles are packed much more
tightly, allowing as many as 80,000 bits per inch. Oxide coatings allow only
20,000 bits per inch.
The future promises more innovations in data storage media, such as the glass
platters recently introduced in some 2.5-inch drives. Glass offers a smoother
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surface and better rigidity than aluminum or ceramic. It is very useful for platters that spin faster than the standard 3,600 RPM of the 1980’s.
Spinning ‘round
Platters are mounted on an axle called the spindle. A drive may contain more
than one platter, yet all platters are mounted on a single spindle. The spindle
usually rotates the disks in a counter-clockwise direction.
The spindle is turned by a brushless, direct-drive (no gears or belts), direct-current (DC) electric motor. This motor may be built into the spindle or may
reside below it.
STOP
The motor that resides below the spindle is called a pancake motor because of its flat shape.
Depending on the specific drive, hard drive motors spin the disks at 5,400
RPM (which is typical), 7,200 RPM, or even 10,000 RPM. With higher spindle
rates come higher noise and heat levels but also higher performance.
Head parking
Safe Landing
Zone
Platter
Arm
Figure 9. Safe landing zone for head parking
If a head were to physically contact a platter it could destroy data, remove
magnetic particles from the disk surface, and cause irreparable damage.
While the platter is spinning, the read/write heads are kept off the platter by
the air cushion the spinning creates. However, heads must be moved away
from the data area of a platter when the hard drive is turned off. This process
is called head parking. Portable and laptop computers also have a “sleep”
mode that parks the head after a certain period of inactivity.
STOP
This air cushion created by spinning is known as the Bernoulli Effect.
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A head is parked when it is moved over an area on the platter that is used only
as a “parking space” or “landing zone.” This area is typically at the center of
the platter near the spindle (Figure 9). Head parking avoids accidental contact
of head and platter, and is particularly important if the drive is being moved.
Some drives park the heads entirely off the platters, preventing any chance of
accidental contact from shock. Typically these drives also push a small wedge
between the pairs of heads to keep them from slamming into each other.
Head parking only reduces the chances of the head contacting the platter. To
eliminate the possibility of accidental contact, many newer drives feature
autolocking. Autolocking physically locks a parked head over the landing
zone, preventing any movement.
Many drives that use a stepper actuator need utility software, often provided
by the manufacturer, to tell the controller to park the head on computer
power-down. This is obviously not as safe as an automatic device built into
the drive itself. Some stepper actuator drives come with an autopark mechanism built into the controller.
STOP
Most hard disk drives manufactured since 1986 are autoparking.
Form Factor
Form factor is a fancy way of referring to the platter’s diameter. A hard drive
with a form factor of 3.5 inches has one or more platters that are 3.5 inches in
diameter. Standardized form factors allow drives to be installed on a wide
variety of platforms with minimal changes.
The size of the entire housing unit, which also contains the mechanics and
controller, is much larger than its form factor measurement. The housing unit
or frame is the foundation to which all other parts are connected. This frame
needs to be rigid so that components within the drive are not hurt on installation or during operation. Most housing units are made from aluminum alloys.
Technological refinements in disk capacity have reduced the number of platters needed in high-capacity drives. Half-height models (1.625-inches tall for
5.25-inch drives) require fewer platters, and take up less vertical space in the
casing. One-third-height, low-profile drives (1-inch tall for 2.5- and 3.5-inch
drives) are readily available. Laptop computers that need high-capacity storage
can take advantage of the compact size of 2.5-inch hard drives.
The more common hard drive sizes at the end of 1996 include:
• full-height 5.25-inch drives with capacities of up to 23 GB
• half-height 3.5-inch drives with capacities of up to 10 GB
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• one third-height 3.5-inch drives with capacities of up to 5 GB
• 2.5-inch drives with capacities of up to 3 GB
The trend has been to migrate most internal drives for Macintosh and PC to
the 3.5 inch form-factor because of its price-per-megabyte advantages. Fiveand-a-quarter-inch drives are used only for the highest capacity applications.
Eight-inch drives are virtually extinct.
STOP
The Macintosh II, Macintosh IIx, Macintosh IIfx and some older PCs were built to accommodate
internal 5.25-inch half-height drives if a user wished to install one.
Smaller, 2.5-inch drives are gaining in popularity, primarily for laptops and
notebooks. These 2.5-inch drives are available in a variety of heights from
19mm to 12.5mm. The most common size for external hard drives is
3.5 inches, although you can find 5.25-inch, 8-inch, and 14-inch drives in
some mainframe or workstation applications.
Figure 10. A 2.5-inch drive
In 1996, JTS Corporation introduced a 3.0-inch form factor disk, suitable for
notebook computers. Three-inch platters have 70 percent more surface area
than 2.5-inch platters, allowing them to hold more data.
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Generally, the trend in the marketplace is toward smaller and smaller drives.
Higher densities make it possible to develop small, high-volume platters. Platters have gone from 14-inch to 8-inch to 5.25-inch to 3.5-inch to 2.5-inch (and
back up to 3.0-inch, in the instance of drives created for notebooks). Even
smaller 1.8-inch drives have been developed for use in personal digital assistant, palmtop, or laptop computers. The 1.8-inch drives typically come in a
PCMCIA type II or III credit-card sized form factor. Even smaller 1.3-inch
drives were made at one time but proved unpopular. The smaller form factor
drives were not popular because of their high cost per megabyte.
STOP
PCMCIA stands for Personal Computer Memory Card International Association. It’s a joint effort
of various special interest groups aimed at setting a standard for memory cards used in PCs.
PCMCIA cards add improved computer memory capacity or enhance connectivity to external
networks and services.
Because storage capacity is the paramount concern of most users, the actual
size of a drive is important to consider only when buying an internal drive—it
has to fit inside the computer’s casing. You are limited to a certain drive size
that may not offer the capacity and performance you want. You should worry
about the dimensions of a large external drive only if you have limited space
or you want your computer system to be portable.
STOP
Look out for Bigfoot! In early 1996, Quantum Corporation announced the release of a low-cost,
5.25-inch form factor drive, called Bigfoot, that can be plugged into a bay used for a CD-ROM.
The larger form factor is Quantum’s way of dealing with the constant market demand for
increased storage—up until Bigfoot, manufacturers increased storage by increasing areal density
on a disk or adding more disks and heads. Quantum anticipates that Bigfoot will support only
Fast ATA-2, spin at 3,600 RPM, and have a 15.5 ms access time.
In selecting a drive there are other considerations beyond the drive’s physical
dimensions. Smaller drives cost more per megabyte than full-size drives and,
despite their denser platters, do not deliver better performance. Instead, they
may sometimes prove to be both slower and less accurate, as the heads have
to move more often and there is less room on the controller for a memory
cache or other performance-enhancing options.
NOTE
For more information on caching, see “Caching” on page 52.
Another disadvantage to internal drives is they use the computer’s own power
supply. This may adversely affect the computer, particularly if it is a “loaded”
machine. An external disk drive avoids this problem by using its own power
supply, allowing it to be easily moved from one computer to another.
STOP
42
An example of a “loaded” machine is a computer that has many internal devices and expansion
cards, such as an accelerator or graphics card. These also consume power from the computer’s
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internal transformer and generate heat, reducing the efficiency of the computer’s overall
operation.
Operations
Formatting
Before a hard disk can be written to, it must be formatted. Formatting’s basic
and critical function is to lay down tracks and sectors.
Tracks are the invisible concentric circles of magnetic “grooves” within
which data is physically stored on the platter. Sectors are subdivisions of
tracks and are usually defined to contain exactly 4,096 bits of data (that is, 512
bytes times eight bits per byte). Besides the 512 bytes (or more) of data, a typical disk sector contains the following information:
•
•
•
•
•
•
•
synchronization information
defective sector flag
sector header information
sector header CRC (cyclic redundancy check)
intrasector gap (separates the sector ID information from the data track)
error correcting code (ECC)
intersector gap (separates the preceding sector from the next sector)
There is a lot of information in a sector. The drive needs it for housekeeping.
It is not accessible to the user. Some hard drive manufacturers factor these
housekeeping bytes into total capacity, so the drive capacity available to the
consumer is actually less than the stated capacity of the drive.
IBM has developed a technique, known as NO-ID sector format, that reduces
this overhead, resulting in capacity increases of up to 10 percent. It uses the
servo control system to locate physical sectors and a defect map in RAM to
find logical sectors.
Formatting specifies alternate sectors, also called spares. The controller will
utilize a spare while formatting when the read/write head encounters a bad, or
corrupted, data block on the platter.
STOP
Data blocks are the smallest chunk of memory accessed or transferred by the disk drive. They are
usually 512 bytes, although they can be multiples of 512 bytes.
To support larger logical block sizes, drives automatically block and deblock
physical sectors in the current block size. A data block allocated on the spare
sector will correspond to the now unusable data block. The number of spares
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per track is configurable. It’s possible to eliminate spares altogether. This frees
up that sector for storage. However, we recommend having at least one spare
per track. Typically, one spare is the default setting of a preformatted drive.
Drives usually keep a lookup table of spares in their local RAM. Consequently, the number of spares that can be handled is fixed, limited by the size
of this RAM.
By putting tracks and sectors in place, formatting sets up the hard drive’s optimal organizational structure. It assigns logical data blocks to physical areas on
the platter, skips over any defective areas it encounters, and keeps a grown
defects list of the bad areas so that the controller knows not to write data to
them.
STOP
“Logical” is used to distinguish an abstract from a physical representation. Data is not physical;
its representation is made physical via the read/write head magnetizing portions of the platter
into recognizable patterns. For more information on logical blocks, logical units, and logical unit
numbers, see Chapter 5, “All About SCSI.”
Assigning logical blocks to physical sectors maximizes the drive’s ability to
access information via interleaving, optimal block size, and other software
configurable aspects of the hard drive’s operation.
NOTE
Interleaving is discussed in “Interleaving” on page 48.
Encoding
Once you have a fully functioning hard drive with all components cooperating, how does your computer store information on it? And how does it find
and read this data after it is translated into magnetic patterns?
Information is not written as one contiguous stream in each track, but is broken up among sectors. The read/write head dips in and out of the sectors to
read and write data as needed.
Encoding enables the head to read and write data on a moving target in fractions of a second, and to access that data again just as quickly. Encoding refers
to two things:
• the protocol of laying down data as a sequence of on’s and off’s
• the process that lets the drive head constantly monitor its own position
along a track
You might think of encoding as the digital equivalent of putting sprocket
holes in a film strip. As the number of digital sprockets are tallied, an accurate
measure of how far the head has moved may be determined by equating time
with distance. This is possible because the speed of a platter’s rotation is a
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known value. If one value—timing—is known, the crucial value—location—
can also be determined.
Variable zone recording
The outer tracks of a platter are of greater circumference and therefore physically longer than the inner tracks. Consequently, the outer tracks offer more
real estate for data storage. In IDE and SCSI storage devices, controllers can
take advantage of the outer tracks’ greater storage capacity in a process called
variable zone recording. Variable zone recording keeps data density constant
throughout the disk by increasing the number of sectors on longer outer
tracks. This is possible because data is accessed at a high-level by specifying a
block number and not cylinder, head, sector.
STOP
In pre-IDE/SCSI drives, such as ST-506 drives, all tracks are typically assigned the same capacity.
With variable zone recording, tracks are grouped into zones. The tracks in
each zone are allotted a specific number of sectors. The outer-zone tracks
receive a higher number of sectors than the tracks in the inner zones. In addition to increasing the capacity of the platter, zone sectoring reduces access
time and increases transfer rates because the head covers less ground to access
the same amount of information on the platter.
The number of zones and the number of sectors per zone vary according to the
size of the platter. Generally, drives with smaller form factors have fewer
zones, but other considerations, including the type of media used and how it
was formatted, also affect this.
Encoding processes
On a platter spinning at 7,200 RPM, a read/write head has barely one-thousandth of a second to read a sector of information. At this speed, and with all
this informational ground to cover, the head could easily lose its place. Encoding processes were developed to keep track of the head’s location on the disk.
STOP
Advanced encoding methods also enable greater data densities without requiring more space.
Timing mechanisms correlate a platter’s constant speed with the distance
traveled to yield a precise calculation of the head’s position over the platter.
Frequency Modulation (FM)
Frequency modulation was the first timing mechanism. Every other magnetic
domain represented a clock pulse. This method had the disadvantage of using
half of a disk’s storage capacity just for the timing information.
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Modified Frequency Modulation (MFM)
Modified frequency modulation solved this problem by moving the timing
information onto its own track. During the original low-level formatting process of an MFM drive, the synchronization bytes are added to the data sectors.
Synchronization bytes are magnetic data that marks location and time.
Because MFM results in twice as much data storage as the original FM, it is
also known as double-density recording.
Run Length Limited (RLL)
Run Length Limited (RLL) encoding was borrowed from the world of mainframe computers. It increased drive storage capacity beyond MFM.
As you might guess with a concept borrowed from mainframes, the theory
behind RLL is more complicated than any normal computer user needs to
know. Basically, RLL uses only a few selected optimal bit patterns, determined from magnetic flux changes in storage media. This makes it possible
for RLL to use less disk space than MFM to store a bit. It actually uses smaller
magnetic domains, or “bit cells.” The read/write head of a specialized RLL
controller can create bit cells one-third smaller than those used with MFM
encoding. This requires a drive with a more accurate read/write head and
higher density (or thinner) tracks. RLL also uses a special code that can read
16-bit patterns of information, rather than the eight bits used by MFM. Some
MFM drives can use RLL encoding and controllers, but it is not advisable on
older drives.
There are several types of RLL codes. The distinguish themselves from each
other by the number of zero bits each can store as a consecutive string before
inserting a one bit.
STOP
Bit is a contraction formed of “binary” and “digit.” All computer information is represented as a
unique combination of the binary digits 0 and 1, which are also called “ons” and “offs.”
• 2,7 RLL allows from two to seven consecutive zero bits. This results in a
50 percent increase in drive capacity over standard MFM.
• 3,9 RLL allows from three to nine consecutive zero bits. This method,
also called Advanced Run Length Limited (ARLL), results in a 100 percent
increase in drive capacity over standard MFM.
• 0,4,4 RLL allows consecutive one bits in a data stream. There can be up to
four zeros between ones in certain data sequences. This allows a
25 percent or more increase in capacity over 3,9 RLL encoding.
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Data detection
Data is detected in hard drives typically with a technique known as peak
detection. Peak detection has been used for data detection since the 1960s. It
involves detecting data at high speeds by using a data-encoding method that
spaces the signal during reads. The analog detection circuits can then scan
each peak in series. The problem with this encoding scheme is that it generates a lot of overhead, slowing performance.
A newer technique is known as partial response maximum likelihood
(PRML). This began to appear in drives in 1990. Instead of spacing out analog
peaks as in peak detection, digital filtering is used to compensate for signal
overlap. After filtering, the scheme identifies what are the likeliest sequence
of data bits written to the media. This scheme still uses RLL encoding but can
utilize advanced (0,4,4) encoding for greater density. The digital ones can take
place without all the digital zero spacers. A 25 percent or better improvement
in bit density can be achieved. Transfer rates increase by 25 to 50 percent.
PRML causes larger errors, so error correction code needs to be longer (144-bit)
and faster on these drives.
Extended PRML (EPRML) offers read channels with refined equalization to
achieve 10 percent or better gains in areal density over PRML. EPRML began
to appear in drives in 1996.
Error correction
Data stored on platters inevitably develops errors. Drive manufacturers have
devised effective techniques that minimize the possibility of data loss. One of
these techniques is error correcting codes (ECC), which store additional check
information at the end of each data block. These extra bytes are used to verify
the integrity of the block and to correct for minor errors.
Each hard drive manufacturing company uses its own proprietary ECC software technique. Many drives can correct an error burst up to 22 bits long by
using 11 bytes of ECC data per 512-byte block. More advanced drives can correct even more bits per burst. These ECC data bytes are read when using
Read/Write Long commands on IDE and SCSI drives. All this error correction
is transparent to the user and happens internally in the drive. Bits can be set
in Mode Page 1 of SCSI drives, to alert the driver that ECC was needed to correct data flaws.
NOTE
You can learn more about mode pages in Chapter 5, “All About SCSI.”
When you hear drive heads doing a hard seek (a hard seek usually makes a
cha-chunking noise), it’s probably because retries are occurring. Retries cause
the head to seek back to track zero and retry the data access. A drive will try
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this multiple times. The number of times is defined in a SCSI drive’s Mode
Page 1 settings. If you hear the cha-chunking noise, you probably should back
up all your data and test the drive’s media for defects.
Interleaving
1:1 Interleaving
6:1 Interleaving
Figure 11. Examples of 1:1 and 6:1 interleaving
The central processing unit (CPU) of some computers can’t handle data as fast
as the hard drive controller can access the data on the spinning platter. By the
time the CPU digests the information that the head has just read from one
sector and orders the head to access the next sector’s information, that second
sector on the spinning disk may have already passed by. To read it, the head
must wait a full platter rotation for that sector to once again pass beneath it.
A hard drive’s interleaving, set during low-level formatting, overcomes the
CPU’s slower response capability by matching data transfer rates between the
read/write head and the Input/Output (I/O) port of the computer.
Interleaving describes how sectors are arranged on the platter so the head can
read them in the fastest possible sequential order. The information allotted to
ordered sectors does not follow the sectors’ sequential numerical order. Information is placed on sectors that are not physically contiguous, so the head
doesn’t have to wait a full rotation for a missed sector to come by again.
On faster machines, a smaller interleave can be accommodated. Nowadays, a
1:1 interleave factor is fairly standard (Figure 11).
• A 1:1 interleave (actually contiguous sectors with no interleave) provides
the fastest data access on faster machines, such as the Macintosh II family
or newer or 80386/20, 80486, and Pentium and Power PCs.
• A 2:1 interleave factor is optimal on the Macintosh SE or 80286/10 or
faster machines.
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• A 3:1 interleave factor is optimal on the Macintosh Plus or 8086 or 80286/
6 machines.
Interleaving to optimize performance became such a big issue on PCs that
several programs, such as SpinRite™, were written to re-interleave drives
without losing data.
With interleaving, what is optimal depends on whether the drive has a track
buffer. A track buffer stores the contents of an entire track that passes under a
head even if the I/O request isn’t for the entire range. Early ST506 and ESDI
drives had track buffers on the controller card, while modern IDE and SCSI
drives have them on the drive itself. A 1:1 interleave is supported on all computers for drives with track buffers. Most modern drives (those produced after
1994) actually ignore the interleave value specified during a format and
always use 1:1.
Sector skew
Track one
Track two
Track three
Sector four
on track three
Track four
Tracks one through four
equal one cylinder
Figure 12. Sector skew offset
Sector skew improves data access by optimizing the placement of information
on adjacent tracks, in much the same way interleaving optimizes head movement within a track. It does this by taking into account the time it takes the
head to move to another track and the distance a sector will travel in that
time (due to the platter’s rotational speed), and then offsetting the sector numbering on the next track according to those calculations.
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Suppose the read/write head has just finished writing to the last sector on one
track. Given a 1:1 interleave, the head is ready to write to the first sector of
the next track. If this takes too long on a revolution of the disk, the head will
arrive too late in spite of the interleaving. To solve this, the numbering for the
sectors of the next track is offset by one or more positions. This ensures that
when the head travels to the track, the correct sector has not already passed.
Figure 12 shows a typical four-platter hard drive with a sector skew. Tracks
are laid down in a cylindrical order on multi-platter hard drives. The first platter holds the first track, the second platter holds the next, and so on down the
cylinder.
The concept can also be applied to a controller accessing another platter in the
drive, with a different read/write head. The second platter would be skewed as
many sectors as necessary to ensure that the proper sector is at the head when
the controller is ready to access that platter.
Data transfer rates
Data transfer rates are described in megabytes, megabits, or megahertz (millions of bits) per second. Data transfer rates are determined by the density of
data on the platter and how fast that data can be transferred to the host computer. The read channel on the head and the host interface to the computer
are the biggest factors limiting data transfer performance. Read channels are
being improved, as are links between read channels and controllers.
The sustained data transfer rate is not affected by the host interface. Sustained
data transfer rate is the average rate at which a drive can transfer data. Sustained transfer is produced by averaging because most drives have data densities that vary according to where data is located on the disk. Tracks toward
the inner perimeter hold less data than tracks on the outer perimeter. A typical drive with 3200 tracks per inch has 36,352 to 57,856 bytes per track.
The sustained data transfer rate on a hard drive has lagged behind the host
interface data transfer rate, and will probably continue to do so for the foreseeable future.
Table 1 illustrates operational speeds and data densities typical in 1996:
Table 1. Typical operational speeds and data density in 1996
50
Category
Description
Speed
7200 rotations per minute (RPM) ÷ 60 seconds = 120 revolutions per second
Average Formatted Track Capacity
45,000 bytes
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Table 1. Typical operational speeds and data density in 1996 (Continued)
Category
Description
Average Sustained Data Transfer Rate
5.15 MB/s (more accurately from 4.16 to 6.59 MB/s)
To/From Host
40 MB/s burst (this is on an Ultra SCSI device)
Transfer rates have not kept up with the fast pace set by CPU performance.
This is illustrated in Table 2.
Table 2. Comparison of transfer rates and CPU speeds
Improvement
ratio
Year
Transfer rates
CPU performance
1984
1 MB/s (ST225N)
0.5 MIPS (68000, 8086)
2:1
1990
3 MB/s (3105S)
50 MIPS (68040, 80386)
1:16.6
1995
8 MB/s (ST12450W)
200 MIPS (Power PC 604, Pentium Pro)
1:25
Disk performance appears to be improving at a rate of 20 to 30 percent a year.
Conversely, CPU performance has improved 200 times in 11 years. The disk
performance bottleneck has slowed total computer system performance
growth.
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Caching
Figure 13. Data flow in memory caches
Caching is a popular method of accelerating operations by temporarily storing
frequently accessed data on an intermediate medium that is faster than the
source medium. Caching can improve the real performance of many diverse
operations within the computer.
• Disk data can be cached into RAM.
• CD-ROM data can be cached onto hard disks and/or into RAM.
Disk caching minimizes overhead and access time while maximizing transfer
rate in data retrieval by storing “popular,” or likely to be used, information
where it can be accessed quickly.
STOP
52
Overhead is the operational delay incurred during command execution; the SCSI bus is the main
culprit. When data is requested by the CPU, the request must go through the SCSI call chain,
wait for the bus, and wait for the drive’s head to locate the data. Access time is seek time, plus
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latency, plus the time it takes the head to read a sector. For more information about the SCSI call
chain, see Chapter 5, “All About SCSI.”
By storing recently used data in cache, a CPU request for this data will be
served more quickly because it need not cause an actual transfer off the disk.
There are three common types of cache present in most computers (Figure 14):
• CPU cache resides on or near the CPU.
• Software disk cache resides in the CPU’s RAM.
• Hard drive cache resides on the hard drive’s controller board.
CPU cache
Software disk cache
Hard drive cache
Figure 14. Data transfer rates increase the closer cache is to the CPU
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The closer the information is to the CPU, the more quickly it can be accessed:
• Data stored in RAM is the closest (CPU and software disk cache).
• Data stored in the disk controller’s RAM is farther away (hard disk’s
cache); access to the hard disk cache is slower than access to RAM cache,
mostly due to data transfer on the SCSI bus.
• Data stored on the disk itself is the farthest away. In addition to the time
needed to send the information on the SCSI bus, the access time and data
transfer rates of the hard drive itself take a toll.
CPU cache
Cache in or external to the CPU can hold a part of a program or data so that it
may be accessed quickly while the CPU is executing the program. CPU cache
is fairly small because it is located in or near the CPU, where real estate is
scarce and costs are high. CPU caches are constructed with expensive highspeed memory, so typically they are limited in size to about 4 to 256 KB.
Most modern high-speed processors have secondary cache external to the
CPU to speed up main memory performance. Cache that is external to the
CPU is also known as Level 2 cache. Level 2 caches range in size from 256 to
1024 KB. Typically they are constructed of high-performance sub-10 ns static
RAM cache chips.
Software disk cache
Software disk cache, used for storing the most frequently accessed disk information, speeds data retrieval but limits the amount of RAM available for the
program itself. Software disk cache on the computer is usually configurable.
Typically it can be adjusted in size and enabled or disabled. It can also be
increased through the installation of additional memory.
The amount of RAM you should set aside for the software disk cache depends
on several considerations:
• How much RAM is available?
The smallest software disk cache setting for System 7.5 on the Macintosh
is 32 KB. If extra memory is available, set the RAM cache to a higher
value. Apple has recommended 32 KB of cache per megabyte of RAM in
the machine. The best way to find the optimum value is to run
applications with the RAM cache at different settings. Start with 96 KB
and increase it until performance is maximized. The optimum value will
be the one that results in the fastest data access while maintaining
enough RAM to run the program.
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• How much is needed to run an application?
There is also a diminishing return with increasing cache size. As more
and more memory is allocated for the cache, more and more overhead is
required to maintain the cache. Applications such as color publishing
stream large amounts of data, so set the software disk cache to a small
number like 32 or 64 KB for these applications. With applications such as
digital video, performance can suffer when you use very small caches. Set
a maximum size of 256 KB. Avoid virtual memory whenever possible.
• What sort of application is being run?
Some programs make better use of a software disk cache than others. A
word processing program loads files only as it needs them and repeatedly
uses the data. Here, a software disk cache would almost certainly speed up
data transfer. Spreadsheet programs usually load the entire spreadsheet at
once, so a software disk cache is of little use.
Software disk caches help minimize IDE or SCSI bus traffic by servicing I/O
requests within the computer itself.
Microsoft Windows for Workgroups 3.11™ users should enable 32-bit file
access, which handles disk caching for Windows®. Users with at least 16 MB
of RAM will be advised by the operating system to use 4 MB for a disk cache.
Previous versions of Windows 3.1 utilize caching loaded by DOS’s SMARTdrive utility program. SMARTdrive is a real mode disk cache, so it does not
greatly accelerate Windows operations. Windows 95® and NT™ automatically
manage the size of the disk cache and dynamically size it depending on load.
STOP
Windows 95 also lets you control read-ahead cache size.
Hard drive cache
Hard drive cache, located on the controller board, is used to store the most
recently requested disk information or anticipated data. Although it is slower
than RAM cache, it has the advantage of freeing up the computer’s RAM for
running programs. Hard disk cache is typically 256 KB to 1024 KB. Some
drives have caches that are configurable using commercial disk utility software. Configurable parameters typically include:
•
•
•
•
•
enable/disable hard disk cache
prefetch size
write cache enable
buffer size (See “Track buffering” below.)
number of segments
Most caches can be divided into memory segments. Segmenting a cache is
particularly useful in that it allows for organizing cache information
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according to the way you use your system. For example, you can segment
a cache in two: one for an application, one for an application’s data files. A
single segment would only allow one area to be cached at a time. Some
caching algorithms vary the number of cache segments to adapt to
changing conditions.
Track buffering
Track buffering takes advantage of the principle of locality. When data is
requested from a particular sector, studies have proved an 80 percent likelihood that the CPU will soon be asking for information located in adjacent sectors. In anticipation, the controller will read the entire track and store it in its
RAM. This full track read-ahead buffer speeds up information retrieval for
almost all applications, although there are exceptions. If an application uses
many small data transfers from various locations on the platter, as do database
programs, having the controller read too far ahead may actually slow down
the process; it will be reading and storing too much information that will
never be requested by the program.
If a track buffer is not present, a program reading sequential sectors on the
disk will have to wait an entire revolution for each successive sector access.
Prefetch
Prefetch, or Read Look-Ahead, is similar to buffering, except it can read ahead
to the next track. These larger reads get more data ready for the CPU’s next
request, thus speeding up access time. The parameters controlling prefetch on
many drives can be customized to maximize performance, including the number of sectors it will prefetch.
Read caching
Read caching utilizes more RAM more efficiently than track buffering. Usually a system has enough RAM to store several tracks of data. Each time a disk
request is made, the cache algorithm checks to see if it’s in RAM cache. If the
data is in RAM cache, it is returned right away. If the data is not in RAM
cache, it is read from disk, copied to the cache, and transferred to the host.
This continues until the cache is full. When it is full, there are different methods of selecting which data should be replaced as new data is added:
• a random entry
• the least recently used
• the oldest entry
A variant to read cache is read ahead cache, where an entire track of data is
read and stored in cache when a single sector of that track is requested (as
with track buffering). All further transfers within the track come from cache.
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Write caching
In write caching, the controller:
• transfers a write request to its own cache
• tells the CPU that the task is complete
• completes the write at a later time
This speeds operations because, ordinarily, the controller would not tell the
CPU it had finished the command until it actually had, and the CPU would
have to wait for notification before going on to the next command.
Write caching is a recent advance in improving disk performance. It holds
great promise for speeding random small-chunk writes. For example, assume
the following average times to complete a 4 KB random write:
Issuing the command (command overhead)
1 millisecond (ms)
Data transfer
1 ms
Seek time
14 ms
Latency
8.3 ms
Issuing a Command Complete message
1 ms
Total transfer time
25.3 ms
Figure 15. Time it takes to complete a 4 KB random write
With write caching, the CPU saves 22.3 ms by not being affected by latency
and seek time. The total time spent is three ms. The downside is that information can be lost if there is a power off before data has been written to disk.
Drivers should flush the cache when the computer is shutting down. An uninterruptible power supply (UPS) is vital when you use write caching.
STOP
A UPS is an intermediary device connected to a power source and a computer. When there is a
power failure, the UPS uses a battery and power circuits to produce the voltage required to keep
the computer running. Depending upon the size of the battery and the power requirements of the
computer, a UPS can last anywhere from a few minutes to several hours.
Multiple sectors of data could also be combined into a single write, saving lots
of latency. Writes could also be sorted into ascending or descending order and
performed in the fastest way possible.
A safer variation is the write-through cache, where data destined for the drive
is written to the disk but also stored in the cache. If this data is needed again,
it can be read from the cache instead of going to disk. This is often combined
with read caching to accelerate disk performance.
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Write caching is present in most modern hard drives and also in many operating systems. SMARTdrive 4.1 and newer, included with DOS 6, includes both
read and write caching. Each of which can be configured or disabled.
Hardware caching controllers
Some vendors offer hardware-based caching disk controllers that combine an
IDE or SCSI interface with lots of RAM slots for cache. They can be costly
because you must populate them with expensive RAM. This RAM is dedicated to the controller and cannot be used by your applications. Disk caching
is built into most operating systems, and an additional cache could get in the
way. It is also very difficult to design a general cache algorithm that is good for
a variety of uses. Cached data from RAM on the computer also arrives much
faster than cached data stored on a caching disk controller expansion card.
RAM drives
The ultimate hard drive would be made up entirely of instant-access RAM.
However, this would be prohibitively expensive, extremely volatile, and
would need a back-up power source, such as a battery. There are many PCMCIA cards designed to store data by using nonvolatile flash memory chips. At
one time, people believed that this type of storage would replace hard drives;
but high cost prevented this. Flash memory costs about 50 times more than a
typical 2.5-inch hard drive. The drives write data about five times slower than
they read data. They are available only in very small capacities.
NOTE
For more information about PCMCIA cards, see Chapter 8, “All About Expansion Buses.”
Thermal recalibration
Many hard drives need to recalibrate their heads every couple of minutes to
compensate for heat buildup that causes components to expand and contract.
This thermal recalibration (t-cal) doesn’t affect the drive’s normal operations
but can adversely affect applications such as audio or video digitization and
playback. T-cal can last hundreds of milliseconds. If a drive cannot service a
request during a t-cal, a dropout of video or audio could occur.
An AV-ready drive needs to provide an uninterrupted sustained data transfer
rate. Modern drives, or ones with an AV designation, are designed to overcome
this limitation by using special algorithms to defer t-cal until there is no
activity or by performing t-cal one head at a time. Drives with embedded
servo also avoid the need for t-cal.
STOP
58
Error-correction can cause AV problems. Handling it in the drive hardware helps avoid them.
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Drivers
The final piece in the Operations puzzle is the device driver. The driver translates computer operating system requests into the SCSI or IDE commands
that tell the drive how best to carry out the computer’s orders.
NOTE
For more information about drivers, see “Device drivers” on page 181.
When Things Go Wrong
Despite all the excellent engineering that we’ve just described, hard drives can
fail. When the drive stops working it’s called a “crash.” Although a crash can
be the result of software problems, it may also be due to a mechanical problem in the drive itself. Keep in mind that the majority of failures are due to
really minor glitches. A minor change in a disk sector could render your entire
machine useless. When things go wrong, try not to assume the worst.
Common causes of failure are discussed below. Figure 16 provides an overview of the most frequent causes of system failure.
Percent
100
90
80
70
60
50
40
30
20
10
0
Human Error
Software
Bus
Media
CPU
Figure 16. Most frequent causes of system failure
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Software errors
In the case of software errors, the drive is still functioning and the data can
probably be rescued using a file recovery program, such as Norton Utilities.
The two most common causes of software errors are:
• poorly written software
• viruses
To reduce the number of possible causes for error, disable as much third-party
hardware and software as possible. This will allow you and the various companies’ technical support departments to diagnose the problem faster.
Many things can go wrong, but what’s important is that you are alert to the
warning signals, such as:
• Upon boot-up, the screen displays some sort of visual message, such as a
blinking question mark (PC) or a Sad Mac (Macintosh), or a dialog box
with an error message (Macintosh and PCs).
• Upon boot-up, the computer doesn’t recognize a disk and asks if you want
to initialize it or says the operating system is missing (Macintosh and
PCs).
!
Don’t choose to initialize. This will wipe out all data, applications, and anything else you had
stored on your hard drive.
• Upon boot-up, the disk does not mount, or crashes midway during boot
(Macintosh and PCs).
• An application repeatedly crashes when launched (Macintosh and PCs).
• Opening a folder crashes the system (Macintosh and PCs).
• Copying a file crashes the system (Macintosh and PCs).
• Copying a file returns a write or read error (Macintosh and PCs).
See Chapter 10, “Troubleshooting,” for some suggestions on what to do.
Hardware errors
Hardware crashes, or hard crashes, come in as many varieties as there are
parts in the hard disk drive; four are the most common:
•
•
•
•
60
media errors
controller damage
head crashes
spindle failure
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Media errors
Errors can result from bad sectors on the platter that are not successfully
mapped out by the drive.
The disk will need to be tested to first find and then map out the bad blocks. If
the corrupted information is in the directory, driver partition, or partition
map, the drive will not know where to find the data stored on the platter. The
disk will be unreadable and may have to be reformatted. Because there is limited physical damage to the platter or the hardware with media errors, much
of the data is probably intact and can be recovered with a utility program.
Some formatting software allows you to configure bad-block mapping and
reallocation. Drives usually maintain a fixed number of spare sectors on each
track. If these are all used up, then they look to a cylinder-based spare. Operating systems, such as DOS, map out bad sectors when they are high-level formatting a drive or running a utility such as SCANDISK.
STOP
If your system encounters a bad block, it is often best for you to perform a full backup, then
reformat the drive.
Look for programs that do a variety of tests to find disk problems. Sequential,
data pattern, and butterfly tests are useful for finding media errors. Testing
the RAM and controller on the drive are also useful tests.
Controller damage
Controller damage occurs when one of the controller’s parts—such as a chip
or capacitor—fails to perform. All the data is intact, and there is no damage to
the platter. Possible causes include a voltage spike, which could blow out a
chip, and environmental damage due to extreme heat or cold. You’ll need to
send the drive to the manufacturer for repair.
Head crashes
The typical, and most graphic, head-crash scenario is when platter wobble or a
voltage drop causes the read/write head to contact the platter surface. Due to
the platter’s high speed and delicate surface, the head can plow through a lot
of information very quickly. Several tracks could be destroyed in one crash.
That data is lost forever. If the drive is still working, fragments of media can
get between the head and the disk and cause further damage to the magnetic
surface. If this happens, you need to replace the damaged head and platter. If
you wish to recover the salvageable data, you would also need to pay for
expensive data recovery.
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Spindle failure
The spindle is a sealed unit with lubricated bearings. It should not need any
maintenance, yet approximately 15 percent of hard drives eventually fail
when friction-produced heat burns away the lubrication. (When a manufacturer uses the wrong lubricant, or too much of it, it is said to cause a “stiction” problem.) The bearings grind against the spindle and its casing; it can
slow down, or seize up and lock in place. You may notice a grinding noise
when the drive is running, or a failure to spin up when the drive is turned on.
If the disks are intact, they can be removed in a “clean room” and installed in
a functioning drive. This service is very expensive.
Stiction
Stiction is short for static friction. It happens when the heads of the drive are
stuck to the platters of the drive. When you try to power up the drive, the
heads cannot break free. Stiction is usually caused by improper lubricants on
the drive platter.
Temperature problems
Some drives develop problems only after a certain interval of power-on time.
This usually happens when there might be a fan cooling problem with the
subsystem or when there is an unreliable part on the disk controller. Many
modern drives spin at 7200 RPM or faster and utilize many watts of power.
They usually need forced air cooling to keep their operating temperature
within design guidelines.
Drives Don’t Have to Die Quickly!
Saving data
The safest approach to saving your data is to rely on data duplication, rather
than data recovery. This means having all your essential data backed up on
another storage device, so that it’s ready to be placed either on the damaged
drive when it’s up and running again, or on a replacement drive.
Copying files over to floppies is the most popular method of backing up data.
It’s a good method, but other more reliable, more automatic, faster, and—
unfortunately—more expensive methods exist for those who are serious about
protecting their data.
Some users have come to rely on tape drives to do incremental backups; that
is, a software utility reads whatever information has been added to the hard
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drive during that work session, and copies it to a high-capacity, but slow,
sequential access tape drive.
NOTE
See Chapter 4, “Other Types of Storage Devices,” for more information.
Incremental backups only back up files that have been changed since the last
backup, so they occur quickly (although the first backup must be a full
backup, which takes time). This technique can be used with a second hard
drive or removable drive, either manually or with an automatic utility program.
Some users prefer differential backups. A differential backup is a combination
of an initial full backup and subsequent backups of all files that have changed
since the last full backup (not just the changed parts of those files). This
allows for easier restoration of your drive to its original state.
Corporations typically have the means for centralized backup. They usually
ask users to back up to a server or load a background agent that allows a central backup server to pull all the files that need to be backed up.
Whatever approach you take, try to automate the process so you do not have
to remember to back up your data. You should back up as often as you feel
comfortable. Most people back up at the end of every day, others back up once
a week. Backup media should be stored in a safe place away from magnetic
fields, liquids, smoke, and high temperatures. Make sure you keep some backups off-site, so natural disasters cannot wipe out all of your data. Also make
sure you test your backups by restoring some of your backed up files. Its
shocking how many times bugs in backup software crop up.
Disk duplexing and disk mirroring
Disk duplexing and disk mirroring were developed to provide automatic
backup for network servers—computers that store data for a large group of
connected PCs. But the techniques are applicable to single-user data safety as
well. Both methods are expensive because twice the number of drives is
required to obtain the same capacity.
Disk duplexing
Disk duplexing, as its name suggests, is using one drive to duplicate the read/
write moves of another. This creates an identical copy of the original. This is
accomplished through a dedicated set-up that has two drives and two separate
drive interfaces and host adapters, each with its own power supply. Because
the host computer has two separate SCSI ports—the standard port and usually
a card-based controller board—there is minimal reduction in system performance. Data is both read from and written to both drives. When reading, the
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data from the two drives may be compared to verify data integrity. If the main
drive or controller fails for any reason, the second is activated automatically,
and the user is notified of the failure. Using duplexed drives, in effect, puts
one’s data eggs into two baskets.
Disk mirroring
In disk mirroring a single disk controller is connected to two drives. All controller activity is performed twice; first to one drive, then the other. If one
drive fails, the other one takes over. Because data is read from one drive but
written to both, there is no reduction of performance during read operations
and about a 50 percent reduction during write operations. With mirroring, in
contrast to duplexing, if the host adapter fails, the system will go down.
You still need to maintain backups when doing mirroring. You need to protect
against theft, fire, or software problems.
RAID, or Redundant Array of Independent Disks
RAID takes the concept of duplication one step further by linking a number of
drives together. Many variations on this technique have been developed. Most
of them use a “check disk,” which keeps information about the other linked
drives so that data can be reconstructed in the event of drive failure.
For detailed information on RAID, see Chapter 3, “RAID Technology.”
But When You Gotta Go …
Unlike other parts of the computer, hard drives have many moving parts,
making them more susceptible to failure. Components that fail often include
the fan and the power supply. They fail because they include moving parts and
operate at high temperatures. Unfortunately, all disks will eventually die—it’s
only a matter of time.
New techniques help predict when a drive is likely to die and provide early
warning messages to users. Early warnings allow a user to back up data prior
to a drive’s failure. With drives getting larger and larger every year, this
becomes increasingly important for keeping data safe. Telltale signs, such as
many recoverable errors, many retries, or slowing performance, are monitored
and reported.
Predictable failures are characterized by a slow degradation in an attribute
over time due to component problems. Slow degradation allows the attribute
to be monitored and a warning threshold level established.
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Some attributes that could be monitored include:
•
•
•
•
•
•
•
•
•
•
•
•
•
head flying height
data throughput performance
spin-up time
re-allocated sector count
seek error rate
seek time performance
spin try recount
drive calibration retry count
channel noise
signal coherence
signal amplitude
excessive bad sectors
excessive soft errors
IBM developed a technology known as Predictive Failure Analysis (PFA®) that
measured several drive attributes, including head flight height, and provided a
warning that a failure could occur.
Compaq developed a technology known as IntelliSafe™ similar to PFA. It
measured attributes and provided a threshold for warning levels. Compaq put
this in the public domain in 1995. This lead a variety of drive manufacturers
to develop the SMART standard.
Self-Monitoring, Analysis and Reporting Technology (SMART) was developed to
help predict reliability and provide early warnings to inevitable failures.
SMART was designed to try to provide 24 hours of warning before drive failure. SMART technology can be extended to other devices such as tape and
CD-ROM drives. SMART warnings require drives that support the protocol
and driver software that monitors this vital information.
SMART is encompassed by the SFF-8035 standard for ATA/IDE drives, and the
X3T10/94-190 document for SCSI devices.
NOTE
Not all failures are predictable. Backups are always necessary. Additionally, physical damage is
expensive to repair, so practice basic maintenance and be aware of what can damage your drive.
Drive failure can result from a number of easily-preventable factors.
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Preventative measures
Table 3 lists and describes preventative measures you can take to prolong the
life of your drive:
Table 3. Prolonging disk life and maintaining data integrity
66
Measure
Explanation
Backup
You must religiously back up the contents of your drive onto another storage
medium, be it floppies, tape, or optical. This will insure you against loss of data due
to hard disk crashes, accidental erasure, errant software or hardware, or viruses.
Think about it this way: if your drive died today, would you be able to reconstruct its
contents? Make sure you test some of your backups to ensure they are restorable.
Utility Software
In addition to backup software, you should own a disk recovery tool to repair disk
problems. Most of these recovery packages include useful utilities to mirror
directories, optimize your drive, and undelete files. It’s handy to have a low-level
disk utility program that can test data. Virus eradication software is very useful.
Emergency Boot Disk
Make an alternate bootable floppy, hard drive, or CD-ROM to use for startup in case
your primary drive is damaged. Place important utility programs such as disk
recovery tools and hard drive utilities on this emergency disk.
Dust Control
Dust control is one of the most important aspects of your drive’s environment. A
fleck of dust—even smoke particles—in the wrong place can crash your drive.
Keep the your drive area as dust-free as possible, so no dust can block its air flow.
Surge Suppressor
Power spikes or surges can be minimized by a surge suppressor, a device that
smooths the current sent to your drive and that will, when properly fused, shield it
and the controller from a killer voltage spike. Voltage irregularities are
commonplace—every time a refrigerator or other large electrical device comes on
or off, for example—so a surge suppressor is a necessity.
UPS
You may wish to consider acquiring a UPS, or uninterruptible power supply, to allow
you precious time to back-up your data in the event of a power outage. A typical
UPS supplies 1,200 watts of power and should buy your system 30 minutes.
On/Off Policy
Some people think that turning your drive off at every opportunity will help prolong
its life. That is a tough call: switching drives on and off causes more wear and tear;
a drive draws the most current during spin-up, and the motor is under the greatest
stress at that time. We think this is a good rule of thumb for most systems: shut it
down if it’s going to be unused for more than a couple of hours. Also don’t power
down then power up the drive right away. Give the drive a minute to settle. Make
sure there is no disk activity when you shut down your computer.
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Table 3. Prolonging disk life and maintaining data integrity (Continued)
Measure
Explanation
Temperature Control
A good flow of air is crucial to avoid overheating the computer’s or external drive’s
power supply. Because the components of hard drives operate at very small
tolerances, it doesn’t take much of a shift in temperature to throw things off kilter.
To make matters worse, because the hard drive is a sealed unit, when heated it acts
like a pressure cooker. To compensate for temperature changes, drives have autocompensators that regularly conduct seek tests and adjust the movements of the
head actuators accordingly. Be sure to keep your drive operating in a temperate
environment: Most hard drives are rated for operation between 50˚ and 122˚
Fahrenheit, 8 and 80 percent relative humidity, and –1,000 and 10,000 feet
altitude.
Drive Placement
Hard drives are designed to operate in certain physical orientations, and upside
down is not one of them. A drive operated upside down will get “used to” this
orientation and may not work properly when righted. Most drives can be operated
sideways, but should not have the power supply situated below the drive; otherwise,
the drive could get “cooked” by the power supply.
Data recovery
If your hard disk has a serious hardware crash and you need to recover data,
you will need to send your drive to a place that can perform data recovery.
These firms perform a variety of services. They can try replacing the controller card on the drive to see if there was a part that was damaged. This usually
fixes 75 percent of the failures.
If there is a mechanical failure, they can open up the drive in a Class 100 clean
room and attempt to repair any head, spindle, or platter damage. Be prepared
to spend lots of money. It’s far cheaper to keep frequent backups.
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Defragmentation
Fragmented Data
(represented by black patches)
Defragmented Data
Figure 17. Fragmented and defragmented disks
A simple method to reduce some of the wear on the read/write head and actuator is to defragment the drive. This will also help the drive achieve its maximum performance potential.
When a computer writes to a platter, it begins writing to the first available
location. If it runs out of space before it runs out of data, it moves to the next
available location, which may not be on a contiguous block. Spreading a file
across non-contiguous blocks is called file fragmentation.
When a drive head reads a fragmented file, it has to jump about the platter to
locate and deliver the fragmented data. The extra movement of the heads
increases wear on the drive’s components.
Fragmentation is not very apparent when a drive is new or freshly formatted.
It gets worse over time as the head grabs whatever sector space is available.
When new applications are installed on the drive, they may start out in a fragmented state: if the drive was already fragmented, added material may be
placed wherever the write operation finds patches of free space. It only gets
worse from here. If your hard drive is taking longer to load an application or
read a file, or if the status light goes on and off instead of staying on, your
drive is probably overly fragmented.
There are two ways to defragment a drive. The first is with a dedicated optimizer. These software utilities can reorganize files on the drive and put them
in contiguous sectors. The second method involves copying the contents of
your hard drive onto a second, essentially empty (although formatted) hard
drive, and then recopying the files back onto your hard drive. Either method
will defragment your drive, thus reducing the work of the read/write head and
actuator, and optimizing its access time.
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Although defragmentation programs are not destructive, be sure to back up all
data before you defragment. As with any operation that writes data to the
disk, various factors—such as a power surge or outage—could corrupt the
data.
Need a New Drive?
Once you become familiar with all the potential capabilities of a drive, you
may find yourself dissatisfied with the limitations of your present hard drive.
What should you look for in a new drive?
Indicators of durability and reliability
Most sales literature that discusses the attributes of a disk drive will include
at least three indicators of durability and reliability:
• Mean Time Between Failures (MTBF)
• Mean Time to Repair (MTTR)
• Shock Ratings
STOP
A failure is any problem that prevents a drive from performing a specified operation. Failures that
occur during an OEM integration, such as when an OEM manufacturer qualifies or encloses a
drive, are not counted.
Mean time between failures (MTBF)
Start with the drive’s MTBF rating. This vendor-supplied specification indicates how long a drive should last. MTBF ratings can range up to and beyond
1 million hours. It’s important to understand the meaning of this rating when
you purchase a drive. The drive holds all your data, so you really want to purchase one that is reliable.
An MTBF rating of 50,000 hours indicates that half (or the mean) of the drives
with that rating will fail within 50,000 hours, or 5.7 years.
50,000 hours
24 hours a day × 365 days a year
= 5.7 years
Figure 18. Interpretation of the MTBF rating
A 100,000 hour MTBF drive should last twice as long! Approximately eight
percent of drives fail each year. After one year, 92 percent are still operating;
after two years, 84 percent are still operating, and so on. MTBF ratings of
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1 million hours are not uncommon; however, this does not actually mean the
drive will run for over 100 years.
STOP
IBM has an interesting method for calculating MTBF. They define it as the mean of a distribution
of product life times. Their defined group of drives include:
• drives that have not reached end-of-life (typically five to seven years)
• drives that are operated within a specified temperature range under specified conditions
• drives that have not been abused or damaged
They divide the total operating hours of the defined group of drives within a specified time period
by the total number of failures occurring within that time period.
A drive with an MTBF of 1,000,000 is very unlikely to last that long because it will have reached
its defined end-of-life before then and thus been eliminated for consideration within the MTBF
calculation. To overcome this limitation, IBM suggests that the entire MTBF rating be applied to
a string of drives, each brought into play as the preceding drive reaches end-of-life. The MTBF
calculation would suggest that the total running time of all these drives would reach (or surpass)
the MTBF calculation before an increase in the likelihood of failure. For such drives, you might
want to discover their scheduled end-of-life or look at their warranties to decide if the company
will stand behind them for a satisfactory amount of time.
Drive manufacturers determine MTBF by running only a couple of thousand
hours of tests on a representative population of drives in an optimal environment and then extrapolating the results. “Corrected” for the real world,
MTBFs would drop about 35 percent. Rarely do MTBF ratings include power
cycling, access testing, and variations in temperature. If they did, we would
still be waiting for a rating on most drives now on the market. However, the
testing is rigorous—punishing shocks, temperature extremes, constant read/
write operations—and it pushes the drives well beyond what they will likely
encounter in a lifetime of use. MTBF ratings are typically geared for office
conditions. A dust-free, environmentally controlled “clean room” can
increase MTBF ratings by 100,000 hours or more.
MTBF ratings are not standardized. Manufacturers use different methods for
establishing ratings. They have mainly been used for marketing purposes.
MTBF ratings should serve only as a guideline.
Here are three ways to calculate MTBF; all result in different ratings:
• Combined Life Expectancy
Combined life expectancy uses the combined life expectancy of all the
parts used to make a drive. It relies primarily on Mil Spec 217 or the
BellCore publication. This method does not include head/disk failures.
This is the most conservative method, generating MTBFs in the 30,000 to
50,000 range. It was in use in the early days but has since been abandoned.
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• Field Rate of Return (FRR)
MTBF = (No. of drives shipped per month × Power-on hours per month) ÷ Drives that fail per month
Figure 19. Formula for determining MTBF by Field Rate of Return
This only relates to the failure rate of the installed base, not the average
life span of each drive. An MTBF of 1,000,000 means that out of 10,000
drives, you can expect one drive to fail every 100 hours:
1,000,000 MTBF = (10,000 drives × 100 hours) ÷ 1 failure
Figure 20. Interpretation of the MTBF rating based on FRR
• On-Going Reliability Tests (ORT)
Manufacturer’s randomly select drives at the plant and test them. ORT is
useful in combination with combined life expectancy and FRR to
determine MTBF ratings on older drives.
The bottom line is the warranty on the drive. The longer the warranty, the
more confident the vendor is in the reliability of the drive.
Mean time to repair (MTTR)
Mean Time to Repair is a vendor-supplied figure for the average length of time
a typical repair would take for that drive. If a drive had been repaired five
times over the period of a year and the times involved in repair were two,
one, one, three, and three hours respectively, the MTTR for that device would
be two hours (10 hours ÷ 5 repairs = 2 hours). It is intended to indicate the
modularity of the drive and the quality of its parts.
Shock ratings
Shock ratings are a way of comparing the overall ruggedness of a drive. Unlike
the MTBF ratings, which are extrapolated predictions, shock ratings are the
result of actual tests and are therefore considered much more accurate and
reliable. Shock ratings are measured in g’s; a g is equivalent to the pull of
earth’s gravity. The average rating for hard drives is from 5 to 20 g’s when
operating. Drives in the 1.8-inch form factor have operating shock ratings in
the 200 g’s range. 200 g’s equates to a drop of only one foot. Bernoulli drives
shine in this area: they are rated at over 1,000 g’s due to the use of flexible
Chapter 2: All About Drives
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disks. Bernoulli’s are useful for portable and laptop computers. Newer drives
include shock sensors that shut down the drive when acceleration is sensed.
NOTE
For more information on Bernoulli drives, see “Bernoulli drives” on page 101.
Other statistics to consider
An average seek time of less than 10 milliseconds (ms) is respectably fast.
Sub-12 ms is typical.
The sustained data transfer rate should be at least 5 MB/s, or at least as fast as
your computer can handle. Vendors like to promote their drive’s “burst” data
transfer rate (10 MB/s up to 100 MB/s). This is the fastest that data can be
moved at one instance under optimal conditions, which means data transferred from cache. Sustained rate is the average data transfer rate over the
entire disk (5 MB/s is average; 100 MB/s is high). Sustained data transfer rate
includes burst from the cache (optimal) and cross-cylinder accesses (suboptimal).
If you’re a real speed demon, invest in an add-on SCSI accelerator card and
RAID to improve throughput. High-end applications, such as color publishing
and digital video, demand the fastest throughput available. Raw, uncompressed, full-frame, 60 field per second video requires 27 MB/s, while manipulating 100 MB color scans requires as much performance as possible. More
performance equals better video quality (less compression) or more jobs performed per hour.
Make sure your drive’s cabling interface accommodates future expandability.
Capacity
Choose a drive with a high formatted capacity. Hundreds of megabytes isn’t
very much anymore, considering the size of today’s popular applications and
files. Many operating systems alone require 50 MB of space. An office suite of
applications may require an additional 100 MB. Utilities and games could
require another 100 MB. Graphic programs can generate individual files that
are hundreds of MB. One minute of high-quality digital video can occupy
200 MB. One minute of CD-quality audio occupies 10 MB.
Technology is bringing down the price of higher capacity drives. You may be
surprised at the price difference between two models with the same capacity.
Get 50 percent more capacity than you think you need.
Keep in mind that most drive manufacturers measure megabytes in millions
of bytes (1,000,000 bytes instead of 220 = 1,048,576 bytes). Capacities are
increased by five percent from this fact alone. On a 1000 MB drive, the difference is 48 MB. Operating systems do not measure capacity this way, so your
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operating system probably displays a capacity that is less than what you
thought the drive holds.
Other considerations
Get a drive with a voice-coil actuator. It provides superior head positioning,
and automatic parking for added safety. Also, the drive should accept what are
called step pulses. (Ironically, this is even more crucial for voice-coil actuators
than for stepper motors.) A step pulse moves the head one track over, and
speeds access time by quickly positioning the head over the correct track.
Consider SCSI-3 drives rather than SCSI-2 drives to take advantage of vendor’s
implementations of current and imminent improvements. You will need a
SCSI-3 compatible controller or host.
NOTE
See Chapter 5, “All About SCSI,” for more information.
Consider a drive with advanced RAM caching features. Write caching—featured on several drives—is a tremendous advance in caching technology.
Trends in Hard Drives
Smaller platters
As the density of hard disks and the sensitivity of read/write heads has
increased, hard drives have gotten physically smaller. The gap between the
read/write heads and the platter is getting narrower. Because of this, the magnetic flux has a shorter distance to travel and the magnetic domain on the
disk can be smaller. This results in more bits per inch on each track.
STOP
Magnetic flux is the magnetic exchange between the read/write head and the platter. Magnetic
flux allows the head to write and read data.
Improvements in servo data recording and head movement are allowing for
disks with even more tracks per inch. One GB, 2.5-inch drives combine 6,000
tracks per inch and over 100,000 bits per inch to produce densities of more
than 1600 MB per square inch. Disk size (referred to as its form factor) has
gone from 8 inches to 5.25 inches to 3.5 inches. Disks as small as 2.5 inches
and 1.8 inches are now in use, typically in laptop and notebook-size computers. The 3.5-inch drive has replaced the 5.25-inch as the industry standard.
NOTE
See “Anatomy of a hard drive” on page 27 for more information.
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In general, the larger the drive (in dimension, not capacity) the greater the
speed. The smaller the drive, the lower the cost and the power consumption.
But note that currently the 3.5-inch drive seems to represent the price break.
It offers the lowest cost when figured on a per-megabyte basis. Surprisingly,
drives smaller than 3.5 inches are more expensive than larger drives, but may
assume the price-break position in the near future.
Smaller drives
Computers are getting smaller. Desktops have become laptops which have
become notebooks. Real estate is now becoming a significant concern; the
insides of these boxes are getting cramped. Smaller drives make better use of
available space. Two half-height drives can be installed in the space where one
full-height drive (1.6 inches tall) used to fit in a desktop computer. Thirdheight drives, standing one inch tall, are currently the most popular. For
extremely tight casing requirements—such as for laptop and notebook computers—17 mm height drives (2.5-inch form factor) have been developed.
Although the smaller drives are currently more expensive per megabyte, the
prices are sure to come down as their sales and use increase.
Faster, smaller controllers
The controller chip sets of newer drives (the “brains” of the controller) are
composed of fewer and faster integrated circuits, providing greater processing
power at higher operating frequencies while requiring less physical space in
the drive itself. Most 16-bit based controllers use either the Intel 80186 family
of microcontrollers or the Motorola 6811 family. Most drive vendors stay with
a particular processor family to ensure firmware compatibility. With the
emergence of high-speed serial SCSI, vendors are forced to move to faster 32bit processors and 32-bit memory subsystems. The chip sets on newer 2.5inch and smaller drives incorporate variable zone recording and dedicated
servo capabilities to increase disk density and capacity. Advanced semiconductor technology such as multichip-module (MCM) and chip-on-board (COB)
packaging allows multiple unpackaged chips to be placed in tight areas.
NOTE
See “Anatomy of a hard drive” on page 27 and “Encoding” on page 44 for more information. For a
discussion of chips, see “SCSI Integrated Circuits (Chips)” on page 135.
Despite these advances, the space limitations of smaller controllers means
that there is less room for the sophisticated circuits, such as large caches, that
are supported by larger drives. These functions may have to be relocated to
the computer.
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Connector size is slow to change
Strange as it seems, the shrinking disk size may be limited by the hard drive’s
connectors. A standard 50-pin SCSI connector measures 2.69 inches wide and
0.25 inches thick. It is already a challenge fitting these connectors on smaller
drives.
NOTE
For more information, see “Connectors” on page 152.
A possible solution is to use high-density “D-sub” connectors, as defined in
the SCSI-3 specification. These connectors pack the pins more tightly, and
measure a trim 1.44 inches wide and a quarter-inch thick. Another solution,
already in limited practice, is to reduce the number of pins on the connector.
This is possible because many of the pins are connected to ground wires, and
in practice not all the ground wires may be needed. Forty-pin arrangements
are used for 2.5-inch drives.
Eighty-pin SCA connectors are also gaining in acceptance. These support fast
and wide SCSI-2 and include power and spindle synchronization and reduce
the amount of cabling required. These are good for disk array applications
where drives have to be hot-swapped and less cabling is desired.
STOP
“SCA” stands for single connector attachment. It is a high-density connector that carries every
type of signal passed along a SCSI bus, including SCSI ID, LED, and spindle synchronization
information. It is particularly suitable for hot-swapping—removing and inserting removable
drives while the system is powered on—due to its timed pins. Timed pins allow for removing
power first and ground last when a drive is disconnected, and attaching ground first and power last
when a drive is plugged in.
Faster, lighter heads and actuators
The read/write heads on hard drives are becoming smaller and lighter. The
lower the mass of the head and actuator, the more easily and quickly it can be
moved across the disk. Unfortunately, if a head and actuator are too small and
light, they will be unstable. These components may soon reach their physical
limits.
STOP
Force equals mass times acceleration. The faster the head moves, the greater the force exerted on
it. If the head is made any lighter, it will break apart.
For this reason, advances in seek time performance—the time it takes the
head to physically arrive at the location where it can read and write data—will
be minimal. Some drives increase seek performance by “short-stroking” a
higher capacity drive. Short stroking is using only half of a higher capacity
drive to achieve faster seek times by requiring the drive to seek across only
half the platter. Overall, seek time has only improved from about 16.5 to 8 ms
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in the last ten years, while processor speeds have increased about 1000 times.
The laws of physics start to get in the way of reducing seek times any further.
It’s clear that storage systems have become a limiting factor in increasing a
computer system’s performance.
Other potential areas for advancements in drive performance include:
•
•
•
•
•
increased medium rotation rate
larger caches
higher densities
faster and wider transfer rates
more drive intelligence
Green drives
Of increasing importance is the need to produce environmentally correct
products. Environmental concerns have already resulted in the development
of hard drives that support a low-power sleep mode. Some drives even have a
deep sleep mode where everything is turned off. Newer drives are coming out
that require 3V instead of 5V. In Europe, there are laws that require this. In the
US there is a standard, known as Energy Star, that specifies power management in the computer. In effect, power management means that the drive will
spin down after a certain amount of inactivity. The EPA estimates that a system that powers down during inactivity could save the user $70 to $90 a year.
Energy consumption is extremely important to portable computing users.
NOTE
You’ll want to balance your power management configuration with your desire to prevent wearand-tear on your drive. See “On/Off Policy” in Table 3 on page 66.
All digital drives
Drives are becoming more digitally oriented every year. Older drives possessed many analog components. Newer drives include:
•
•
•
•
digital read channels
digital servo control
digital spindle control
digital processors
Going all digital makes drives more programmable. It permits a finer degree of
control over the specific physical interface variables. Some drives even go further by adding low-cost digital signal processors (DSP) to perform mathematical functions faster.
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Holographic memory
After decades of research, the first holographic memory system appeared in
1995. Holographic memory is a variation of conventional holography. Two
laser beams are focused in the system at different angles and optically interfere on a light-sensitive film. One beam carries the data, which can be either a
representation of the digital data stream or a pictorial image. By using a scanner to change the angle or position of the other beam, multiple views of the
image are recorded. Thousands of these holograms can be recorded on a single
piece of film.
The advantage to holographic recording is that a large quantity of data is
stored in parallel in an image, making for very high data transfer rates. The
first system from Holoplex, the $10,000 HM-100, retrieved 26 MB/s. Future
versions promise to be up to 10 times faster and up to 100 times larger in storage capacity.
Large volumes
Whatever type of drive you pick, you can be assured that you are among many
people needing lots of capacity. About 90 million hard drives were produced in
1995 (Table 4).
Table 4. Worldwide volume of drives
1995
1996 (est.)
1997 (est.)
1998 (est.)
500 MB–1 GB
36.3 M
32.8 M
25 M
20 M
1 GB–2 GB
15.5 M
32.5 M
41 M
20 M
All capacities
90 M
118 M
140 M
160 M
Drive capacity will continue to grow, doubling in size every 18 months—a
60 percent annual rate of increase. Previously, capacity had increased
30 percent a year. Drive product life cycles are about 9 months. Current and
predicted high-capacity drives are illustrated in Table 5.
Table 5. Current and future capacities of 3.5-inch drives
Chapter 2: All About Drives
1995
1996
1997
1998
1999
8 GB
16 GB
24 GB
32 GB
40 GB
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The amount of storage needed on an average server will continue to grow, as
illustrated in Table 6.
Table 6. Amount of storage on an average file server
1995
1996
1997
1998
1999
2000
8 GB
20 GB
50 GB
125 GB
300 GB
500 GB
Storage will continue to become more affordable due to improved technology
and ongoing price wars. Table 7 illustrates the historical and predicted cost
per megabyte of storage space.
Table 7. Historical and predicted cost per MB
STOP
1980
1990
1995
2000
$30/MB
$10/MB
$0.25/MB
$0.05/MB
Consolidation has pared down the number of drive companies. Ten to twelve percent per quarter
decreases are typical.
Drives will continue to become faster, as illustrated in Table 8.
Table 8. Escalation of typical drive speeds in RPM
78
1990
1995
2000
3600
7200
10,800
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3
R A I D Te c h n o l o g y
What is RAID?
RAID stands for Redundant Array of Independent Disks. It was developed at
the University of California at Berkeley in the mid-1980s. It was designed to
provide greater performance, capacity, and reliability by coordinating the
read/write activities among a series of linked drives. It described a way to link
several drives together so that they behaved and appeared as one large drive.
RAID was built upon existing technology rather than developed as a radical
new product.
STOP
RAID originally stood for redundant array of inexpensive drives because the drives used were not
that expensive. Later it changed to independent disks. Although these systems were inexpensive
considering their capacity and data security features, they were more expensive than single drive
solutions because they contained more advanced software and hardware technology.
RAID provides an alternative to SLED (Single Large Expensive Drives) as a
high-capacity, high-performance storage method with the added benefit of
data protection. Most RAID systems implement the latest versions of SCSI
and serial SCSI to maximize data transfer rates. RAIDs typically provide very
fast sustained transfer rates by greatly reducing latency and seek time. This
could increase performance by up to 25 percent.
NOTE
RAID ensures the availability of the storage array and is not a substitute for backup. Data could
be destroyed by accidental erasure or viruses, and lost forever if you’re not backing up regularly.
RAIDs are employed almost exclusively by high-end users who need very
large, fast, and reliable data storage systems. The application is often mission
critical, where losing any data would cost thousands of dollars. RAID technology is typically used for:
• file servers
• database servers
• high-end users of digital video and color publishing applications
RAID has become an important area of storage. Businesses have adopted
RAID for its reliability and its performance aspects. The RAID market has
been growing at about 30 percent per year. Sales in 1994 totaled about
$3.5 billion. They are projected to double to $7 billion in 1997.
Chapter 3: RAID Technology
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What to look for
RAID’s key aspects include:
•
•
•
•
•
•
•
availability
reliability
capacity
expandability
performance
manageability
cost
Different RAID levels offer different degrees of fault-tolerance. Some levels
may add increased performance and capacity without increasing reliability.
Different RAID levels are expandable in different ways. Some require the
addition of only one drive; others require the addition of two to five drives at a
time. Performance varies with each RAID level. It’s important to define the
intended application and related performance requirements to make a match
with the appropriate RAID level. RAID can be very expensive. The intended
application must justify the need.
Specific features
Table 9 lists some features of RAID subsystems:
Table 9. Features of RAID subsystems
80
Feature
Explanation
Parity
RAID systems that include parity allow the system to continue to run if one of the drives has failed. It
allows for the reconstruction of the data on the failed drive. Parity is a data transmission system in
which the number of “1” bits must add up to an odd or even number. SCSI uses odd parity. This
means the sum of all ones in a byte plus its parity bit will always be odd.
Caching
Some RAID controllers, especially ones for levels 3, 4, and 5, have RAM that is used for prefetching
data or for write caching. Write caching helps alleviate the penalty paid for writing in some of the
higher RAID levels. More sophisticated write caching includes battery backup that can maintain data
for hours in case of a power loss. Some have a mirrored cache for redundancy.
Hot Spare
These are extra drives that are fully spun up but will be used only if one of the other drives fails.
Warm Spare
Some RAID systems require that a system go off-line but not power-down to replace components such
as drives.
Multiple Hosts
Some RAID subsystems allow multiple host adapters or computers to be hooked up at the same time.
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Table 9. Features of RAID subsystems (Continued)
Feature
Explanation
Redundant Controllers
Some RAID subsystems include two RAID controllers. Should one fail, the other takes over. Some
systems have both controllers active; others have one active and one in standby mode.
Hot Swappable Drives
Various RAID subsystems include drives in canisters, shuttles, or carriers that can be removed and
plugged in while the system is running (Figure 21).
Figure 21. RAID subsystem with a hot-swap drive
Hot Swappable Power Supplies
Power supplies on some RAID subsystems can be hot swapped and load shared. Load sharing means
that when both power supplies are operable, they share the load of supplying power to the system. If
one should fail, the other assumes the full load. Hot swap capability allows you to swap out the
inoperable power supply with an operable spare while the system continues to run. This setup gives
you N+1 fault tolerance (N is the number of power supplies required by the system, and 1 is the
spare). Hot swappable power supplies should be used in conjunction with uninterruptible power
supplies (UPS) and power conditioners to ensure a continuous flow of power during swaps.
Figure 22. Hot-swappable power supply
Redundant fans
Some subsystems include multiple fans. Should one fan fail, the others will cool the system. Monitor
fans to detect overheating caused by failures.
Setup Software
RAIDs can be set up with setup software, LCD front panels, or serial terminal-based connections.
Monitoring Software
More advanced RAID subsystems include monitoring software to check subsystem status across a
network. Some will notify via pager, simple network management protocol (with TCP/IP), or e-mail.
Chapter 3: RAID Technology
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Table 9. Features of RAID subsystems (Continued)
Feature
Explanation
Configurable Rebuild
Some RAID subsystems have a configurable rate of data rebuild.
Ranks
Some RAID subsystems allow you to construct multiple groups of disks into RAID sub-units. Each rank
could have a different RAID configuration but be controlled by the same controller.
Active Backplane
More advanced RAID subsystems allow you to hot plug drives. Such subsystems have an active
backplane that you plug drives into. The backplane provides drives with power and data connections
even when they are pulled or plugged in on the fly. Without an active backplane, plugging in a drive
while the system is operating could kill the SCSI bus.
Drive Channels
The more channels to the drives in a RAID system, the greater the fault tolerance and the faster the
performance. Drive channels are not made equal. Some offer much faster performance than others.
XOR Engine
RAID 4 and 5 systems need to generate parity through the use of an XOR routine. The XOR routine is
designed to create information that allows a RAID to be recovered in the event of a failure. Higherend RAID subsystems include a built-in custom XOR hardware engine that speeds up generation of
parity. Others rely on an on-board processor to do the XOR calculation. For more information on XOR,
see “RAID 5 and Fault Tolerance” on page 89.
Spindle synchronization is available on some RAID systems (mostly RAID 3).
The spindles of all the RAID drives are synchronized to spin in unison, not
just at the same speed. Each platter of each drive is in precisely the right position relative to the other drives. This nearly eliminates latency because the
head on drive B is over the appropriate sector on its platter just as the head on
drive A finishes reading from or writing to its platter.
RAID Levels—Overview
RAID subsystems differ from each other in the variety of features offered and
in the variety of disk and data storage configurations available. The primary
factors that define the RAID level of a subsystem are:
• the way data is written to the disk array
• whether there is some form of data redundancy
• whether and how the array is expandable
There are six standard RAID levels: RAID level zero through RAID level five.
Table 10 provides an overview of those, plus RAID level 6.
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Common
Name
Disk Striping
Mirroring
–
RAID Level 3;
Parallel
Transfer
Disks with
Parity
–
RAID Level 5
RAID Level 6
RAID
Level
0
Chapter 3: RAID Technology
1
2
3
4
5
6
As RAID Level 5, but with additional
independently computed redundant
information.
Data sectors are distributed as with disk
striping. Redundant information is
interspersed with user data.
Data sectors are distributed as with disk
striping. Redundant information is stored on
a dedicated parity disk.
Each data sector is subdivided and distributed
across all data disks. Redundant info
normally stored on a dedicated parity disk.
Data protected by a Hamming Code (a type
of error tracking algorithm). Redundant info
is distributed across m disks (see next
column).
All data is replicated on a matching disk.
Data distributed across the disks in the array.
No redundant information provided.
Description
n+2
n+1
n+1
n+1
n+m
2, 4, 6…
at least 2
Disks
Required
Highest of all listed alternatives.
Much higher than a single disk;
comparable to RAID Level 2, 3, or 4.
Much higher than a single disk;
comparable to RAID Level 2, 3, or 5.
Much higher than a single disk;
comparable to RAID Level 2, 4, or 5.
Much higher than single disk,
comparable to RAID level 3, 4, 5.
Higher than RAID Level 2, 3, 4, 5,
lower than 6.
Lower than single disk.
Data Reliability
Similar to disk striping for read,
significantly lower than RAID Level 5
for write.
Similar to disk striping for read,
significantly lower than single disk
for write.
Similar to disk striping for read,
significantly lower than single disk
for write.
Highest of all listed alternatives.
Highest of all listed alternatives.
Higher than single disk for read;
similar to single disk for write.
Very high.
Data Transfer Capacity
Table 10. Comparison of common RAID levels
Similar to disk striping for read,
significantly lower than RAID Level 5
for write.
Similar to disk striping for read,
generally lower than single disk for
write.
Similar to disk striping for read,
significantly lower than single disk
for write.
Up to twice that of a single disk.
Up to twice that of a single disk.
Up to twice that of a single disk for
read; similar to single disk for write.
Very high for both read and write.
Maximum I/O Rate
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RAID Level 0
6d L0 RAID
Blocks 0 to 7
Blocks 8 to 15
Blocks 16 to 23
Disk 1
Disk 2
Disk 3
Disk 4
Disk 5
Disk 6
Segment 1
Segment 7
Segment 13
Segment 2
Segment 8
Segment 14
Segment 3
Segment 9
Segment 15
Segment 4
Segment 10
Segment 16
Segment 5
Segment 11
Segment 17
Segment 6
Segment 12
Segment 18
Figure 23. Six-drive, level 0 RAID
RAID level 0, also known as data striping, is an array of drives that transfer
data in parallel. Data is spread out among the drives one segment at a time.
STOP
A segment is a predetermined number of blocks.
Level 0 provides higher performance while remaining transparent to the user.
The capacity of the array equals the sum of the capacities of the drives. For
example, if you have six 1 GB drives striped, you have a virtual 6 GB volume.
Level 0 provides no data redundancy. No redundancy means no parity for error
recovery over and above that provided by a normal drive.
Drive 1
Sledge cutaway
Drive 2
Figure 24. RAID 0 array
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Level 0 is easy to expand by adding one extra drive at a time. Generally, more
drives equals more performance. Performance increases until the bus is saturated, in which case more buses could be used. Striping two drives generally
increases performance about 1.8 times over a single drive. A drive that
achieved 10 MB/s on its own would perform at about 18 MB/s in a RAID
level 4 array. The smaller performance gain from doubling is due to command
and disconnect overhead. The maximum bandwidth of the bus must accommodate these transfer rates. In this example, the bus would have to accommodate a burst transfer rate of at least 20 MB/s. Without the security of data
redundancy, you must avoid potential data loss problems due to inadequate
bandwidth.
RAID level 0 is popular with users of digital video, image processing, and
color publishing applications where the highest performance is very important. Broadcast quality compressed video requires about 12 MB/s sustained
data transfer rate, far more than a typical inexpensive single drive could offer.
!
The lack of data redundancy makes level 0 unsuitable for mission-critical applications.
The RAID controller at level zero is generally a SCSI host adapter that came
with or was added to the computer.
RAID 0 also has been implemented with removable drives, such as Winchester cartridge or magneto-optical, allowing a fast drive unlimited capacity.
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RAID Level 1
6d level1 RAID
Blocks 0 to 7
Blocks 8 to 15
Blocks 16 to 23
Disk 1
Disk 2
Disk 3
Disk 4
Disk 5
Disk 6
Segment 1
Segment 4
Segment 7
Segment 2
Segment 5
Segment 8
Segment 3
Segment 6
Segment 9
Segment 1
Segment 4
Segment 7
Segment 2
Segment 5
Segment 8
Segment 3
Segment 6
Segment 9
Figure 25. Six-drive, level 1 RAID
RAID level 1 is basic disk mirroring. It offers only a little extra performance
over that of a single drive. With RAID level 1, the data written to one drive is
written at the same time to a second drive. If one drive fails, the mirror of its
data exists on the other drive. Because of this redundancy, the usable storage
capacity of level 1 is half the total capacity of the drives used. This makes it
an expensive solution. Drives must be added two at a time to expand capacity.
The RAID controller used for level 1 is generally a SCSI host adapter that
came with or was added to the computer.
RAID Level 2
RAID level 2 is a patented architecture of Thinking Machines, Inc. It uses sector bit interleaved data and multiple, dedicated parity disks. Data is interleaved across drives, so less overall storage space is required for data backup.
The parity disks avoid the cost of the duplicate-disk structure required for
data mirroring by RAID level 1.
NOTE
For more information on interleaving, see “Interleaving” on page 48.
In RAID level 2, all of the array disks are synchronized, so that all disks must
be accessed in parallel. This is good for supercomputer applications that
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require large file transfers at high bandwidth. It’s not good for applications
that call for many small reads and writes.
RAID level 2 is set up with a fixed number of drives, each reading a fixed
number of data bits from each drive. Adding another drive would entail redesigning the system.
RAID Level 3
Composite Block 0
Composite Block 1
Composite Block 2
Composite Block 3
Composite Block 4
Composite Block 5
Composite Blocks...
RAID 3
Disk 1
Disk 2
Disk 3
Disk 4
Disk 5
Disk 6
Block 0(1)
Block 1(1)
Block 2(1)
Block 3(1)
Block 4(1)
Block 5(1)
Block 0(2)
Block 1(2)
Block 2(2)
Block 3(2)
Block 4(2)
Block 5(2)
Block 0(3)
Block 1(3)
Block 2(3)
Block 3(3)
Block 4(3)
Block 5(3)
Block 0(4)
Block 1(4)
Block 2(4)
Block 3(4)
Block 4(4)
Block 5(4)
Block 0(5)
Block 1(5)
Block 2(5)
Block 3(5)
Block 4(5)
Block 5(5)
Parity
Parity
Parity
Parity
Parity
Parity
Figure 26. Level 3 RAID
RAID level 3 is an array of disk drives that transfer data in parallel, while one
redundant drive functions as the parity check disk. Data is interleaved at the
byte level. A single block requires a transfer from every drive. Together they
work as one virtual disk.
STOP
RAID level 3 is also known as parallel disk array, or bit or byte-based striping with parity.
Level 3 is noted for fast data transfer rates of large files such as in graphic
applications and digital video. However it is even slower than a single drive
on small transfers. Level 3 has higher performance over a single drive as well
as increased reliability.
STOP
With level 3, if a data drive fails, the controller rebuilds the data on the fly, using a combination of
the data on the other drives and the parity drive.
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From 83 to 90 percent of the total disk space is available for storage, depending on the number of drives in the array. However, the parity check disk still
makes it 10 to 20 percent more costly than using an equal number of single
drives. With RAID level 3, it’s easy to add extra drives to expand capacity.
The RAID controller at level 3 is generally a stand-alone SCSI-SCSI disk array
controller board that resides in the storage subsystem.
RAID Level 4
RAID level 4 uses an independent disk array. Independence means the drives
don’t function in parallel. With RAID 4, data follows independent paths to the
individual drives. Data is striped across the drives in units of blocks. One
drive is dedicated to parity. This method spreads data across many disks, providing higher performance. Writing is slow because reads from drives must be
done to calculate parity information. Level 4 is noted for fast data transfer
rates of large files such as in graphic applications and digital video.
The RAID controller at level 4 is generally a stand-alone SCSI-SCSI disk array
controller board that resides in the storage subsystem.
RAID Level 5
Blocks 0 to 7
Blocks 8 to 15
Blocks 16 to 23
Blocks 24 to 31
Blocks 32 to 39
Blocks 40 to 47
RAID 5
Disk 1
Disk 2
Disk 3
Segment 1
Segment 5
Segment 9
Segment 13
Parity
Segment 21
Segment 2
Segment 6
Segment 10
Parity
Segment 17
Segment 22
Segment 3
Segment 7
Parity
Segment 14
Segment 18
Segment 23
Disk 4
Disk 5
Segment 4
Parity
Segment 11
Segment 15
Segment 19
Segment 24
Parity
Segment 8
Segment 12
Segment 16
Segment 20
Parity
Auto-swap
Figure 27. Level 5 RAID
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RAID level 5 also uses an independent disk array. It provides the highest level
of data security through data redundancy in the form of parity bits striped
across disks in the array (Figure 27). Level 5 is similar to level 4 except:
• Parity is spread over some or all of the array disks rather than on one
dedicated parity disk.
• More than one drive can be written to concurrently.
• It’s faster on small transfers.
• It’s slower on large transfers.
• It’s slower when writing data.
Writing is slow because reads from drives must be done to calculate parity
information. Small writes are fast because (for example) an eight-drive system
can write to four drives at the same time. Performance and data rates are
higher than a single drive. Spindle synchronization is optional on level 5
arrays.
It’s easy to add drives to level 5. This level requires at least three drives. Ten
to twenty percent of capacity is used for parity overhead. Under RAID level 5,
data lost on a failed drive can be reconstructed on a spare drive.
The RAID controller at level 5 is generally a stand-alone SCSI-SCSI disk array
controller board that resides in the storage subsystem.
Level 5 is great for file-servers, database servers, and internet servers. However, it doesn’t provide the performance that video and publishing applications need.
RAID 5 and Fault Tolerance
Fault tolerance is the ability of a system to continue operating even when one
(or more) of its component parts have failed. RAID 5 uses a Boolean logic function called XOR to generate parity information and to reconstruct lost data.
STOP
“XOR” stands for “exclusive or.” The symbol “⊗” is the logical symbol for “XOR.”
Parity information is generated by comparing the data from one disk to the
data from another, bit-by-bit, across all data disks of an array (Figure 28). The
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XOR function sets a parity bit to 1 if and only if either of the compared data
bits is 1, but not both.
RAID 5 XOR
RAID 5 XOR A
Figure 28. Creating parity data on RAID 5 using XOR
Assume that a disk array with six disk mechanisms has been set up so that
Disks One through Five are initialized for RAID 5 and are considered member
drives; Disk Six is allocated as the auto-swap unit (Figure 29).
Blocks 0 to 7
Blocks 8 to 15
Blocks 16 to 23
Blocks 24 to 31
Blocks 32 to 39
Blocks 40 to 47
RAID 5
Disk 1
Disk 2
Disk 3
Segment 1
Segment 5
Segment 9
Segment 13
Parity
Segment 21
Segment 2
Segment 6
Segment 10
Parity
Segment 17
Segment 22
Segment 3
Segment 7
Parity
Segment 14
Segment 18
Segment 23
Disk 4
Disk 5
Segment 4
Parity
Segment 11
Segment 15
Segment 19
Segment 24
Parity
Segment 8
Segment 12
Segment 16
Segment 20
Parity
Auto-swap
Figure 29. Level 5 RAID
Suppose now that a 2 MB data file is saved from the host computer to the
RAID system. The file is first received by the RAID controller board, whose
functions include:
• the creation of parity data
• the management of all I/O operations to the six disk mechanisms
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The RAID controller board creates an additional 500 KB of parity data from
the original 2 MB of file data, and interleaves all data (both parity and file data)
in segments across all member drives (Figure 30).
STOP
A 2 MB file consists of 500 segments of data with an additional 125 segments of parity data.
RAID 5
Disk 1
Computer
Sees:
Disk 2
Disk 3
Disk 4
Disk 5
Segment 1
Segment 2
Segment 3
d.o. RAID 5
Segment 4
Segment 5
Segment 6
Segment 7
Figure 30. Data organization in RAID level 5
File and parity data are saved to the member drives in the following fashion:
•
•
•
•
•
STOP
Segment 1 is written to block addresses 0 through 7 on Disk One.
Segment 2 is written to block addresses 0 through 7 on Disk Two.
Segment 3 is written to block addresses 0 through 7 on Disk Three.
Segment 4 is written to block addresses 0 through 7 on Disk Four.
Parity data is written to block addresses 0 through 7 on Disk Five.
The information written to block addresses 0 through 7 on Disks One, Two, Three, and Four is
processed through an XOR algorithm to create parity data (also one segment in size). This parity
data is written to block addresses 0 through 7 on Disk Five.
•
•
•
•
•
Segment 5 is written to block addresses 8 through 15 on Disk One.
Segment 6 is written to block addresses 8 through 15 on Disk Two.
Segment 7 is written to block addresses 8 through 15 on Disk Three.
Segment 8 is written to block addresses 8 through 15 on Disk Five.
Parity data is written to block addresses 8 through 15 on Disk Four.
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STOP
Parity data is created from the same information written to block addresses 8 through 15 on Disks
One, Two, Three, and Five, and is written to block addresses 8 through 15 on Disk Four.
This process is repeated until all 500 data file segments and 125 parity segments are interleaved equally among member drives.
For this example, assume that Disk Three fails. The data that was stored on
Disk Three is automatically rebuilt, segment by segment, from the information stored on the remaining disks. The first segment is rebuilt by taking the
data from block addresses 0 through 7 on Disks One, Two, and Four and the
parity information from block addresses 0 through 7 on Disk 5 and processing
them in an XOR algorithm (Figure 31).
RAID 5 Reconstruction
DATA
DATA
RAID 5 XOR D
DATA
PARITY (XOR)
RECOVERED DATA
Figure 31. RAID 5 reconstruction
The resulting segment of data is an exact duplicate of what was stored on
block addresses 0 through 7 on Disk Three. It is written to block addresses 0
through 7 on Disk Six.
This process is repeated until all the data that was on Disk Three is rebuilt
onto Disk Six. The lost parity information that was on Disk Three is also
rebuilt by processing the data on the remaining four member disks through
the XOR algorithm.
When the rebuild is complete, Disk Six's role has been transformed from
auto-swap drive to an active member drive in the RAID 5 array. During or
after the rebuild process, Disk Three may be pulled from the RAID system. A
replacement drive canister may be inserted during or after the rebuild process
(this is known as a hot-swap). The RAID controller board automatically allocates the replacement drive canister as the auto-swap unit.
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Up to two disks can fail
After the failure of Disk Three, its contents were automatically rebuilt to the
online spare, Disk Six. Disk Three is removed, and after contacting your vendor for an RMA number, it should be returned for service. The RAID system
still provides optimal performance at this point, as it still has five member
drives to stripe data across (Drives One, Two, Four, Five, and Six). Although
the level of fault tolerance has been diminished, it is not eliminated.
Now suppose Disk One fails. The RAID system remains operational, even
though only four drives are on-line. Performance will suffer at this point
because the RAID controller board has to use the XOR algorithm to recreate
the lost data segments on-the-fly for each read and write command it receives,
rather than service the requests directly.
Increasing fault tolerance
Fault tolerance can be increased by purchasing a seventh drive canister for use
as an emergency spare. When the seventh drive canister is inserted into the
drive bay of a removed failed drive, the RAID controller board automatically
allocates it for use as the online spare.
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Other RAID Levels
Some vendors have created additional RAID levels that are proprietary or nonstandard. Some of these have been adopted by the RAID Advisory Board as a
standard. Table 11 lists several examples of non-standard or proprietary RAID
levels.
Table 11. Other RAID levels
STOP
RAID Level
Description
RAID 6
This level is an enhanced version of RAID 5 that includes two sets of
parity data rather than one (RAID Advisory Board definition).
RAID 0+1 or 10
This is a combination of RAID 0 and 1. Basically, drives are striped
together like RAID 0, then mirrored onto another set of drives like RAID
1. Some companies have called this RAID 6.
AutoRAID
Developed by Hewlett-Packard in 1995, this technology automatically
switches between RAID 0, 1, and 5, depending on how the array is
accessed. This system is easy to expand.
Tape Arrays
RAID 0, 1, and 3 have been implemented using tape drives. RAID 0
allows tapes to be striped for maximum backup performance. RAID 1
allows 2 copies of a tape to be made at one time, preventing data loss
due to media failure or loss. RAID 3 allows for enhanced backup
performance but also redundancy if one tape is lost.
Removable Arrays
RAID 0 and 1 have been implemented using removable drives such as
optical, SyQuest, or Jaz drives. This allows performance to be maximized
or fault tolerance to be achieved. Because the media offers infinite
storage capacities, projects can be archived and transported easily.
Some enterprises are marketing layered—or “RAID on RAID”—products as RAID 10, RAID 53
and RAID 77.
Hardware-Based Disk Arrays
The drives of a hardware-based disk array are connected to a RAID controller
board installed inside the disk array chassis. The host computer recognizes
the RAID controller board as the SCSI storage device, and not the drives connected to it. Read and write commands sent from the computer are processed
by the RAID controller board, which in turn sends the appropriate read and
write commands to the drives. Overall performance is enhanced when RAID
management tasks are executed by a processor on a controller board because
the host computer is freed from input/output overhead and the disk array
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itself is freed from the host computer’s server and application loads. These
arrays are sometimes known as SCSI-SCSI array controllers.
Bus-Based Hardware Disk Arrays
Bus-based hardware disk arrays are controlled by a RAID host adapter that
resides in the computer. These are typically PCI-based boards that include
multiple SCSI channels to drives and a RISC-based processor. Their advantage
is a direct, high-speed connection to the host bus. These generally outperform
platform-independent arrays and cost half as much. This type of controller
typically requires a PC with many drive bays or an active hot-swappable backplane. Their disadvantage is that they are platform-specific.
Software-Based Disk Arrays
The drives of a software-based disk array are connected directly to the host
computer and its SCSI bus(es). The member drives may be connected in a variety of methods, such as the dual SCSI bus setup of a Quadra 900/950 or Power
Macintosh 8100/8500/9500, or they can be daisy-chained onto a single SCSI
accelerator card. Regardless, it is the host computer and software drivers that
manage the read and write commands to the member disks of a software
array. Although software-based arrays are not as versatile as hardware-based
arrays, they are much simpler and less expensive.
Specialized RAID software performs the setup of the software-based disk
array, initializing each of the member drives with a RAID driver. During general RAID usage, the member drive’s RAID driver tells the host computer’s
system software how to read and write data to it. This will differ depending on
whether the disk array volume has been initialized for RAID 0 (striping),
RAID 1 (mirroring), or standard HFS usage.
STOP
HFS stands for Hierarchical File System. It is used by the Apple Macintosh System 7 for storing
files and folders on a hard disk.
SAF-TE
SAF-TE stands for SCSI Accessed Fault-Tolerant Enclosures. Conner started
this specification to address the need for a standard method of communicating
with fault-tolerant enclosures. It defined a set of SCSI commands that check
the status of fans, power supplies, and temperature. A small computer with its
own SCSI ID monitors the enclosure and tells the main computer how it’s
doing. Other standards for enclosure monitoring include I2C and AEMI.
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RAID Advisory Board (RAB)
The RAID Advisory Board is an association of suppliers and consumers of
RAID-related products. It was formed to:
•
•
•
•
•
•
promote RAID and related technologies
write test plans
measure performance
stimulate standardization efforts
share resources among members
help the RAID industry grow
You can contact them at 507-931-0967 for more information.
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4
O t h e r Ty p e s o f S t o r a g e D e v i c e s
Hard disk drives are not the only type of storage device. There are many different types that offer their own sets of performance and operational advantages.
Table 12 provides an overview of alternative storage.
Table 12. Overview of other types of storage devices
Device
Description
See …
Removable Drive
A hard drive and head assembly in a rigid cartridge that plugs into a “mother” unit.
page 98
Removable Cartridge Drive
One or more hard drive platters in a self-contained rigid case that can be removed
from the main drive unit.
page 99
Zip Drive
A drive containing a 3.5” flexible disk that spins within a cushion of filtered air.
page 101
Bernoulli Drive
A drive containing a 5.25” flexible disk that spins within a cushion of filtered air.
page 101
Magneto-Optical Drive
These use a high-powered laser to write and a low-powered laser to read.
page 102
Phase Change Optical Drive
The surface coating of phase change media changes from an amorphous to a
crystalline state, depending on how much it is heated by a laser.
page 104
Floptical Drive
These drives use optical and magnetic technology. Servo data is perforated into the
disk. Magnetic data is located between these optical grooves.
page 105
CD-ROM Drive
CD-ROM drives use laser beams to read CDs.
page 106
CD-Recordable (CD-R) Drive
A high-intensity laser changes a dye in the surface coating of the CD to reflect a lowintensity read laser as a pit would on a CD-ROM.
page 109
Write-Once, Read-Many (WORM)
Some drives prevent over-writing or erasing data through a series of software flags
and special data bits. Others change the physical properties of the media.
page 118
CD-ROM Rewritable Drive
A new standard proposed to create a rewritable compact disc.
page 118
Jukebox
A large chassis contains multiple optical cartridges, tapes, or CD-ROM discs.
page 119
Digital Versatile Disc (DVD) Drive
Provides for up to 4.7 GB of storage on a 120-mm CD-ROM-sized optical disc.
page 120
Tape Drives (DAT and DLT)
These Drives use magnetic recording tape to store data in a fashion similar to audio
tape recorders. The information is usually stored in digital form.
page 122
Alternative storage devices are discussed in greater detail below.
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Removable drives
Removable
Figure 32. Removable canister drive
Removable canister drives combine the capacity of a hard drive with the convenience of carrying or storing multiple units. A removable drive is essentially a hard drive and its entire head assembly in a rigid cartridge module that
plugs into a “mother” unit. The removable part contains the platter and read/
write head. The mother unit contains a power supply and may contain a hard
drive controller for communicating with the computer.
NOTE
98
Do not confuse removable drives with removable cartridge drives. For information on removable
cartridge drives, see page 99.
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Table 13. Advantages/disadvantages of removable drives
Advantages
Disadvantages
• high capacity (up to tens of gigabytes, infinite
storage capability with multiple drives)
• performance (equivalent to a regular hard drive)
• functional versatility (arrays can be created)
• portability of data
• durability (cartridge modules are made of sturdy
construction, although they are still hard drives and
should be handled as carefully)
• high cost
• limited compatibility due to vendor-unique designs
• limited number of offerings compared with other
types of drives
• vulnerability
Subject to wear from repeated insertions. Not as
sturdy as fixed drives, subject to damage in transit.
• low shock resistance compared to other storage
Removable cartridge drives
Spindle
Sliding access door
Drive heads (in main chassis)
Removable Cartridge
Magnetic media disk (inside cartridge)
Write/Protect tab
Figure 33. Removable hard cartridge system
Removable cartridge drives are devices with one or more hard drive platters in
a self-contained rigid case that can be removed from the main drive unit.
When the cartridge is inserted into the chassis unit, a door in the cartridge
opens automatically. The read/write heads in the chassis unit are moved over
the spinning platter(s) inside the cartridge and float on a cushion of air, just as
in most fixed hard drives. When the cartridge is removed:
• The heads withdraw.
• The opening to the platters closes.
• The cartridge is ejected from the drive unit.
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Removable cartridge drives are great for capacity-intensive activities. They
are very popular among desktop publishing, graphic design, and multimedia
gurus because they’re easily transportable, can be used instead of hard drives,
and have a wide customer base, making them excellent transportable media
for jobs going to service bureaus.
The most popular base mechanisms are manufactured by SyQuest Inc. and
Iomega. These companies use various designs that have some similarities.
STOP
Iomega and SyQuest disks are not interchangeable.
SyQuest Inc. has been making cartridges for a number of years and is considered fairly reliable. They offer both 3.5” and 5.25” disk versions that vary in
capacity, compatibility, and performance. SyQuest also has a 1.8-inch PCMCIA-based removable drive.
STOP
PCMCIA stands for Personal Computer Memory Card International Association. It’s a joint effort
of various special interest groups aimed at setting a standard for memory cards used in PCs.
PCMCIA cards add improved computer memory capacity or enhance connectivity to external
networks and services.
Iomega Jaz platters, while newer to the market, incorporate a few design features that could be an improvement over the original design of the SyQuest
units, particularly in keeping dust away from the platter surface. The initial
Jaz drive accommodates Jaz media in 540 MB and 1 GB capacities, making it a
great medium for storing large data files.
There are other manufacturers of similar systems. However, there are no formal international standards for media interchange on drives of this type;
therefore, it’s not possible to mix the media of one manufacturer with the
drive of another.
Table 14. Advantages/disadvantages of removable cartridge drives
Advantages
•
•
•
•
•
100
portability of media
speed (as fast as most hard disk drives)
cost (some cartridges are relatively inexpensive)
capacity (theoretically infinite)
universality (wide installed base)
Disadvantages
• limited compatibility and questionable future
compatibility
• reliability somewhat lower than a fixed hard drive
(frequent use makes cartridges vulnerable to
exposure to dust and other contaminants)
• relatively low shock rating
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Zip drives
Zip drives are 3.5-inch removable media drives that work on a principle somewhat similar to Bernoulli drives. They offer about one-tenth the capacity per
cartridge of Jaz drives, but still enough capacity for many users.
STOP
Like the Zip and Bernoulli drives, Jaz drives are manufactured by Iomega.
Zip drives are also substantially slower than Jaz drives and substantially less
expensive. With a sub-$200 price point, they are affordable to many users.
Table 15. Advantages/disadvantages of Zip drives
Advantages
Disadvantages
• capacity (enough to accommodate a wide range of
user needs)
• low cost
• speed (slower than Jaz drives)
• low capacity compared to Jaz drives
Bernoulli drives
Bernoulli
Figure 34. Bernoulli cartridge
Bernoulli drives use 5.25-inch cartridges containing a flexible disk that spins
within a cushion of filtered air. The drives get their name from a phenomenon
called the Bernoulli Effect. The Bernoulli Effect is observed when the velocity
of a fluid over a surface is increased and the pressure of that fluid on the surChapter 4: Other Types of Storage Devices
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face decreases. In the instance of Bernoulli drives, the fluid involved is air.
The reduced air pressure draws the disk toward the read/write head. If a dust
particle enters the filter between the head and the platter, the platter’s flexibility allows room for the particle to escape, and the filtered air blows it away.
Table 16. Advantages/disadvantages of Bernoulli drives
Advantages
Disadvantages
• flexing capability apparently makes the drive
almost immune to a head crash (a Bernoulli drive
has a shock rating of 1,000 g’s)
• capacity similar to SyQuest units
• high reliability ratings (MTBF)
• reasonably high marks for speed and data security
• relatively low cost cartridges
• theoretically infinite storage capability
•
•
•
•
•
slightly slower speed than SyQuest
more expensive hardware than SyQuest
small installed base
limited capacity per cartridge
only one vendor
Magneto-optical drives
MO
Figure 35. Magneto-optical drive and optical cartridge
Magneto-optical (erasable and multifunction optical) drives use 3.5- or 5.25inch cartridges that look like and have similar capacities to some WORM cartridges (write-once, read-many; see page 118). The difference in MO technology is that data can be stored, retrieved, manipulated, or deleted, over and
over (write-many, read-many).
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The recording surface of an MO disc is similar to the surfaces of other magnetic media cartridges except that an MO’s disc surface is covered with a protective polymer coating. The magnetic particles on the recording surface are
also somewhat different in that they consist of metals with relatively low
coercivity. This means the surface metals must be heated to about 400˚F in
order to switch their magnetic polarity (as is necessary for write operations).
Writing is done in two passes:
1. On the first pass (the erase pass), the laser heats the particles that represent those data bits that need to be changed, while the magnetic head
sets their polarity to the “0” position.
2. On the second pass, the polarity of the magnet is reversed while the
laser heats up only those particles that need to represent “1s.”
A low-powered laser, which does not heat the disk significantly, is used to
read the disk, taking advantage of the Kerr effect.
STOP
The Kerr effect states that when a polarized light is reflected off a metal surface in a magnetic
field, the polarity of the light is rotated clockwise or counterclockwise, depending on the field’s
polarity. The drive detects this rotation and interprets it as data.
Some optical drives feature direct overwrite, significantly improving performance. LIMM-DOW (light intensity modulation method direct over-write)
allows data to be written in a single pass. This new method of writing requires
new drives and media. The first 3.5-inch LIMM-DOW 640 MB drive was
shipped by Fujitsu in 1996. The first 5.25-inch LIMM-DOW 2.6 GB drive was
shipped by Nikon in 1996. In late 1997, a 7 GB version should be introduced.
By the year 2000, 5.25 magneto-optical drives should be approaching a 10 GB
capacity.
The magneto-optical disc is sturdier than other magnetic media. The magnetic particles do not deteriorate or fade as quickly. Information is recorded in
a special translucent material. An MO disk looks somewhat like an audio CD.
There are both 5.25-inch and 3.5-inch ISO magneto-optical drives. These have
slightly faster transfer rates, spin rates, and seek times than Bernoulli drives
because there is less real estate to navigate, but they hold less data. MO cartridges are also less expensive than hard drives.
Some vendors have created proprietary standards with varying capacities and
success rates. Beware of these products when looking for a new drive. Indus-
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try-standard support allows for backwards compatibility and lower operating
costs not always found in proprietary standard products.
Table 17. Advantages/disadvantages of magneto-optical drives
Advantages
• very high capacity (up to several gigabytes)
• reliability (media stable up to 30 years)
• durability (impervious to heat up to 300˚ F; not
affected by magnetic sources; impervious to dust)
• compatibility (ISO standard media)
• versatility (most higher capacity drives read/write
smaller capacity cartridges)
Disadvantages
• high cost
• slow performance (compared to magnetic media
drives; also writing is done in multiple passes, so it
is slower than reading)
• incompatibility due to non-standardization among
manufacturers
• low spin rate (less than 3,600 RPM, increased
latency)
Phase change optical drives
Phase change optical drives, like magneto-optical drives, change the physical
nature of the disk to record data, but phase change drives write in one pass
instead of the two passes necessary for magneto-optical drives. Phase change
drives use a medium in which the data-storing material can be changed from
an amorphous to a crystalline state, depending on how much it is heated by a
laser. At a lower level, the laser heats the media to the point where it becomes
crystalline (if it is already crystalline, it remains so). At a higher level, the
laser melts the media, which returns to the amorphous state when it cools.
The reflectivity of the crystalline spots is different than that of the amorphous
ones. The disk is read using a low-level laser, similar to ones used on magneto-optical drives.
Table 18. Advantages/disadvantages of phase change optical drives
Advantages
• speed
By writing in a single pass, phase change drives
provide all the advantages of magneto-optical
drives plus the added speed resulting from one pass
writing, which makes it almost twice as fast.
104
Disadvantages
• limited life of media
The repeated heating of the media wears it down.
The disks for some phase change drives are rated at
100,000 read/write cycles, while magneto-optical
drives are rated at 1,000,000 cycles.
• high cost of hardware (more than MO)
• slow seek times
• no ISO standard
• very small installed base
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Floptical drives
Floptical drives marry optical and magnetic technology. The floptical media
follows standard, high-density DD and HD, 3.5-inch floppy diskette technology. As with optical discs, the servo data is perforated into the disk and then
used by an optical system to position the read/write head. The magnetic data
is located between these optical servo grooves. The embedded servo is indelible. It cannot be destroyed or corrupted. It allows the head to follow the
eccentricities of the media. This increases track density from typically 135
tracks per inch to over 1,000. Formatted capacity is usually between 20 and
100 MB, compared to a standard floppy’s 1.44 MB.
The read/write heads have two different head gaps:
• a narrow gap for use with high density floptical disks
• a wide gap for use with standard floppy disks
The drive senses which type of disk has been inserted and automatically uses
the appropriate head gap and servo system. This is a low-end product that may
replace current floppy drives but not hard drives.
STOP
In 1996, Compaq and 3M released information about an LS-120 floptical disk. It has 120 MB
capacity, 500 KB/s data transfer rate, 70 millisecond average seek time, rotational speed of
720 RPM, and track density of 2490 tpi.
Table 19. Advantages/disadvantages of floptical drives
Advantages
• higher densities than standard floppy disks
• compatibility with standard 3.5 floppy disks
• high capacities when used with floptical discs
Chapter 4: Other Types of Storage Devices
Disadvantages
• slow performance (slower than hard drives: 135 ms
seek time compared to less than 20 ms for
Winchester-type drives)
• vulnerability of media to wear
• high cost
At present, floptical drives are about two and a half
times more expensive than standard floppy drives.
• durability (media not that sturdy compared to
Bernoulli)
• slow start/stop times
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CD-ROM drives
CD-ROM
Figure 36. 6Plex CD-ROM drive
CD-ROM (Compact Disc, Read-Only Memory) drives use laser beams to read
compact discs (CDs). A CD is a combination of a reflective aluminum platter,
a polycarbonate layer stamped with microscopic pits and lands, and a clear
lacquer coating, which protects both the stamped polycarbonate (data side)
and the aluminum platter (label side).
A pit or land represents an “on” value in binary code; the transition between
them signifies “off.” The laser beam in a CD-ROM drive shines through the
polycarbonate coating, bounces off the surface of the aluminum platter, and
returns to be interpreted. The label side could get scratched and, depending
upon the severity of the scratch, the disk could still be read. If the side with
the pits got scratched, it is more likely that the disk could no longer be read. It
would depend on the depth, length, and location of the scratch. CDs should
have a shelf life of about 50 years.
Data is arranged on the 650 MB disc in data blocks of 2048 bytes. Each block
has 304 additional bytes for error correction and overhead. This allows for an
error rate of one per 1013 bytes, which is extremely low.
STOP
BLER is an acronym for block error rate. BLER is a measurement of how many errors are detected
in the error correction code (ECC) blocks on media. BLER is usually expressed in numbers of
errors per second. Different CDs have different BLERs. This is attributable both to quality of
manufacturing and care taken with handling. BLER can increase from scratches, finger prints, and
environmental degradation.
Drive performance is measured in terms of data transfer rate and access time.
The basic transfer rate of a CD-ROM is 150 KB/s. Drives faster than this basic
speed are designated double-speed, quad-speed, or Nx-speed drives. A quadspeed drive transfers data at approximately 600 KB/s. The current limit of CDROM technology appears to be around 10x, due to mechanical and economic
constraints.
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Drive performance varies according to the location of data on the disc. Singlespeed drives run at 200 RPM at the outside edge and 500 RPM at the inside.
Access time for CD-ROMs is measured as it is for hard disks. Access time is
in the hundreds of milliseconds, making CD-ROM drives 20 to 40 times
slower than hard disk drives. Most CD-ROM drives have a RAM cache on
board to speed up loading. Further performance improvements can be gained
by using third-party software that extends caching.
CD-ROM drives are available in both IDE and SCSI interfaces. Because the
devices are so slow, IDE is a perfect low-cost interface for these drives.
Data is structured in one of several data formats named after colored books
such as Orange Book or Yellow Book.
NOTE
See “Compact Disc Recordable drives (CD-R)” on page 109 for more detail on disc format
standards.
CD-ROMs are available in several file system formats. The most common are
ISO 9660, for PC or cross-platform CDs, and HFS, for Macintosh CDs. Table
20 lists CD file system formats and describes some pros and cons for each.
Table 20. Pros and cons of CD-ROM file system formats
File system
Pros
Cons
HFS (Hierarchical File System)
Standard Macintosh interface.
Not usable on most non-Macintosh OSs.
ISO 9660 Level 1
Usable with almost all computer operating
systems.
• Maximum file name length is eight
characters with a three-character
extension.
• Directory nesting is limited to eight total
characters. Defining a file in a directory or
path is limited to 256 characters.
ISO 9660 Levels 2 and 3
• Provides ISO compatibility.
• File name can be up to 30 characters,
including three-character extension.
Rock-Ridge
• Provides versatile file naming.
• Based on ISO 9660 Level 2.
Hybrid
• Compatible with most operating systems.
• Provides HFS support on Macintosh, ISO
for non-Macintosh systems.
• Limits disc capacity slightly due to
duplicated directories.
• Requires extra set-up time.
Generic
Allows exact duplication of a device or volume,
including partitions unusable in the native OS.
• Features are not often needed.
• Requires special driver to mount.
Chapter 4: Other Types of Storage Devices
• Directory nesting is limited to eight total
characters.
• Defining a file in a directory or path is
limited to 256 characters.
Supported almost solely in Unix environments.
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Table 20. Pros and cons of CD-ROM file system formats (Continued)
File system
Pros
Cons
Generic XA
Generic with extended architecture.
• Features are not often needed.
• Less widely supported than Generic.
CDRFS
• Provides support for packet-written discs.
• Necessary driver is freely available.
• Not ISO “standard,” but ISO “compatible.”
• Requires separate driver or OS support.
CD-UDF
• “Universal” file system.
• Extensible to non-CD media.
• Provides packet support.
• Not ISO “standard,” but ISO “compatible.”
• Requires separate driver or OS support.
CDs are a standard way to publish large amounts of information, such as databases, directories, and graphic and font libraries for desktop publishing and
multimedia uses. The cost to stamp out a CD-ROM has fallen below $1.00
each when done in large quantities.
Common stereo audio CD players, while based on the same laser-etched pits
technology, use different electronics than CD-ROM drives. They cannot be
used as a data storage and retrieval device for the computer. However, computer CD-ROM drives can also play audio CDs.
There are many CD-ROM drives on the market, sometimes making it difficult to chose the model that will best suit your needs. Table 21 lists some of
the features you should look for when you shop for a CD-ROM drive.
Table 21. Features to look for in CD-ROM drives
108
Feature
Why you want this
Multiple Platters
Some drives can hold 4, 7, or many more CD-ROMs at one time
and switch between them on the fly.
Digital Audio Extraction
Some CD-ROM drives can transfer digital audio from a compact
disc to the computer for saving into files.
PhotoCD Compatibility
Some drives are incompatible with the Kodak PhotoCD format.
Multisession Compatibility
Some CD-ROM drives cannot read CD-ROM’s that have multiple
sessions on them.
Tray vs. Caddy Loading
Different CD-ROM drives load the disk differently. Some use a
caddy; others have a tray onto which you place the CD.
CD-Extra Support
Some CD-ROM drives cannot read audio disks with data tracks.
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In 1994, 22 million CD-ROM drives shipped. Approximately 33 million were
shipped in 1995, making CD-ROM drives commonplace on virtually all new
shipping computers.
Table 22. Advantages/disadvantages of CD-ROM drives
Advantages
Disadvantages
• standardization
It is a standard format supported by all personal
and workstation computers.
• low cost hardware and media
• wide availability of titles
• slow access time
• read-only capability
Compact Disc Recordable drives (CD-R)
CD-R
Figure 37. CD-Recorder
The desktop CD-Recordable drive (CD-R) market is relatively new. The viability of CD-R technology has resulted from a combination of factors:
• technical refinements
• material cost reductions
• growth in the installed base of CD-ROM readers
This technological evolution is bringing about a revolution within many areas
of personal computer usage, including:
• electronic publishing
• multimedia
• imaging and archiving
This revolution along with continued reductions in price are going to lead to
one of the steepest growth curves ever experienced by a data storage product.
We are at the beginning of a huge opportunity for those who will manufacture,
resell, integrate, or use CD-R technology.
Chapter 4: Other Types of Storage Devices
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CD-R has been the longest anticipated and perhaps most frequently requested
data storage technology of the past ten years. Shortly after audio CDs were
introduced in 1983, users began requesting a means to record their own audio
mixes to CDs. When CD-ROM was introduced for computer data distribution
in 1984, demand for writing to CD formats on the desktop increased further.
Rewritable magneto-optical drives were released in 1987 and one of the most
common end-user questions became, “Why can’t this drive read and write
CD-ROM discs?”
Finally, the pricing, feature set, and software support of CD-R drives has
reached the point where it is practical, affordable, and easy for a typical enduser to create a CD-ROM disc on a personal computer. This capability, coupled with explosive growth of the installed base of CD-ROM readers over the
past three years, ensures the CD-R drive market huge growth for many years
to come. Analyst reports consistently position CD-R as the fastest growing
segment of the data storage market.
CD-Recordable is an international standard sometimes referred to as “Orange
Book, Part II.” This standard defines the technology by which CDs will be
written—by means of laser rather than stamping—so that the discs may be
created quickly with relatively low investment by the creator. It also ensures
that those discs will conform to the standards created for each disc format
(such as audio disc and CD-i) and will thus be compatible with all drives and
software designed to use CDs of that format. Because of this standard, it is
possible for anyone with a fairly decent personal computer to choose a CD-R
drive and media from more than 15 manufacturers, buy off-the-shelf software,
and create a CD that is readable by any CD-ROM reader in the world.
NOTE
For more information on standards, see Table 23 on page 113.
Standardization also sets up competitive situations where all manufacturers
find themselves on a level playing field assured of a broad potential market.
This in turn leads to steadily decreasing costs, improvements in performance,
and disciplined market growth.
Typical CD mastering
The typical CD or CD-ROM disc is created at a factory or “mastering facility”
on a piece of equipment called a stamper. Stampers are capable of producing
large volumes of discs very quickly. But there are disadvantages, too: stamping
is analogous to paper printing on a large scale. The equipment is specialized
and very expensive. It is designed for high-volume reproduction with a relatively long and costly set-up process. Trained personnel are required to operate and set-up the equipment. The whole process is cost-effective only when a
large quantity of discs is required. Such a process requires that the disc’s
author:
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•
•
•
•
•
•
•
send data off-site to the mastering facility
wait for set-up
wait for creation of a check disc
wait for the check disc to be shipped
review the check disc
provide approval
reship the check disc to the manufacturer
Finally, a few days after approval, the discs are produced and shipped back to
the author. This process requires considerable preparation, involves numerous
possibilities for delay, and introduces additional costs, such as shipping and
handling.
Desktop CD mastering
CD-R technology moves CD mastering onto the Desktop. Just as the Macintosh computer and laser printers revolutionized the paper publishing world
with Desktop Publishing, personal computers and CD-R solutions will radically change electronic publishing by decreasing costs and time-to-market
while increasing flexibility. Mastering can be done on-site, on a Desktop, from
creation to testing. Stamping will remain very important, and will even grow,
because those facilities will still be required for mass duplication of discs.
CD-R media is similar, but not identical to, a “normal” CD or CD-ROM disc.
Both media use a polycarbonate substrate, covered by one or more other layers, all of which are encased in a protective layer of lacquer. Normal CDs have
only one additional layer, a reflective aluminum layer that is sandwiched
between the substrate and the protective coating. CD-R discs also have a dye
layer that lies between the substrate and the reflective layer. Most CD-R discs
use gold for the reflective layer because of its higher reflectivity, resistance to
corrosion, and greater ability to protect the dye.
Protective layer
Reflective layer
Pit
Dye layer
Land
Pregroove
compare CD & CD-R
PC substrate
CD-ROM
CD-R
Figure 38. Comparison of CD-ROM and CD-R media
Chapter 4: Other Types of Storage Devices
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On a conventional CD, data, represented digitally by 1s and 0s, is recorded as
pits and lands (flat spots) in the pre-grooves of the media. On a CD-R disc,
rather than a physical pit or depression in the groove, a dye is used to change
the reflection of the laser in the same way that a pit would. This dye is activated by a laser beam of a somewhat higher intensity than that used to read
data. The use of this dye allows discs created by a laser to be just as readable
on conventional CD readers as discs created by a stamper.
Two types of dye are used in CD-R media:
• cyanine
• phthalocyanine
The proponents of phthalocyanine claim that media using that dye has a
longer shelf life based on extended life testing. Proponents of cyanine dye
claim that it is more tolerant of CD-R drives whose lasers are on the edge of
being within specified ratings. To date, no conclusive impartial tests have
been published on this topic. In reality, the actual chemical formulation of the
dyes and other coatings varies somewhat with each manufacturer. Some
media are clearly superior to others. CD recorders and readers should work
equally well with either dye. However, some recorders do seem to work better
with one manufacturer’s media than another. When in doubt, use only the
media recommended by the drive manufacturer.
CD-R discs come in three capacities, 650 MB, 550 MB, and 150 MB. Because
they can also be used to record audio data, the capacity is sometimes
described in minutes: 74, 63, or 18.
It is very important that recording surface (the underside) of CD-R discs not be
touched, scratched, or contaminated in any way until after the disc is completely recorded. Oil, grease, and other contaminants can prevent data from
being written accurately to the disc. After the disc has been written, it can be
treated like any normal CD.
STOP
The recording side of a CD-R disc typically has no label or printing on it.
Currently, most CD-R drives require that the media be put into caddies. In the
future, most mechanisms will probably change to tray loading. In the meantime, make sure you have caddies if you want to write CDs.
Fortunately for the user, while CD-R technology has been growing in popularity, CD-R media prices have been shrinking. In 1993, the street price for a single blank disc was at least $20. In 1995 was $10. The price will likely drop to
$5 and below in the near future.
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CD-Recordable drives are very similar to CD readers. In fact, many recorders
are based substantially on manufacturers’ reader mechanisms. However, the
differences are both important and complex. The primary differences include:
• the laser diode
• the electronics
• the firmware
Unlike CD readers, recorders need a laser diode that is capable of emitting
beams of two different intensities. The read beam is the same as a CD-ROM
readers. The write beam is more powerful since it’s used to create a chemical
reaction rather than just a reflection. Other circuitry that allows the drive to
receive incoming data and perform error correction is also necessary. CD-R
drives need larger RAM buffers to prevent data underruns and firmware that
supports all of these new features.
Virtually all CD-R drives use a SCSI interface. SCSI offers a good combination
of performance, software features and versatility, plug-and-play installation,
and relatively low cost. CD-R drives appeared with other interfaces, such as
ATAPI, in 1996.
NOTE
For more information on ATAPI, see Chapter 7, “Other Storage Interfaces.”
There are many different formats for compact discs. Many of these formats
are also international standards. These standards are often named for the color
of the book in which their specifications are published. Table 23 lists CD formats and standards with an explanation of their applications.
Table 23. Overview of CD formats and standards
CD Format
General Information
Standard
CD-DA
Compact Disc Digital Audio. The format used for conventional audio CDs. It is readable
on all audio CD players and most CD-ROM readers as well as a wide variety of other
multimedia devices.
Red Book
CD-ROM
Compact Disc Read-Only Memory. The format used for almost all CD-ROM applications.
It is readable on all CD-ROM drives and some multimedia devices. (See page 106.)
Yellow Book
CD-ROM XA
CD-ROM Extended Architecture. A seldom-used CD format that allows for the
interleaving of data, audio, video, and graphics for smoother playback of multimedia
content. It is only readable on XA-ready (i.e. XA compatible drives). Most—but not
all—readers sold after 1991 are XA-ready.
Yellow Book
CD-i
CD interactive. Used for some home entertainment systems. Philips, which developed
the standard, has been the most vocal proponent of this technology. Like CD-G, many
readers support the format but there are few computers or applications that allow the
discs to be used.
Green Book
Chapter 4: Other Types of Storage Devices
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Table 23. Overview of CD formats and standards (Continued)
CD Format
General Information
Standard
CD Bridge
This format is used for Kodak Photo CDs and Video CDs. Many readers support the
format but it has not yet become important globally. Software support and demand for
discs in these formats is still somewhat immature. The standard for PhotoCD was
created and is enforced by Kodak. Standards are proposed, but not in place.
Proposed:
CD-G
CD Graphics. The format used for Karaoke CDs. The format is readable on a variety of
devices, but most of them are consumer audio components. While the discs may be
readable on CD-ROM drives, special software is required to see the graphics.
White Book
CD-R
CD-Recordable. The format used for CDs that can be written to.
Orange Book, Part II
CD-Extra
A new standard for combining audio and data on a disc in such a way that the audio
may be played on any consumer CD player, while both the audio and the data can be
enjoyed on a computer with a compatible CD-ROM drive. This standard is very new and
many CD-ROM and CD-R drives do not support it.
Blue Book
White Book
Blue Book (see CD-Extra)
There are several different methods of writing to a CD-R disc. The method to
use depends on the following criteria:
• what the drive supports
• the application for which the disc it is being created
• the audience it is intended to reach
What follows is a description of the various methods of writing to a CD-R
disc.
Disc-at-Once
PMA
PCA
Lead In
Data
Lead Out
DAO
center
Figure 39. Order of information on a DAO CD
Disc-at-once, also known as uninterrupted write and DAO, is a writing
method in which the entire disc is written in one uninterrupted session. Data
cannot be added to the disc later. A DAO disc is Red and Yellow Book compliant. DAO is 100 percent compatible with all CD-ROM drives. It is used for
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creating final discs for electronic publishing, audio, and any other application
where complete compatibility is crucial.
STOP
A session is defined as a group of data containing a Lead In area, a Data area, and a Lead Out area.
A session can be recorded in one sitting or can be recorded incrementally, depending on the
formatting software’s capability. Each session carries about 15 MB of overhead for the Lead In and
Lead Out areas.
Incremental
Lead In
PMA
PCA
Track 1
gap
Track 2
gap
Lead Out
Incremental
center
link
link
Figure 40. Order of information on an incremental CD
Incremental recording is a method in which data can be added to a CD-R disc
in segments. There may be, but does not have to be, interruptions in the data
flow between segments. Data can be added using track-at-once (TAO) or
packet writing and single-session or multisession recording. These discs are
Orange Book compliant.
NOTE
See below for more information on TAO, Packet, and single- or multisession recording.
Track-at-Once
With track-at-once (TAO), the user can copy one track of data at a time to a
CD-R disc. Up to—but no more than—99 tracks of data can be stored on a
disc, space permitting. TAO recording can be used with either single-session
or multisession recording. The finalized disc is 100 percent backward compatible.
STOP
A track is a logical group of data, not a physical track.
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Packet
Lead In
PMA
PCA
Track 1
gap
Track 2
gap
Lead Out
Packet
link
center
link
link link link
Figure 41. Order of information on a packet CD
The user can copy a packet of data at a time to a CD-R disc. Like a track, a
packet is a logical group of data. However, while a disc can hold no more than
99 tracks, it can hold up to 1,000 packets, space permitting.
About 20 percent of each packet is overhead space. This method can be used
in single-session or multisession recording. It eliminates buffer underrun
problems—where the source of data supply is unable to keep pace with the
CD-R device’s recording speed—allowing reliable recording from slow
sources. It also lessens the need for a dedicated source or buffer drive.
Packet writing is still undergoing standardization and is not yet widely supported. The compatibility of packet-written CDs will not be known until the
standard is complete. It is fairly certain, however, that packet writing will not
be well-suited for writing audio discs.
Multisession
PMA
PCA
1st Volume
Data
2nd Volume
Data
Multisession
center Lead In 1
Lead Out 1
link
Lead In 2
Lead Out 2
Figure 42. Order of information on a multisession CD
A multisession disc is a CD-R disc containing more than one session. Each
session carries 15 MB of overhead for the Lead In and Lead Out areas. A multisession-capable writer is necessary for writing to the disc. A multisession
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capable reader is necessary for reading the disc, although the first session can
be read by any CD-ROM drive.
STOP
A session is one, contiguous, spiraling string of data written to, or stamped into, a disc. There
may be more than one session on a disc. A track is a portion, possibly all, of a session. A session
may contain many tracks, but a track may not contain a session.
An audio CD will have several tracks that are stamped into the disc as one, contiguous, spiraling
string of audio data—the silence between tracks is data that makes no sound. If a conventional
CD contains digital and audio data, it is stamped with at least two tracks in one session, that is
one, contiguous, spiraling string of data: first the digital data, then some silent data, then the
audio data.
If a CD is made with two sessions, the boundary between the sessions also forms a boundary
between tracks. The two sessions are recorded as two, contiguous, spiraling strings of data, one
following the other. The boundary between the sessions is not soundless data in this case, but a
small bit of smooth, unmarred, CD surface. Sessions are physically separate where tracks are
separated by silent data.
Multivolume/Multisession
Multivolume/multisession is a format where each session appears as a separate volume of data. On a PC, different volumes can be displayed as different
drive letters. On a Mac, different volumes can be displayed as different icons.
This is the most common type of multisession disc.
Kodak® Multisession
TOC Pointer
PMA
PCA
Kodak
center Lead In 1
1st Session
Data
Lead Out 1
2nd Session
Data
link
Lead In 2
Lead Out 2
Figure 43. Order of information on a Kodak multisession CD
Kodak multisession is a format where all sessions appear as one unique volume. On a PC, the disc can be displayed as one drive letter regardless of how
many sessions are on the disc. On a Mac, the disc can be displayed as one volume icon.
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WORM drives
WORM stands for “write-once read-many.” There are many different types of
WORM drives. They come in various form factors with various media formulations. They use a number of different writing techniques. Examples of
WORM drives include 12-inch platter drives from companies such as Sony
and Hitachi, multifunction magneto-optical drives from many different manufacturers, phase change optical drives designed by Panasonic, and the currently popular CD-Recordable drives.
Some WORM drives, including many multifunction drives, prevent over-writing or erasing data through a series of software flags and special data bits that
tell the firmware of the drive not to write to certain data blocks. Others, such
as the 12-inch drives and CD-R, are ablative—meaning the writing process
changes the physical properties of the media in a way that cannot be undone.
STOP
One meaning of ablation is reduction or dissipation, as by melting. The “ablative” in “ablative
WORM” refers to the creation of permanent pits in the media surface.
Ablative WORM is the ultimate archive device, because the information is
written physically and therefore is less prone to incidental data loss from
things such as magnetism and viruses. The primary benefit, however, is that
the data cannot be tampered with. Ablative WORM media is an excellent
choice for storing legal, medical, and other important documents.
Table 24. Advantages/disadvantages of WORM drives
Advantages
Disadvantages
• durability
Disks can last up to 100 years.
• high capacity—from 650MB for CD-R up to 15GB
for a 12” platter
• high cost media and hardware
CD-R is an exception to this.
• data inflexibility
As the name suggests, once data is written, it is
there for good, occupying disk space.
• slow access time relative to hard discs
• lack of standards
• lack of interchangeability
• incompatibility
CD-ROM Rewritable drives (CD-RW)
CD-ROM Rewritable is a new proposed standard for creating a rewritable
compact disc. Unlike CD-R, CD-RW discs can be reused many times. CD-RW
specifies a 650 MB drive with an access time in the 300 ms area. CD-ROM
drives capable of reading these disks will be called MultiRead-capable and will
be available in 1997. Older CD-ROM drives will not be capable of reading this
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format. Most DVD-ROM drives will also be able to read this format. In October 1996, Sony and Philips announced the first physical and logical file format
specification for the new standard. The CD-RW physical format specification
is set forth in Orange Book, Part III. While the logical data format is based on
UDF (Universal Disk Format) version 1.5.
STOP
Originally, this technology was called CD-ROM Erasable (CD-E). On October 25, 1995, Philips
announced the release of the first draft of Orange Book, Part III. Version 1.0, which contains the
CD-E specification, was set to follow in the first quarter of 1996. According to the specification,
CD-E discs were to be readable in CD-ROM drives, readable and writable in CD-R drives, and
readable, writable, and rewritable in CD-E drives. CD-E drives were to be capable of reading all
existing disc formats. Ricoh developed media for CD-E using an aluminum reflective layer, rather
than the CD-R’s gold. Additionally, it was to save data using phase change technology. For more
information on phase change technology, see “Phase change optical drives” on page 104.
Jukeboxes
Jukeboxes are similar to the music machines often found in restaurants and
bars. A large chassis contains multiple optical cartridges, tapes, or CD-ROM
discs that get shuffled around by a mechanical device. A particular platter is
fetched and moved to the enclosed drive when the data on that cartridge has
been requested.
Jukeboxes have differing capacity for the number of drives in the unit and
number of disks supported. The more drives in the unit, the faster data can be
accessed and the more data can be accessed by multiple users at one time.
Table 25. Advantages/disadvantages of jukeboxes
Advantages
• massive storage capacity, usually many gigabytes
• multiple-user access
Suitable for networked computers and large
operations that need centralized archival storage.
Chapter 4: Other Types of Storage Devices
Disadvantages
• high hardware costs and possibly high maintenance
costs
• durability
More moving parts to wear out, so more things can
go wrong.
• slow access time
Slow when going from one cartridge another.
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Digital Versatile Disc drives
DVD
Figure 44. Comparison of single- and dual-layer DVDs
Digital Versatile Disc (DVD) is a new standard designed to be the successor to
CD-ROM discs and VHS video cassettes. The DVD standard was first
announced in 1995. At that time the 650 MB limit to CD-ROM capacity was
becoming a barrier. Due to advances in multimedia, programs were growing
beyond 650 MB. VHS, with its costly cartridge, terrible picture quality, and
slow duplication times, has been looking for a successor for a while.
The DVD standard provides for up to 4.7 GB of storage on a 120-mm, CDROM-size optical disk (DVD-5). In the consumer format, DVD-VIDEO, it has
room for up to 133 minutes of high-quality, 500-line, MPEG-2, compressed
video with AC-3 Dolby Digital surround sound. The format achieves this
capacity boost by using a smaller dot pitch allowing it to use a shorter wavelength red laser (650/635 nanometers).
STOP
Dolby AC-3 is audio decoding that allows for 5.1 channels of digital surround sound.
DVD discs use two bonded 0.6 mm substrates, reducing the distance between
the disc surface and the physical pits. Lasers don’t have to penetrate as far as
they do on CD-ROMs, so pits can be more finely positioned. Also with two
substrates, a separate layer could be added. It would be read by a different lens,
allowing for increased capacity.
Other parts of the specification call for a dual-layer disk, DVD-9, with up to
8.5 GB capacity. Also specified are dual-sided flippable disks, DVD-10 and
DVD-18, with capacities up to 9.4 and 17 GB. Write-once DVD versions,
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known as DVD-R, are specified at up to 3.8 GB capacity. DVD-RAM specifies
a rewritable version that provides more than 2.6 GB per side, utilizing phase
change technology.
When used as a CD-ROM, DVD is called DVD-ROM. DVD-ROM may utilize
the Universal Disk Format (UDF) ISO 13346 standard for formatting data on
discs. This standard has been spearheaded by the Optical Storage Technology
Association (OSTA). This would allow disks to be read/written on any platform, ensuring full data interchange.
Most DVD products will be backward compatible with current CD-ROM
discs because they include two separate optical pickup lenses. Initial drives
should be about twice the price of current CD-ROM or CD Players. Future
versions should rapidly fall in price. DVD drives included in computer systems will require MPEG-2 and AC-3 decoders to play DVD discs. These
decoders could be placed on the drive or on an expansion card.
Because the DVD disc is higher density, it offers a basic quad speed transfer
rate of 1.35 MB/s, equivalent to a 9x CD-ROM drive. The system’s peak data
rate is 10.8 Mb/s with an average sustained data rate of 4.94 Mb/s.
When used in consumer applications, this type of disc provides up to 100 minutes of D-1 quality digital video with Dolby AC-3 digital surround sound.
Movies can now be stamped out quickly and cheaply. DVD disks can be manufactured using very few changes to current CD production techniques.
Due to copyright issues raised by the motion picture industry, DVD was not
released as planned in 1996. The computer and motion picture industries
finalized an encryption scheme in October 1996 that allowed the first DVD
products to be shipped that same month. The first DVD-ROM drives for computers will ship in 1997.
Chapter 4: Other Types of Storage Devices
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Tape drives
DAT
Figure 45. DAT and DAT autoloader tape drives
Tape drives use magnetic recording tape to store data in a fashion similar to
audio tape recorders. One difference is that information is usually stored in
digital form. The primary difference between the different types of tape drives
is their capacity and the type of tape cartridge they use (such as 4 mm digital
audio tape, 8 mm tape, quarter inch tape [QIC], Travan, and half-inch DLT).
STOP
Travan is a subset of the mini-cartridge format QIC (quarter-inch cartridge). At .315 inches,
Travan tape is slightly wider than standard QIC. It allows a 750 foot tape length, which translates
to larger capacity—up to 10 GB for proposed TR-5—and data transfer rates up to 1.2 MB/s.
Most tape drives include a compression feature that compresses data written
to (or decompresses data read from) the tape drive on-the-fly. Compression
ratios range from 4:1 for spreadsheets, to 2.4:1 for text files, to 1.3:1 for graphics files. The amount of compression you get depends on the type of data.
The most common tape drives are the low-cost QIC tape drives. They populate almost 80 percent of the market. These QIC drives are very popular on
PCs. Next in popularity is the 4 mm DAT drive with about 15 percent market
share, with the remainder in the 8 mm and DLT markets.
Digital Data Storage (DDS)
DDS is a high-performance, high-capacity, streaming tape device designed for
use on mid-range and high-end computing systems. They are used for local
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and network backups or archiving. They allow you to back up a larger data
capacity at a higher speed than many other types of tape drives.
Digital Data Storage (DDS) is a standard format for 4 mm tapes using helical
scan recording. It was developed by Hewlett-Packard and Sony. Originally
developed to replace the audio cassette, DDS has found homes in both proaudio and computer areas.
• The first generation standard is DDS-1 (or simply DDS).
• Data compression was added to DDS-1 to produce the DDS-DC standard.
• Further enhancements, notably narrower tracks and thinner tape, led to
DDS-2, the third generation. DDS-2 provides twice the capacity of DDS-1.
• DDS-3 will appear in 1996, offering higher capacity and faster data
transfer rates due to the use of PRML read/write channels and a higher
density metal particle MP++ tape.
STOP
PRML stands for partial-response maximum-likelihood. It is a technique used by a read/write
head in a drive mechanism for detecting data. Instead of spacing out analog peaks as in peak
detection, digital filtering is used to compensate for signal overlap. After filtering, the scheme
identifies what are the likeliest sequence of data bits written to the media.
Table 26. Comparison of features of DDS-1 through 4
Feature
DDS-1
DDS-2
DDS-3
DDS-4
Tape Length
60 or 90 m
120 m
125 m
180 m
Native Capacity
1.3 or 2 GB
4 GB
12 GB
24 GB
Compressed Cap
2.6 - 4 GB
4 - 8 GB
12 - 24 GB
24–48 GB
Channel
8,10 mod
8,10 mod
8,10 PRML
8,10 PRML
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Digital Linear Tape (DLT)
6.0
10
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Figure 46. DLT tape drive
DLT is a high performance, high capacity, streaming cartridge tape device. It’s
often used for data backup or archiving. The design often includes:
• dual-channel read/write head
• Digital Lempel-Ziv (DLZ) high-efficiency data compression
• tape mark directory for maximum throughput and minimum access time
DLT allows you to back up more data at higher speeds than other personal
computer-oriented tape products. In non-compressed mode, the drives have a
maximum sustained transfer rate of 1.5 MB/s. In compressed mode, the maximum transfer rate is 3 MB/s write and 3.5 MB/s read.
Capacities range all the up to 40 GB on a single tape in compressed mode.
Table 27. Advantages/disadvantages of tape drives
Advantages
• low cost
Most inexpensive way to back up data.
• potential capacity
Multiple cartridges increase capacity.
• fast transfer rates
Transfer rates up to 1.6 MB/s and beyond possible.
124
Disadvantages
• slow access times
Due to slow access and sequential access,
inappropriate for on the fly, primary data storage.
• reliability
Not a very reliable medium over the long term.
Subject to warping and shrinking.
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5
All About SCSI
Overview
Chapter 5 begins with SCSI background and basics, skims the surface of
SCSI’s technical waters—occasionally diving a bit deeper into computer-specific surf—and finally deposits you firmly at the shores of information beyond
and outside the SCSI scope.
If you’re unfamiliar with SCSI, you’ll want to read Chapter 5 from beginning
to end. Even SCSI gurus will benefit from this refresher. You’ll get the most
out of it if you’ve already read through Chapter 2, “All About Drives.”
SCSI Is Simple …
Storage products need a way to connect and communicate with the rest of the
computer. SCSI has emerged as a very popular method of interfacing storage
products to a personal computer. SCSI, pronounced “scuzzy,” stands for Small
Computer System Interface. It is a standard specification that defines how
computers and their peripherals connect and communicate with each other.
SCSI is best known as the interface for Apple Macintosh hard disk drives.
In June 1986 SCSI-1 was adopted as an American National Standard by the
American National Standards Institute (ANSI). It qualified as ANSI standard
X3.131-1986.
STOP
ANSI is a private, nonprofit membership organization that performs two functions:
• It coordinates the United States’ voluntary consensus standards system.
• It approves American National Standards.
ANSI ensures that a single set of non-conflicting American National Standards are developed by
ANSI-accredited standards-developers, and that all interests concerned have the opportunity to
participate in the development process. ANSI does not develop standards. However,
• It provides the means for determining the need for standards.
• It ensures that qualified organizations develop those standards.
• It coordinates standards approval.
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ANSI established the X3 category for information technology. Some of the current technologies
that fall under the ANSI X3 category are listed in Table 28.
Table 28. ANSI X3 categories for current technologies
ANSI Category
X3T10.1
Technology
Serial Storage Architecture (SSA)
X3T11
Fibre Channel, HIPPI and IPI
X3T12
Fibre Distributed Data Interface (FDDI)
X3T13
Advanced Technology Attachment (ATA)
and Advanced Technology Attachment
Packet Interface (ATAPI)
ANSI follows these steps to set standards:
1.
2.
3.
4.
Study group assembles to develop project proposal (six months).
Work group develops draft of proposed standard (six months)
Technical committee reviews and (possibly) approves draft (one year).
Standard is submitted to ISO (International Standards Organization) for installation.
The entire process can take from two to ten years.
ANSI’s endorsement of particular guidelines, though not binding, exerts a powerful standardizing
influence in the computer industry.
… and Complex
SCSI defines a command set that improves data exchange and enables use of
several different kinds of peripheral devices with one or more host computers.
The SCSI specification defines an input/output (I/O) interface that is variously
described as:
•
•
•
•
•
•
•
•
nonproprietary
device-independent
flexible
low- and high-level
intelligent
eight-bit
parallel
peer-to-peer
These terms are explored in the sections that follow.
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Essential Terminology
SCSI jargon is esoteric but critical to understanding the underlying concepts.
Here is terminology to acquaint you with some of the fundamentals. You’ll
also find these and other useful terms in the Glossary.
• Bus
The wires that provide the physical means for conveying electrical
signals. Think of a bus as a conveyance for electrical signals rather than
people.
• Host
The computer—specifically, its central processing unit (CPU) and SCSI
chip.
• Initiator
A SCSI device, usually the computer—or more specifically its SCSI chip—
capable of initiating an operation. In multiple-host systems initiators
(hosts) may also be targets.
STOP
A multiple-host system is a system that has more than one computer connected to the SCSI bus.
• Target
A SCSI device, usually the peripheral, that carries out the initiator’s
request.
The Origin of SCSI
A discussion of SCSI’s origins almost begs the issue: SCSI is still evolving.
What’s happening today will no doubt later be considered part of its origin.
SCSI-2 is already in place, and SCSI-3 is under development. Assume all references to SCSI herein mean SCSI-1, the original specifications from 1986.
NOTE
For more information about SCSI-2, see “SCSI-2: A Transition From SCSI-1” on page 193. For
more information about SCSI-3, see “SCSI-3” on page 202.
Once the non-mainframe computer industry got started, its sprawling development defied control. Technologies spawned technologies, which were
brought to market in many forms by manufacturers who pursued specific
goals and niche markets. Some believed their implementation was superior to
all others or hoped to establish a de facto standard that would reap substantial
long-term rewards, perhaps even market domination.
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STOP
There are two types of standards: de facto and agency. De facto standards are set by users over
time. Agency standards are received, reviewed, passed, and published by a standards agency. ANSI
and IEEE are standards agencies.
The good news: without imposed uniformity, creativity was not constrained:
• From a developer’s perspective, it meant they could try anything.
• From a buyer’s perspective, it meant everything was available.
The bad news? Many of these products wouldn’t work with anybody else’s.
This pervasive incompatibility among different vendors’ devices eroded buyer
confidence and effectively limited the growth of the industry. In time, a consensus formed about the need for an open, or nonproprietary, standard that
addressed host-to-peripheral communications. Device communications, particularly given the critical nature of data storage, was too important to ignore.
SCSI History
SCSI time line
Figure 47. SCSI time line
A wide-ranging confluence of factors created SCSI, and not everybody agrees
on all the details. Here is our version:
In the 1960s IBM Corporation developed a bus that later became the most
common way for third-party vendors to connect peripherals to IBM mainframes, most often its Model 360. It was called the block multiplexer or OEM
channel. It became so popular that the federal government made it a processing standard (FIPS 60). It allowed multiple peripherals to intercommunicate at
the same time. Other computer manufacturers objected, alleging that such an
adoption gave IBM an unfair advantage, and took this decision to the courts.
STOP
128
OEM stands for original equipment manufacturer. It applies to a product that is manufactured by
one enterprise then sold to another, where it is bundled and sold with the purchasing enterprise’s
own products.
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The suit was unsuccessful, but the pressure it exerted may have later kept
ANSI, which was looking to create an I/O bus standard, from adopting the
OEM channel as it stood. ANSI, wanting to create a standard for a nonproprietary I/O bus, did consider the OEM channel before beginning work on the
intelligent peripheral interface (IPI) in the early 1980s.
NOTE
For more information on IPI, see Chapter 7, “Other Storage Interfaces.”
About 1981, Shugart Associates, a disk drive maker, joined forces with NCR
Corporation and came up with an interface based on the OEM channel architecture called SASI, for Shugart Associates System Interface.
STOP
The individuals responsible for SASI later went on to found Adaptec, Inc.
Unlike the OEM channel, SASI was intended as a low-cost, peer-to-peer interface. Shugart wanted a logical interface to data on drives, while NCR wanted
extensions for large devices and differential cabling. Three manufacturers—
DTC, Xebec, and Western Digital—embraced SASI and built controllers for it.
SASI eventually evolved into what is known today as SCSI.
STOP
A peer-to-peer interface involves two systems that communicate as equal partners, sharing the
processing and control of the exchange.
In 1982, the ANSI committee founded the X3T9.2 standards accreditation
group to create the SCSI standard. In 1983 the first SCSI chip from NCR
appeared, the 5385, even before the SCSI-1 standard was finalized. A year
later, the 5380 appeared with on-chip drivers, making inexpensive SCSI
implementations possible.
Apple’s SCSI Influence
Apple Computer Inc. hopped on the SCSI bandwagon before any other major
company. They introduced a SCSI-ready Macintosh Plus in 1986. The Macintosh line has relied on SCSI ever since.
STOP
Apple has not relied exclusively on SCSI, however. In 1995 they implemented IDE drives in some
of their newer machines, including PowerBooks. For more information, see “IDE” on page 225.
Apple chose the SCSI interface primarily for its speed, device independence,
and expandability. Formerly—in the Macintosh 128 and 512, and in fact with
most computers manufactured prior to 1986—hard disk drives were connected to an existing port designed for a slow device. This was usually a serial
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port, which was very slow and allowed connection of only one device. Connecting to the floppy drive port was not much better, although it was faster
and allowed connection of two devices. Because neither were designed for
hard drives, an inconvenient boot floppy was necessary to start the drive.
STOP
The serial port is a connector on a computer or peripheral device that can send and receive data
one bit at a time. A floppy port can send a receive data in parallel but provides much slower
performance than a SCSI port.
When Apple began shipping all Macintosh computers with a hard drive port
that was standardized on SCSI—an interface that allowed the attachment of
up to seven devices—a clear signal was sent to third-party vendors that here at
last was something upon which they could standardize. This was a critical
endorsement of the new interface.
Unlike many computers, the Macintosh “speaks” SCSI directly via a dedicated port, or connector. Because SCSI support is built-in, no adapter is
required. This is true even of the low-end Macintoshes with one or no internal
slots for peripherals. Every Macintosh can use basically the same SCSI peripherals.
PCs rarely have SCSI on the motherboard. Typically they require a host bus
adapter card to provide SCSI support and connectors.
A Good Start...
To an ever increasing degree, SCSI works.
• It works for connecting different devices to operate together as part of a
system. This is called interoperability or connectivity.
• It works for boosting I/O performance.
• It works for hardware economy.
Interoperability or connectivity
The SCSI specification enables diverse peripheral devices and one or more
computer systems to work together. As many as seven SCSI devices can be
connected to a computer in what is called a daisy chain.
STOP
130
Devices are distinguished by SCSI ID numbers. SCSI ID determines the priority a device will
have when it seeks to gain control of the bus. For more information on daisy chains, see “Physical
connections” on page 147.
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SCSI is device-independent. It masks the internal operation of a peripheral
from other peripherals and the computer. This way the computer doesn’t need
to know much about them to communicate. SCSI does this by treating peripherals as logical units (rather than physical, or actual units) responding to a
known language, also known as a protocol. SCSI treats devices as storage
abstractions. SCSI’s ability to view its world in abstract terms is a powerful
facilitator of its operations.
Each peripheral can comprise eight logical unit numbers (LUN—the numerical representation of the peripheral’s address). Consequently, the capability to
daisy chain seven peripherals to a host means that you are allowed a total of
56 logical units on a SCSI bus.
STOP
LUNs allow for addressing subunits within a peripheral device. Using a CD-ROM jukebox as an
example, the jukebox is identified by its SCSI ID, and the individual drive mechanisms within
the jukebox are identified by LUNs.
In principle, SCSI is like the United Nations. It’s a universal forum that can
accommodate a diverse world of manufacturers. Through SCSI, manufacturers can offer products that are both unique and compatible with other vendors’ hardware. Even implementations of emerging technologies can be
connected easily to a computer system if they speak SCSI (and more and more
of them do).
Still, device independence is not complete. Special software is required to
fully exploit new or dissimilar device types. This special software is called a
device driver. It enables the host computer (initiator) and a peripheral device
(target) to communicate with each other. For example, a device driver is
needed if a system that previously included only the initiator and a hard disk
drive expands with the addition of a scanner or CD-ROM drive.
STOP
A device driver is the software program that translates commands between the computer’s
operating system and the storage I/O interface. For more information, see “Device drivers” on
page 181.
All devices on the SCSI bus are treated as equals in terms of their capability to
communicate directly with other devices on the bus. Because of this, SCSI is
considered a peer-to-peer interface.
Because SCSI defines standard physical and electrical connections for devices,
connecting the parts of your system is easy.
SCSI also reduces software incompatibility. Drivers must use a SCSI Manager,
or else much of SCSI’s potential would be just be so much dead code. If drivers
bypassed a SCSI Manager, they would have to be rewritten every time a
machine shipped with a different SCSI chip or addressing mode.
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NOTE
For more information, see “SCSI Manager” on page 178, “Device drivers” on page 181, and “Call
Chain” on page 183.
Boosting I/O performance
The SCSI bus operates in parallel. Each byte of information is separated into
bits and sent simultaneously along eight parallel wires in the SCSI cable.
STOP
Bit is a contraction of “binary digit.” A bit is always either a 0 (off) or a 1 (on). It is the basic unit
of information used by a computer. There are eight bits of data per byte. Byte is a contraction of
“binary digit eight.”
Because the SCSI specification describes such wiring—as well as other electrical operations and physical connections—it is considered a low-level interface. Yet SCSI also presents programmers with a nonphysical abstraction (a
logical perspective) that can be manipulated with software. Because the SCSI
specification details this command structure, it is also considered a high-level
interface.
SCSI uses complete logical commands. For example, when accessing data on a
hard disk drive, SCSI lets the initiator request a particular data block without
having knowledge of where that data block physically resides on the drive.
The SCSI drive takes this abstract (or logical) request and translates it into the
specific request required to locate the data block. This reduces the inherent
delay when information changes direction on the bus as devices communicate
along the eight-bit-wide data path. It also reduces the amount of information
that is exchanged, because one command takes care of several operational
aspects at once.
By contrast, the OEM channel had to initiate these commands and responses:
1. Go to X place.
2. Wait to be told X has been found.
3. Act on what resides in X.
SCSI is more efficient and direct. It says:
1. Find X wherever it is, and act upon it in such and such a way.
SCSI is more logical and complete.
A SCSI peripheral’s controller (a SCSI chip) provides intelligence, allowing the
peripheral to store and manipulate data. This frees the host CPU to perform
other tasks. Typical drives have embedded microprocessors on the controller
board, such as Intel’s 80C188 or 80C196, to control these operations. Drives
using servos usually have dedicated servo processors that control actuator
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movement, motor speed, and motor spin up. The SCSI chip can also free up
the CPU by reducing an operation’s dependence on the CPU during I/O.
NOTE
For more information, see “SCSI Integrated Circuits (Chips)” on page 135.
Because only one SCSI device at a time may use the bus to send its signals and
await a response, most SCSI peripheral devices are capable of leaving the bus,
completing a time-consuming operation, and then returning. This is known
as disconnect capability. In the meantime, the host can work with other
devices, or another host can use the bus. The result: multiple operations are
performed simultaneously.
STOP
NOTE
A computer or peripheral using the bus is said to have gained control of the bus, and that’s what
we will use to describe that event. But it may help your understanding to bear in mind the bus-ascarrier analogy, and to think of data as riding the bus between stations (the devices or a device and
the CPU).
See “Signal lines” on page 143 and “Bus Phases” on page 169 for more information.
Hardware economy
SCSI relaxes constraints on the physical limits of the bus. The cable that connects SCSI devices can now reach 25 meters (about 80 feet) if used in a differential configuration. Apple and most PC host adapters, however, use the
single-ended electrical specification, which is more restricted—six meters in
length, or up to about 19 feet.
STOP
Differential SCSI is a SCSI cabling scheme that uses the difference between two lines to indicate
whether a logic “1” or a logic “0” is being encoded. The benefits include longer maximum cable
lengths, greater protection from noise, and faster data transfer rates. For more information, see
“Physical connections” on page 147.
… But It Needs Work
A widely acknowledged problem with SCSI is that its early strength—flexibility—is now a problem. The specification lacks specificity.
Revision 17B of SCSI-1 defined classes and formats of commands, but except
for one required command, Request Sense, SCSI devices were on their own. It
was too easy for vendor-unique commands to proliferate, which they did.
Consequently, few SCSI controllers could plug-and-play with any SCSI
peripheral. This was especially true between 1984 and 1987, and particularly
for devices other than hard drives. Without the benefit (constraint?) of a market “tradition,” implementations of newer technologies proliferated.
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NOTE
See “SCSI Manager” on page 178 for more information on SCSI commands.
There is, however, a more comprehensive de facto standard for commands,
called the Common Command Set (CCS 4B), which was included in SCSI-2.
Since 1988, virtually all hard drive manufacturers have incorporated CCS or a
subset of it, resulting in a reduction or elimination of vendor-unique commands. SCSI-2 also described CCS-like commands for non-hard disk devices.
NOTE
For more information, see “SCSI-2 commands” on page 195.
Who Uses SCSI?
SCSI is becoming increasingly associated with high-end PCs, workstations,
and fast hard drives. It also has virtual exclusivity as the interface for state-ofthe-art storage technologies, such as external magnetic and optical disc drives,
DAT drives, WORM drives, and CD-R drives.
STOP
The “Small” in Small Computer Systems Interface (SCSI) has become a misnomer.
SCSI is also gaining popularity as computer systems continue to evolve into
increasingly complex configurations, connecting such diverse peripherals as:
•
•
•
•
•
•
•
•
•
hard drives
scanners
printers
tape drives
magneto-optical drives
removable cartridge drives
CD-ROM drives
cameras
plotters
Even IBM, long a staunch hold-out, is beginning to offer SCSI on most of its
latest computers.
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SCSI Integrated Circuits (Chips)
Controllers
Figure 48. Single host/multiple controllers
Chip overview
STOP
Manufacturers of SCSI chips include Adaptec, Qlogic, and Symbios Logic.
The computer with SCSI or a SCSI host adapter uses an integrated circuit (IC)
dedicated to handling SCSI operations. This IC is known as a SCSI interface
controller. ICs are also called chips, because all ICs, SCSI or otherwise, reside
on a silicon chip. The SCSI Manager controls the operation of this chip and
provides it with an interface to the operating system software.
NOTE
For more information, see “Call Chain” on page 183 and “SCSI Manager” on page 178.
The latest wave of SCSI chips on add-in cards dramatically reduces the
amount of programming needed to perform a SCSI transaction by performing
in hardware (the chip) many of the functions formerly handled by software.
This frees the computer’s CPU for other tasks.
The SCSI chip shipped in most current computers, however, does require
some assistance, so the CPU is not entirely absolved from SCSI operations.
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Chip sophistication—how much of the SCSI operation the chip can handle by
itself—affects overhead and performance.
STOP
Some machines have a microprocessor dedicated to assisting the SCSI chip.
Among the chip’s more critical responsibilities is managing the timing constraints defined in the SCSI specification, including the signal delays that are
central to changing bus phases. This will be discussed in greater detail in the
sections that follow.
STOP
Communication across the SCSI bus is divided into stages known as phases. There are eight
possible bus phases:
• Bus Free
• Arbitration
•
•
•
•
•
•
Selection
Reselection
Command
Data-In/Out
Status
Message-In/Out
For more information, see “Bus Phases” on page 169.
SCSI chip families
Symbios 53C8X/53C9X family
Symbios was formerly known as NCR. The original Macintosh line relied on
Symbios 5380 and 53C80 chips.
STOP
The “C” in 53C80 is shorthand for CMOS, or Complementary Metal-Oxide Semiconductor.
CMOS chips require less power to run.
The 5380, found in early Macs such as the Macintosh Plus, is fairly simple. It
offers the lowest hardware cost to implement the SCSI specification, and controls the following interactions:
•
•
•
•
•
136
arbitrates for use of the bus
performs retries
selects a target
sends commands
transfers data
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However, it needs the CPU to control many SCSI bus interactions, including:
• instigating the SCSI bus phase transitions
• reading the status
• handling data transfers
All of the interactions that the 5380 controls take place in hardware. The
5380 also operates independently of the main CPU’s clock speed (expressed in
megahertz), which lets Apple use the chip in any Macintosh.
The Mac IIci as well as most other Macs use the 53C80; the Mac IIfx uses a
customized 53C80 array. The IIfx integrates the 53C80 and a DMA controller
on a single custom chip on the motherboard. The IIsi and the LC use the
85C80 chip, which offers on-board serial support in addition to SCSI, minimizing the number of chips needed to create the machine.
STOP
DMA stands for direct memory access. It is a code-based mechanism that enables the transfer of
data to or from a computer’s main memory without using the CPU. For more information, see
“Modes” on page 138.
Later Macs such as the Quadra family utilized the 53C9X family of SCSI
chips. The Quadra840AV/Centris 660AV used the 53C94. The Centris 610/650
and Quadra 605/700/800/900/950 used the 53C96. These chips combined
faster 5 MB/s SCSI-2 performance with highly automated operation, and eliminated the need to tediously drive the chip through each SCSI phase.
Symbios Logic 53C7XX/53C8XX families
The Symbios 53C700 family includes an integrated DMA controller, a separate IC to handle bus sequences, and a two-million-instruction-per-second
RISC (Reduced Instruction Set Computing Chip) I/O processor core, which
can read commands from external memory and control one or more complete
I/O transactions without CPU intervention.
The 53C700 allows the SCSI bus to be closely coupled with the host CPU
without having to add an intelligent host adapter. This minimizes overhead
and avoids degrading the CPU’s performance. The 53C700 family interfaces to
Intel’s 80X86 and Motorola’s 68000 family with virtually no fuss, and can also
be used on host adapter boards and with other ICs.
Most chips available today cannot move data faster than 5 MB/s, although
many peripheral vendors claim their products support the 10 MB/s rates
offered by Fast SCSI-2.
The Symbios 53C710, introduced in 1990, supports Fast SCSI-2’s 10 MB/s
synchronous transfer rates. The 537C20, a 1991 entrant, also meets the
10 MB/s data rates, and supports 16-bit 20 MB/s “Wide SCSI” transfers.
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NOTE
For more information on Fast/Wide SCSI, see “SCSI-2: A Transition From SCSI-1” on page 193.
The 53C8XX family takes the 7XX family one step further by adding a PCI
interface to the chip and by increasing the scripting and performance features.
•
•
•
•
53C810 is a Fast SCSI-2 10 MB/s chip.
53C825A is a Fast/Wide SCSI-2 20 MB/s chip.
53C875 is an Ultra SCSI 40 MB/s chip.
53C895 is an Ultra-2 SCSI 80 MB/s chip.
Qlogic ISP chip family
The Qlogic ISP family is a line of RISC-based SCSI chips designed for high performance. They combine a 7- to 18-MIPS RISC processor with a programmable SCSI back-end.
•
•
•
•
•
STOP
ISP1000 interfaces Fast/Wide SCSI-2 to the Sun S-Bus.
ISP1020 interfaces Fast/Wide SCSI-2 to the PCI bus.
ISP1040 interfaces the 40 MB/s Ultra SCSI to the PCI bus.
ISP1080 interfaces the 80 MB/s Ultra-2 SCSI to the PCI bus.
ISP2100 chip interfaces Fibre channel to the PCI bus.
MIPS stands for million instructions per second. It is a measure of a CPU’s execution speed. RISC
stands for reduced instruction set computing. It speeds up processing by employing as few
computation instructions as possible to complete tasks. It improves on complex instruction set
computing (CISC).
Apple custom SCSI chips
Apple designed a chip called the Curio which combined Ethernet, SCSI, and
Serial Communications. This chip was used in the initial PowerMac family
(6100/7100/8100). It was also used in many PowerBooks. Following the Curio,
Apple designed the MESH chip. It debuted in the first generation of Power
Macintosh with PCI, the PowerMac 7500, 8500, and 9500. It is designed to
provide Fast SCSI-2 10 MB/s performance with minimal overhead.
Modes
The SCSI chips function in one of three communications methods of operation, or modes:
• Normal
• Pseudo-DMA
• DMA
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Normal
In Normal mode the Macintosh’s CPU manages I/O communications. Normal mode is a drain on the CPU, which slows I/O performance. Normal mode
is also known as Polling method, in which the CPU checks that there has
been a Request/Acknowledge handshake with every byte of information that
is transferred.
NOTE
For more information, see “Data Transfer Options” on page 139.
Pseudo-DMA
In Pseudo-DMA (direct memory access) mode, the SCSI chip oversees data
transfer, letting the CPU tend to other tasks. This is also known as the Blind
Data Transfer method because the CPU checks data block-by-block instead of
byte-by-byte. Blind transfers are much quicker than the polling method
because larger chunks of information are transferred at a time.
NOTE
See “SCSI, Apple-Style” on page 173 for a discussion of problems with Blind transfers.
DMA
In DMA (direct memory access) mode, the SCSI chip is programmed to transfer data to and from the SCSI bus directly into main memory, eliminating the
need for CPU intervention.
In practice, the modes operate in tandem, with data transfer starting out in
Normal mode and then turning to Pseudo-DMA or DMA mode. During the
data transfer, the processor checks the status register to ensure that transfer is
occurring.
Data Transfer Options
Essential terminology
• Handshake
To understand data transfer options, it's important first to understand the
concept of the “handshake.” As in human affairs, the SCSI handshake
signifies completion of a deal. Only here, it’s between the initiator and the
target.
Whenever data is transferred between the initiator and a target, it is
marked by a Request/Acknowledge handshake. Either the initiator or
target makes a request, and that request is acknowledged by its recipient.
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NOTE
For more information on handshakes, see the Request and Acknowledge entries in “Control
signals” on page 144.
• Data Transfer Modes
Data transfer occurs in one of two modes, or protocols:
• Asynchronous
• Synchronous
The mode used is determined by the SCSI chip via messages between the
initiator and target, usually after a bus reset.
• Bus Reset
A bus reset is a method of clearing (and consequently stopping) all action
on the SCSI bus and returning to a known, free state.
Asynchronous data transfer
Async
Figure 49. Asynchronous data transfer mode
Asynchronous is a mode of data transfer that requires a Request/Acknowledge
handshake for the movement of every byte of information. It has been used by
many computers for all bus phases.
STOP
However, PCs have used synchronous mode from their inception.
For machines that use the asynchronous mode, it is the data transfer default.
It is in effect at power-up, after a Bus Device Reset message, and after a hard
reset condition.
STOP
140
A Bus Device Reset is a message that is sent to reset the operation of a specific device. A hard
reset is toggling a SCSI line to reset all devices on the SCSI bus.
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The SCSI bus in asynchronous mode offers a transfer rate of up to 5 MB/s. The
SCSI specification doesn’t define an upper limit for the asynchronous transfer
rate. Instead, it defines minimum timing (about 250 nanoseconds) for the
Request and Acknowledge control signals, which control information transfer
in the asynchronous mode.
STOP
Fast SCSI uses a timing of about 100 ns, while Ultra SCSI uses a timing of about 50 ns.
Asynchronous data transfer is effectively limited, however, by cable impedance and other transmission-line problems, including whether the cable is
operating at its optimal temperature and voltages. Asynchronous data transfers slow down as the length of cabling increases. A system with 6 meters of
cabling could have a data transfer rate that is 25 percent slower than a short
one-meter setup. The SCSI-2 specification addresses such cabling limits.
NOTE
For more information, see “Signal lines” on page 143 and “SCSI-2: A Transition From SCSI-1” on
page 193.
Synchronous data transfer
Not all SCSI devices support synchronous data transfer. Synchronous transfers allow a series of bytes to be exchanged before a Request/Acknowledge
handshake is required. The number of Requests and Acknowledges will eventually match, but they are allowed to get ahead of one another. Synchronous
data transfer may only be used in the Data Phase following a Selection Phase
in which the SCSI ID for both the initiator and target are asserted.
NOTE
For more information, see “Bus Phases” on page 169.
Synchronous mode can have a “burst” data transfer rate as fast as 5 MB/s—
40 MB/s in SCSI-2. Sustained rates are much slower. This is quicker than
some interfaces, but not as fast as others that employ different cabling
schemes.
NOTE
See Chapter 7, “Other Storage Interfaces,” for comparison.
Synchronous data transfer rates are not as limited by cabling as asynchronous
data transfer rates.
Negotiating synchronous transfer
The target and initiator must agree to use synchronous data transfer in a negotiation process that establishes certain parameters. These parameters include
the transfer period (which determines transfer rate) and the Request/
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Acknowledge offset, which sets the maximum number of Request signals that
may be sent before a corresponding Acknowledge signal is received. An agreement to use synchronous data transfer remains in effect until:
• a Bus Device Reset message is issued
• a hard reset condition occurs
OR
• one of the devices otherwise elects to modify the agreement
NOTE
For more information, see “Bus Phases” on page 169.
Either the initiator or the target may recognize that negotiation for synchronous data transfer is required. If the initiator recognizes the need, it asserts
the Attention signal and sends the target a Synchronous Data Transfer
Request message that indicates these parameters.
NOTE
For more information, see “Control signals” on page 144 and “Messages” on page 164.
The target chooses one of four options:
• It agrees.
• It stipulates a different transfer period and/or offset.
• It sets the offset to zero. A Request/Acknowledge offset of zero means the
devices will use the asynchronous mode.
OR
• It sends a Message Reject, which also results in the asynchronous mode.
If the target recognizes the need, it sends the initiator a Synchronous Data
Transfer Request message that indicates these parameters.
One caveat needs to be considered when using devices with synchronous data
transfer capability: following a reset or power-on, when a device first receives
a command that requires data transfer, it will begin the negotiation for synchronous data transfer. The host must be able to do the following:
• negotiate for a synchronous or asynchronous data transfer agreement
OR
• handle the negotiation and reject it at some point, thus forcing the
asynchronous transfer protocol
OR
• reject the request, thus forcing the asynchronous transfer protocol
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Signal lines
Host
Computer
Signal
Disk
Figure 50. Signal lines
The SCSI bus is composed of up to 68 lines, depending on the electrical specification used. Of the total, 18 are signal lines: nine transmit data signals; nine
transmit control signals; the rest are connected to ground, except for line 25,
which is left open.
Data signals
Data signal lines convey data, commands, messages, bus status, and SCSI ID.
Data signal lines are called DB (for Data Bus) and are numbered from 0 to 7.
DB7 carries both the data and ID for the device with the highest priority,
which is usually the computer. DB0 represents transmission of the least significant device. Each DB line may carry a different SCSI ID, and thus transmit
data for a different device.
DB0 through DB7, however, adds up to only eight lines. The ninth data-signal
line is reserved for parity, which is a self-checking code that tracks the accuracy of binary digits (zeros and ones).
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STOP
SCSI uses odd parity, which requires that the total of the binary digits must come out odd. With
odd parity, if the total of the digits does not come out odd, error correction is automatically
initiated. Parity is a SCSI bus option in SCSI-1 but is required in SCSI-2. If used, parity must be
supported by all SCSI devices on the bus. It is valid for all Information Transfer phases (Data,
Command, Status, and Message) as well as the Selection and Reselection phases. The Macintosh
models previous to the Quadra 840AV didn’t use parity, while most PC SCSI host adapters do.
Control signals
Control signals direct communications and determine bus phases. There are
nine control signals:
• Control/Data
• Input/Output
• Select
• Busy
• Message
• Request
• Acknowledge
• Attention
• Reset
The control signals are listed and defined in Table 29 on page 145. But first,
it’s important to understand some related terminology.
• Arbitration
How SCSI devices vie for control of the bus during an optional bus phase.
• Assert
Usually assert means to send a control signal. For two control signals,
Control/Data and Input/Output, it means selecting one or the other.
• Negate
Usually negate means to withdraw a control signal or indicate its absence.
• Command Complete
The only message supported by all SCSI devices. Command Complete is
sent by the target to the initiator to indicate it has executed the command
(or a series of linked commands) and that a valid status has been sent to
the initiator.
Command Complete is neutral. It is up to the status byte to indicate
whether or not the command was completed successfully.
NOTE
144
For more information, see “Messages” on page 164.
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The nine control signals
Table 29 lists and describes the nine control signals. The industry-standard
nomenclature is in parenthesis.
Table 29. The nine control signals
Signal
Description
Control/Data (C/D)
The target uses this signal to indicate whether control or data information is being transferred.
If the signal is asserted (true or positive), command, status, or message information is being passed. A negated
signal (false) means it’s data.
Input/Output (I/O)
The target uses this signal to indicate the transfer direction.
Asserted (true) means it moves from initiator to target. Negated (false) means it moves from target to initiator.
Select (SEL)
The initiator asserts Select (and a SCSI Device ID on the DB line) to select a target to perform a command.
In systems that allow the target to disconnect while performing a command, the target also asserts Select to
reconnect to the initiator.
NOTE: A system that allows the target to disconnect while performing a command is called an arbitrating
system. For more information, see “Bus Phases” on page 169.
Busy (BSY)
“Wired-or” or “Or-tied” signal that indicates the bus is in use. A “wired-or” or “or-tied” signal means it takes
simultaneous signals from more than one device to be driven true or positive.
The initiator and the competing targets assert Busy during the Arbitration Phase. During the Selection Phase,
the selected target asserts Busy to acknowledge selection, while the targets that were not selected withdraw.
Message (MSG)
The target asserts Message to indicate that a message is being sent over DB lines.
Request (REQ)
The target asserts Request to initiate a data transfer.
If the bus is in the Information-In Phase (the I/O signal is negated, so information is going from the target to
the initiator), the initiator responds to requests by accepting data from the bus. If it’s Information Out (I/O
signal is asserted, so the flow is initiator to target), the initiator places data on the bus. The initiator then asserts
Acknowledge.
Acknowledge (ACK)
The flip side of the Request/Acknowledge coin.
The initiator asserts Acknowledge to let the target know it’s complied with the request. The Request/
Acknowledge handshake takes place after every information transfer.
For more information on handshakes, see “Data Transfer Options” on page 139.
Attention (ATN)
The initiator asserts Attention to let the target know it has a message available.
If the target supports optional messages, it can ask for the message by going into the Message-Out Phase.
Reset (RST)
“Wired-or” signal that clears the bus. RST creates a Bus Reset condition.
For more information, see “Bus Conditions” on page 158.
Reset may be used by any SCSI device on the bus. Usually, it is asserted only by the initiator, and typically only
during power-up; however, the initiator may use it to reset a device that is not responding.
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NOTE
See “Appendix C: The Deep End,” for the signals asserted during different bus phases.
SCSI ID
The SCSI identification (ID) number identifies the individual SCSI device, and
so is sometimes referred to as a SCSI Device address. It has two purposes:
• It allows SCSI devices to find one another along the bus. An ID allows one
device to distinguish itself and to call another device on the bus, identify
itself, and query the other device for its ID.
• The SCSI ID allows the devices to be ranked, or prioritized, so that the bus
can control which device gains access at a particular time. Those with
higher IDs are admitted before those with lower IDs. This is important
because only one pair of SCSI devices at a time—an initiator and a
target—can be in active communication.
Each SCSI device must get its own unique SCSI ID number, which refers to
the data signal line the SCSI device uses (Table 30).
Table 30. Data buses and related SCSI IDs
Data Bus
SCSI ID
DB0
SCSI ID = 0
DB1
SCSI ID = 1
DB2
SCSI ID = 2
DB3
SCSI ID = 3
DB4
SCSI ID = 4
DB5
SCSI ID = 5
DB6
SCSI ID = 6
DB7
SCSI ID = 7
You do not have to assign the ID numbers sequentially. For example, it’s fine
to have only three SCSI devices that are numbered ID6, ID3, and ID1. The IDs
are not related to the physical order in which the devices are arranged along
the bus. ID7 is usually reserved for the host computer.
!
146
If SCSI IDs conflict, neither device is likely to boot, and either could be damaged. SCSI IDs are
usually changed through software or by accessing a switch on the back of the device.
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DB7 carries the highest priority ID (important for arbitration) and is usually
reserved for the computer; DB6 is next, and so on. If the computer has an
internal hard disk, it is usually connected to DB0, with SCSI ID0.
STOP
The SCSI ID code is transmitted during the Selection and Reselection phases. See “Pin
assignments” on page 153 for more information on how this is implemented.
Apple SCSI peripherals are shipped with pre-assigned SCSI ID numbers. If you
don’t have more than one of the same type of Apple SCSI device on the bus,
there should be no need to change ID numbers. On other companies’ external
peripherals, for both PCs and the Macintosh, IDs are usually set by changing a
thumbwheel or pressing push buttons. Some of these devices may require a
utility program for setting IDs.
Physical connections
SCSI cables
Cabling is often overlooked when putting together a system, and that is a mistake. Sloppy or poor-quality cabling is a common source of many problems. In
fact, much of the work of SCSI-2 and SCSI-3 involves improving SCSI’s external cabling schemes, such as placing all data signal and parity lines on the outside of round cables and putting control signal lines at the center to minimize
the effects of signal noise from the data lines. Some premium quality cables
have used this scheme since the advent of SCSI-1.
NOTE
For more information on upcoming cable implementations, see “SCSI-2 highlights” on page 195.
Internal SCSI cables are usually 50-pin flat ribbon cables. They usually run
from the SCSI host adapter to each drive inside the computer.
For external SCSI cables, to ensure high quality, we recommend the use of
double-shielded, twisted-pair, 28 AWG wiring. REQ and ACK should be
together with GND in the middle. Control signals should be placed out side of
the REQ/ACK layer. The data and parity signals should be on the outside
layer.
NOTE
For more information on REQ, ACK and GND, see “Essential terminology” on page 139.
Devices are connected to the computer on the SCSI bus by daisy-chaining
cables: one SCSI device is plugged into the computer’s SCSI port, and then a
second device is plugged into the first device, and so on, up to a total of seven
devices, not including the computer.
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Cables typically have a characteristic impedance of 100 ohms. We recommend that you use cables with the same impedance, and ideally ones that
come from the same company. Cables from different companies or with dissimilar impedances may create signal noise and other irregularities, and place
an increased burden on proper termination. Cabling becomes more critical as
faster data transfer rates are achieved.
STOP
Impedance refers to a measure of the total opposition to current flow in an alternating current
(AC) circuit. For more information on termination, see “Termination” on page 153.
Because most SCSI devices have two 50-pin ports, standard SCSI cables
include at least one 50-pin connector.
Your system may include three types of SCSI cables:
• SCSI system cable
• peripheral interface cable
• cable extender
SCSI system cable
The SCSI system cable attaches the first SCSI device on the bus to the computer. Macintosh versions have a male 25-pin connector to connect to the
computer, and a 50-pin connector to plug into the SCSI device. This cable is
often referred to as 25-50 cable.
25-50
Figure 51. 25-50 Macintosh SCSI system cable
PC versions have a male 50-pin high-density connector to connect to the SCSI
host adapter, and a 50-pin connector to plug into the SCSI device.
50-50hd
Figure 52. 50-high-density 50 PC SCSI system cable
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Peripheral interface cable
The peripheral interface cable lets you daisy chain SCSI devices, one after the
other, up to a total of seven, not including the computer.
This cable typically has two (male-male) 50-pin connectors. It is referred to as
a 50-50 cable.
50-50
Figure 53. 50-50 peripheral interface cable
Peripheral interface cables are used to daisy chain external devices (Figure 54).
Daisy Chain
System cable
Peripheral interface cables
Figure 54. Daisy chaining external devices
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Cable extender
The cable extender functions as an extension cord. Cable extenders can be
attached to one another (male-female), to a system cable, and to a peripheral
interface cable.
Extender
Figure 55. Cable extender
Systems that support SCSI-2 Fast and Wide have a male, 68-pin, high-density
connector to connect to the SCSI host adapter, and a 68-pin connector to plug
into the wide SCSI device. Some Wide host adapters include a 68-50 cable to
attach narrow SCSI devices to a Wide SCSI bus.
Wide drive
68-pin connector
Figure 56. Internal fast/wide drive with a 68-pin connector
There is also a small length of a cable with connectors, called a stub, that connects internally from the SCSI port to the SCSI device housed inside its box.
There are limits to the length of the SCSI bus, beyond which the signals begin
to deteriorate. A realistic and safe guideline is 3 meters (about 10 feet).
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NOTE
See “SCSI-2 highlights” on page 195 for cabling limitations for fast and wide SCSI.
Electrical specifications
Termination
Single-Ended
(one signal wire)
Differential
(two signal wires)
Figure 57. Termination circuits: single-ended and differential
One of two electrical specifications may be used for the cabling that connects
SCSI devices:
• single-ended
• differential
These cabling schemes are not compatible and therefore cannot be mixed on
the same bus.
Single-ended is much more common industry-wide, because it is less expensive and is adequate for systems that perform at a low level. However, it relies
on tight termination tolerances. Going beyond its narrow margin of error can
cause problems. The Macintosh line uses single-ended exclusively, as do most
PCs. Third-party vendors provide differential support.
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Single-ended
In a single-ended configuration, signals on a cable are differentiated according
to the voltage of one wire (up to as high as 5 volts) relative to a common
ground (0 volts).
A signal line will be recognized as asserted (true) or negative (false) according
to the strength of its signal and whether it is active high or active low:
• Active high signals are asserted when the signal strength is above a
certain voltage, which is typically over 2.5 volts (plus or minus a five
percent margin of error).
• Active low signals are those that are asserted when the voltage falls below
a stated level, which is typically under 0.4 volts. Windows of error may
occur if the bus misinterprets a signal that is close to the cutoff. Proper
termination is crucial in controlling these glitches.
This specification is intended for cables up to six meters long (about 19 feet)
and for connecting internal SCSI devices. Apple ignores this convention by
using single-ended cable to connect everything, both internal and external.
Though outside the SCSI specification, this is not actually a violation of it.
Differential
A differential cable configuration determines signals by contrasting the voltage difference between two wires. It is mainly intended for connecting devices
externally. Differential has the advantage of allowing cable lengths up to 25
meters (about 80 feet). It is also more robust and less subject than single-ended
to signal noise and other termination problems.
However, differential cabling requires more powerful drivers, which in turn
require an additional chip. The device itself and the SCSI port host adapter
must be designed to use differential cabling. This boosts a device’s price. For
these reasons, systems that use a differential electrical specification are less
popular than those that use single-ended.
Connectors
The Macintosh line primarily uses a DB-25 external SCSI connector. (The
exception is the PowerBook, which uses a small, square 30-pin connector.)
Twenty-five pins means the cable has 25 lines for transferring information.
NOTE
152
For more information, see “Pin assignments” on page 153.
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Macintosh SE II and newer Macintosh models provide two SCSI connectors:
• The standard DB-25 external connector, located in the back of the
Macintosh (except PowerBooks)
• An internal, 50-pin, dual-row header connector for internal hard disks,
located inside the Macintosh
STOP
Some Macintosh models have two SCSI chips for dual-channel SCSI. These models include the
Quadra 900/950, PowerMac 8100, and PowerMac 7500/8500/9500. On the Quadra 900/950s, the
second internal SCSI connector is hidden under the power supply. On the others, the internal
SCSI connector is on a different channel than the external.
Early IBM PS/2 models used 60-pin high-density external SCSI cables. Most
PC and Macintosh SCSI host adapters use high-density 50- or 68-pin cables.
Most external SCSI devices have two 50-pin Centronics-style ports that
accommodate the SCSI standard connector described above.
STOP
Centronics is a printer manufacturer whose 50-pin connector has become an industry standard.
Instead of using projecting pins that seated in pin-holes, they designed a connector that mated flat
metal strips. See “SCSI-2 highlights” on page 195 for information on upcoming connector
implementations.
Pin assignments
Termination
Terminator
Figure 58. 68-pin SCSI terminator
Termination means closing off the ends of the circuit. You do this to accomplish two things:
• to absorb signals at each end and thus dampen reflections and signal
resonance
• to create enough difference between low and high signal levels for the
devices on the bus to differentiate between the signals and thus
communicate efficiently
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Devices chained together on a SCSI bus form an electronic signal path. Signals
tend to rebound off the end of the SCSI path and cause interference. Interference can cause problems with your computer and your SCSI devices.
Termination on a SCSI bus:
• preserves high transmission speeds
• cleans up the signal along the length of the bus
• provides a reasonable degree of immunity from electrical noise
Termination is required for the SCSI bus to work. A bad signal anywhere on
the chain can cause all of the SCSI devices, in some cases the computer itself,
to function improperly.
Types of Termination
Physically, terminators generally take three forms. Electrically, terminators
are the same, varying only in where and how they are installed. Table 31 lists
and describes types of termination.
Table 31. Types of termination
Terminator
Description
On-Device Terminators
These are known as resistor packs (or sips or dips). You only remove them to add a
second drive or to add a drive to a system where the motherboard is terminated. They
reside on the SCSI device itself and are almost always removable. Always note the
orientation of the resistor packs before attempting to remove them. They are
polarized and must not be inserted backwards. Some newer drives have resistors
permanently mounted on the drive. On these drives, there is a jumper to enable or
disable termination.
External Terminator Blocks or Plugs
These are short plug-like devices and are inserted between an external hard drive's
SCSI connector and the SCSI cable, or on the second connector if one exists. (See
Figure 58.) Don't confuse these with the on-device terminators. On-device literally
means on the drive mechanism—not on the external connector. Never use the
external terminator when the drive inside a cabinet has terminators installed on it.
Main logic board terminators
These may look like a SIMM or a narrow plug. They are used only when there is no
internal hard drive in the Macintosh. On the Macintosh, main logic board terminators
are inserted into the 50-pin SCSI connector on the main logic board where the cable
for an internal drive would normally connect. These main logic board terminators are
keyed (a polarity notch) and must never be inserted backwards.
The Quadra 800/900/950 have a terminator on the internal drive cable itself. This
means that internal SCSI devices should not be terminated.
Switched Termination
154
Some SCSI peripheral vendors also use switched termination. This means they have a
type of switch that controls whether or not the device is terminated. A switched
termination terminator is actually either an on-device terminator or an external
terminator block or plug, but is controlled electronically via a switch.
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How important is termination?
Terminating resistors, usually referred to as either terminators or resistors,
tell the computer where the SCSI bus begins and ends. They also play a role in
matching the impedance of the various cables on your system (especially
important if devices are daisy-chained). The significance of termination
increases as cable lengths and data transfer rates increase.
Proper termination is critical for ensuring that voltages, and thus signals, are
accurately conveyed on the bus. Improper termination can cause the following problems:
• erratic operation (such as power-up problems)
• signal noise (reflected or unwanted signals or voltages)
• general bus failure
Symptoms of improper termination include:
• hard disk drives that do not spin up immediately upon being powered-on
• drives that spin up but do not mount (do not appear on the Desktop or do
not display a drive letter)
• a system that won’t boot-up
• a system that shows a sad icon (Macintosh only)
• system error
• a system that crashes when copying a lot of data
• a “hang” during data transfer or information access
Termination do’s and don’ts
In most cases SCSI signals must be terminated at the physical start and endpoints of the bus. There should be exactly two terminators:
• one for the first physical device on the bus
• one for the last physical device on the bus
Devices considered to be on the bus include all internal and external devices.
Typically, the SCSI bus starts with the host computer.
Each additional terminator beyond the recommended two increases the signal
intensity and, apart from wreaking havoc with the signals, can cause damage
to sensitive components. Devices in the middle of the bus must have their
terminators removed. We recommend that you return the device to the vendor for this procedure unless the terminator is attached externally.
It is not always easy to tell which type of termination a device requires. Reading the device’s owner’s manual, contacting the vendor, or opening the chassis
and poking around inside for terminators are some ways of telling which type
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of terminator is needed (we do not endorse the latter approach). If a device
requires internal termination, we recommend that it be set by a qualified
technician. If the device requires external termination, most users are qualified to attach an external terminator to one of the external SCSI ports usually
found on the back of the device.
Terminators are available in both 50-pin versions for narrow SCSI and 68-pin
versions for Wide SCSI.
Termination power
Term power
Figure 59. Termination power circuit
A device on the SCSI bus requires terminating resistors and terminator power
for those resistors. Terminator power generally runs in the 4.25V to 5.25V
range, with currents in the 900 to 1500mA range.
In general, the SCSI device is responsible for providing its own terminator
power via a diode that prevents power from flowing back to the SCSI device. If
a device does not provide this power, it may not work properly.
Some of the devices that supply termination power (TPWR) to the SCSI bus
can cause problems if they are used on the internal bus of the Power Macintosh 8100/8150/9150. Any drives attached to this bus should be configured
with TPWR disabled, regardless of whether they are terminated or not.
The SCSI specification requires terminator power to be provided by the SCSI
connector, but the PowerBook family and the old Macintosh Plus depart from
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the ANSI guidelines here. The rest of the Macintosh line can provide terminator power to devices connected to its external SCSI port:
• If the Macintosh detects that an external SCSI device does not provide its
own terminator power, then the Macintosh will provide it.
• If the Macintosh detects that an external device does provide its own
terminator power, then the Macintosh won’t provide it.
When the Macintosh does provide terminator power to externally connected
devices, it draws that power from its own power supply. One possible disadvantage to this is that on a system with many attached devices, it could
weaken the effectiveness of the power supply.
Most Macintosh and PC SCSI host adapters provide terminator power to both
internal and external SCSI devices.
Termination examples
Table 32 provides examples of how to set termination for various system configurations.
Table 32. Examples of termination
System Configuration
Termination
Internal hard disk drive
It must be internally terminated. It is the first
physical device on the bus.
No internal hard drive
The internal SCSI port must be terminated.
One external device on the bus
The last drive must be terminated. It is assumed
to be the last device.
Two SCSI devices daisy-chained to one SCSI port
The device furthest from the host (in terms of
cable length) must be terminated.
Active Termination
As SCSI performance continued to improve, newer methods of termination
were needed to handle data integrity problems that arose with the faster SCSI2 buses. The main problem was double-clocking of REQ and ACK lines,
caused by coupling of parallel signal wires, especially when they were not
shielded.
Active termination attempts to address impedance mismatches by compensating for voltage. The SCSI-2 specification adds a voltage regulator to provide
about 24mA of current into each asserted SCSI line over a considerable range
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of terminator power values. The original SCSI-1 specification provided only
20mA when terminator power was at optimal value.
Forced Perfect Termination (FPT)
FPT was developed by IBM. It uses a resistor/diode network to pull current up
until the diode threshold is crossed. The diode then maintains the voltage at
the desired level. These terminators brought current levels into the 30 to
40mA range. FPT is used on the REQ, ACK, and SEL signals.
NOTE
For more information on REQ, ACK and SEL signals, see “Essential terminology” on page 139.
Active Negation
Active Negation adds drive circuitry to SCSI bus drivers that drive the line
high when it is negated. Driving the lines high ensures that REQ and ACK signals are clearly recognized. For maximum utility, all devices on the bus
should use this technology. Terminators are still needed to absorb the reflections at each end of the bus.
Bus Conditions
A bus condition affects all attached devices and cannot be ignored. It is a critical operation mode and thus overrides other lines. There are two bus conditions:
• Attention
• Reset
Attention condition
The Attention condition results when the initiator signals the target that it
has a message ready. The initiator asserts the Attention control signal during
any phase except Bus Free or Arbitration. The target retrieves the message at
its convenience using a Message-Out Phase. The initiator can assert Attention
only if it supports optional messages.
NOTE
For more information, see “Bus Phases” on page 169.
Reset condition
The Reset condition kicks every device off the bus. It is caused when a SCSI
device asserts the Reset control signal. A Reset condition may occur at any
time, forcing the bus into the Bus Free Phase. Targets may go through a com-
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plete power on self test (POST) sequence and clear out pending or current
operations.
Further, SCSI devices support either one of two reset options:
• Hard
• Soft
As their names imply, Hard is more severe, and Soft is more forgiving.
Hard Reset
The Hard Reset option:
• knocks out all commands in process
• drops all SCSI device reservations (for disconnects and command queuing)
• returns all operating modes to their default conditions
This condition is asserted on the Reset signal line and goes out to all devices.
Soft Reset
In contrast, the Soft Reset option will:
• try to complete commands that were in process
• hold on to all SCSI device reservations
• maintain the SCSI device operating mode
The Soft Reset option has the advantage of allowing the initiator to reset the
bus without affecting other initiators in a multiple-host system.
This condition is communicated via the Bus Device Reset message and is
directed toward a specific device. In this way, it is distinguished from the
other bus conditions.
Executing Commands
Command Descriptor Block
The Command Descriptor Block (CDB) is a six-, ten-, or twelve-byte data
structure residing in the computer’s memory that contains the command code
and other information needed by a target to execute a command.
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A target executes a command in three stages:
1. It gets and decodes command information from the CDB.
2. It transfers or receives data, if required.
3. It sends status information.
NOTE
See also “Bus Phases” on page 169 and “SCSI Command Descriptor Block” on page 295.
Each command is sent from the computer to a device as a CDB. There are several groups of CDBs. Each member in a group has similar offsets (locations in
the CDB where certain parameters can always be found). Group 0 and Group 1
are the most prevalent:
• Group 0 CDBs contain Group 0 commands and are six bytes long.
• Group 1 CDBs contain Group 1 commands and are ten bytes long.
Group 1 commands are more frequently used for larger capacity peripherals
(more than one GB), which have greater addressing needs. There are also
Group 2 commands, which, like Group 1, are ten-byte commands.
Some commands require more information than can be conveyed in even a
ten-byte CDB. For these, the CDB includes a “count byte” that tells the SCSI
hard drive how many bytes it should request from the computer in the data
phase, after it gets the CDB. SCSI-2 provides for a Group 5, 12-byte CDB that
sends more information.
NOTE
See “SCSI Command Descriptor Block” on page 295 for tables that illustrate the content of
different groups of CDBs.
Pointers
Pointers reside in the CDB and keep track of command, status, and data transfers. The target places pointers in each byte of the CDB to track the execution
of command information. There are two types of pointers:
• Current (or active)
• Saved
The target saves the pointer values so it can return to a byte if necessary.
There is only one set of current pointers per initiator, but there may be multiple sets of saved pointers—a total of up to seven sets.
Although SCSI is otherwise optimized for I/O, this command overhead—having to build and process SCSI CDBs—slows throughput because it is performed on every SCSI transaction. Minimizing the number of CDBs will
speed I/O.
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Commands
Table 33 lists and describes general device commands. To understand the
descriptions, you should first understand the following terms:
• Sense Data
Sense Data informs on errors resulting from the previous command from
the same initiator. It is saved until retrieved by that initiator or until
another command is received from that initiator. There is an Extended
Sense format, which provides more detailed information, and a Nonextended Sense format, which supports devices not supported by
Extended Sense.
• Sense Key
The Sense Key bit is contained in the Sense Data. It is an error code
indicating a particular classification of error.
Commands marked with a plus sign (+) are accomplished without disk access.
A command followed by (NON) is not part of the Common Command Set
(support for which is required by SCSI-2).
NOTE
See “SCSI-1 commands and opcodes” on page 298 for the operations codes and sense key
descriptions used with these commands.
Table 33. General device commands
Chapter 5: All About SCSI
Command
Description
Test Unit Ready
Checks if the drive is prepared to accept commands requiring disk access. If it
is, the completion status byte indicates Good and the sense key is set to No
Sense. If it is not ready, the status byte will indicate Check Condition, with the
sense key set to Not Ready.
Rezero Unit
Requests that the drive actuator be moved to the cylinder zero and head zero
position.
Request Sense +
Requests that sense data be sent to the initiator. Sense data is detailed error
information generated by the previous command from the same initiator. It is
saved until it is retrieved by that initiator or until another command is received
from that initiator. There is an Extended Sense format and a Non-extended
Sense format for devices that do not support Extended Sense. The Sense Key
field helps determine why the error occurred.
Format Unit
Assigns logical blocks to physical sectors, optimizing sequential access and
avoiding areas known or found to be defective. Formatting options vary with
different drives. Loss of data typically occurs when this command executes. We
recommend backing up data first.
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Table 33. General device commands (Continued)
Command
Description
Reassign Blocks
Requests the drive to reassign defective logical block(s) to areas of the platter
reserved for this purpose. The drive keeps and updates a known defect list.
Defective logical blocks that have already been reassigned by the automatic
reallocation feature of Mode Select will be reassigned again.
Read
Requests that the target transfer data to the initiator.
Write
Requests that the data transferred to the target be written to the medium. It
specifies the logical block address at which to begin, and the number of
contiguous logical blocks of data to be transferred.
Seek
Requests that the target position its head actuator at a specified logical block
address.
Inquiry +
Requests that target identification information—manufacturer, model, and
parameter specifications—be sent to the initiator. This includes the device’s
model number and various revision levels, as well as whether it contains
removable media and supports the Common Command Set or SCSI-2.
Mode Select +
No disk access required if the Save Parameters bit is not set. Allows the initiator
to specify the operating parameters of the drive, and optionally to save them
for future commands.
A Mode Select command overrides previous parameters. Executing Mode Select
in a multi-initiator system creates a Unit Attention condition for all other
initiators.
Reserve
Requests that a logical unit be reserved for an initiator or some other specified
SCSI device.
The initiator will have exclusive use of the drive until one of the following
conditions exists: the same initiator issues another valid Reservation command
superseding the last command; a Reset message from any initiator is received;
or a hard reset occurs.
No error will occur if an initiator attempts to reserve a logical unit it has
already reserved. Drives already reserved will return a Reservation Conflict
status; however, if disconnection is supported, the drive will queue the Reserve
request and disconnect. If some other initiator issues a Release command, the
reserved drive will ignore it.
Release
Requests that a drive’s reservation be canceled.
The Release message is only valid when issued by the initiator that reserved
the drive. No error will occur if an initiator attempts to release a logical unit
that is not reserved.
Copy
162
Provides the means to copy data from one logical unit to another or the same
logical unit. The logical unit may reside on the same or different SCSI device.
This command requires that the device support disconnect.
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Table 33. General device commands (Continued)
NOTE
Command
Description
Mode Sense +
No disk access is required if Save Parameters are not read. Enables the target
to report its operating parameters to the initiator in either its current, saved,
changeable, or default categories. This command complements Mode Select.
Start/Stop Unit
Requests that the drive be spun up or spun down. Can cause removable media
to be ejected.
Receive Diagnostic Result + (NON)
Requests that the results of the self-test be sent to the initiator.
Send Diagnostic +
Requests that the drive to perform diagnostic tests on itself. This is usually
followed by a Receive Diagnostic Result command.
Prevent/Allow Media Removal +
Enables or disables the ejection of removable media.
Read Capacity (NON)
Requests the target send information regarding the capacity and block size of
the drive.
Read Extended (NON)
Same as Read command, but can access more blocks.
Write Extended (NON)
Same as Write command, but can access more blocks.
Seek Extended (NON)
Same as Seek command, but can access more blocks.
Write and Verify
Requests that the drive write the data it has received from the initiator, and to
verify that it has been correctly written to the medium. Useful for optical
drives.
Verify (NON)
Requests the drive to verify that it has correctly written data on the medium.
Read Defect Data (NON)
Requests that the drive transfer data to the initiator regarding defects on the
medium.
Write Buffer (NON)
A diagnostic command for testing the target’s memory and the SCSI bus
integrity. It is used in conjunction with the Read Buffer command. Neither
command affects data storage.
Read Buffer
See Write Buffer command.
Read Long (NON)
Requests that the drive transfer a sector of data, followed by error-correcting
code (ECC) data, to the initiator. This command is intended for diagnostic
purposes and is usually followed by the Write Long command.
Write Long (NON)
Requests that the data usually transferred by the Read Long command be
written to the specified logical block address. The number and order of bytes
should match those in the Read Long command.
The SCSI-2 specification also introduces new commands. See “SCSI-2 commands” on page 195
for more information.
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Mode Pages
A device’s behavior, or “mode,” is determined by its operating parameters.
These mode parameters are grouped into related pages that are numbered for
easy reference. A mode page is the minimum unit that can be specified by the
Mode Select or Mode Sense command.
NOTE
For more information, see “Commands” on page 161 and “SCSI-2 mode pages” on page 314.
A page consists of multiple parameters that must be specified whenever a
page is invoked: Changeable parameters may be set to any acceptable value.
Nonchangeable parameters must be set to zero or left unchanged.
Although optional, mode pages are of interest to users who wish to customize
the operation of their drive.
The Mode Parameter List is sent by the initiator to the drive during the Data
Out Phase. This list contains the Header, zero or more Block Descriptors, and
the mode pages themselves.
The Header specifies the type of storage medium, if any, being used in the
drive and the length in bytes of all Block Descriptors. When the medium type
is set to zero, which is the default, it indicates that the medium is nonremovable.
The Block Descriptor specifies the logical block length for the drive including
the density code, which is the density of the medium—usually used only for
tape drives, the number of logical blocks on the medium in the density code,
and the block length, which is the length in bytes of each logical block
described by the block descriptor.
The rest of the data consists of one or more mode pages.
Messages
SCSI messages are communications between the initiator and target for the
purpose of interface management. A message can be sent in both directions:
message-out from initiator to target, or message-in from target to initiator.
There are three types of message formats:
• single-byte
• two-byte
• extended (three-byte or longer)
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The messages in Table 34 are defined by the SCSI specification. With the
exception of Command Complete, support for these messages in SCSI-1 is
optional. The SCSI-2 specification mandates support for a few of them and
provides additional content to the definitions of others.
NOTE
“SCSI-1 messages” on page 304 contains the operations codes for all messages listed in Table 34
and lists whether the message moves from initiator to target (out) or target to initiator (in). For
more information on changes introduced in SCSI-2, see “SCSI-2 highlights” on page 195.
Table 34. SCSI messages
Message
Description
Command Complete
Sent by the target to the initiator to indicate:
• It has executed the command (or a series of linked commands)
• A valid status has been sent to the initiator.
Command Complete is neutral. It is up to the status byte to indicate whether or not the
command was completed successfully. All SCSI devices must support the Command Complete
message.
A Command Complete or Disconnect message must precede a target’s releasing the Busy
signal. If the initiator detects that this has happened—as indicated by the resulting Bus Free
Phase—it should consider that an error has occurred.
For more information, see “Control signals” on page 144 and “Bus Phases” on page 169.
Extended Messages
Messages that require more than two bytes to send the necessary information. The messages
supported by the disk drive are:
• Modify Data Pointers (In)
Requests adding signed argument to the value of the current data pointer.
• Synchronous Data Transfer (I/O)
Sent to establish synchronous data transfer and its parameters, including:
– The transfer period, which determines transfer rate.
– The Request/Acknowledge offset, which sets the maximum number of Request
signals that may be sent before a corresponding Acknowledge signal is received.
Zero means the devices are using the asynchronous mode. Other than zero
indicates a synchronous transfer mode has been agreed upon.
Macintoshes previous to the PCI versions don’t support synchronous data transfers
although virtually all Macintosh and PC SCSI host adapters do.
• Wide Data Transfer (I/O)
Establishes an agreement between two SCSI devices on the width of the data path to be
used for Data Phase transfers between two devices.
Initiated whenever it is appropriate to negotiate a new transfer width agreement, or
whenever a previously agreed transfer width agreement may have become invalid. For
example, after:
– A Hard Reset condition
– A Bus Device Reset message
– A power cycle
For more information, see “Data Transfer Options” on page 139.
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Table 34. SCSI messages (Continued)
Message
Save Data Pointer
Description
Sent from the target to tell the initiator to save the present active data pointer for that target.
For more information, see “Executing Commands” on page 159.
Restore Pointers
Directs the initiator to restore saved pointers to the active state.
Disconnect
Sent by the target to the initiator in Arbitrating systems to indicate that it is going to release
the Busy signal and thus disconnect, and that it will be necessary to reconnect later to complete
the operation.
A Disconnect or Command Complete message must precede a target’s releasing the Busy
signal. If the initiator detects that this has not happened—as indicated by the resulting Bus
Free Phase—it should consider that an error has occurred.
Initiator Detected Error
Sent by the initiator if it detects an error. The initiator will retry the operation. The target will
respond with a Check Condition status, which terminates the operation.
Abort
Clears the operation. All pending data and status for the initiator issuing the Abort message are
also cleared, and the initiator and target involved enter the Bus Free Phase. No status or
ending message will be sent for the operation. In multi-initiator systems, the pending data and
status for other initiators will be left intact.
Message Reject
Sent by either the initiator or target to indicate that the last message was either deemed
inappropriate or was for some other reason not implemented.
For the initiator to send this message:
• It asserts the Attention signal.
• Then it sends its Acknowledge for the Request/Acknowledge handshake of the message
that it is rejecting.
For the target to send this message:
• It goes to the Message In Phase and sends it.
• Then it requests additional message bytes from the initiator.
It does this so the initiator knows which message is being rejected.
For more information, see “Bus Phases” on page 169.
No Operation
The initiator’s response to a target’s request for a message, if the initiator has no message to
send.
Message Parity Error
What the initiator tells the target if one or more bytes in the last message received had a parity
error.
For the initiator to send this message:
• It asserts the Attention signal.
• Then it sends its Acknowledge for the Request/Acknowledge handshake of the message
that it is rejecting.
It does this so the target knows which message has the parity error.
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Table 34. SCSI messages (Continued)
Message
Description
Linked Command
Complete
Sent by the target to tell the initiator that a linked command has been executed and a status
byte has been sent. The initiator sets its pointers to the initial state for the next linked
command.
For more information, see “Executing Commands” on page 159.
Linked Command
Complete (With Flag)
Sent by the target to tell the initiator that a linked command (with the flag bit set) has been
executed and a status byte has been sent. The initiator sets its pointers to the initial state for
the next linked command.
Abort Tag
When one or more initiators have multiple I/O processes to be queued by a target, each I/O
process must have its own queue tag. In such circumstances, the Abort Tag message is active
(usable). Otherwise, it is unavailable.
• The initiator sends the Abort Tag message to the target and, following successful receipt,
creates a Bus Free phase.
• The target clears out the identified I/O process whether it has started or is waiting.
• All other pending status, data and commands for other queued or executing processes are
not affected.
Clear Queue
When one or more initiators have multiple I/O processes to be queued by a target, each I/O
process must have its own queue tag. In such circumstances, the Clear Queue message is active
(usable). Otherwise, it may or may not be usable.
• The initiator sends the Clear Queue message to the target and, following successful
receipt, creates a Bus Free phase.
• All I/O processes from all initiators in the queue for the specified logical unit are cleared
from the queue.
• All similarly identified executing processes are stopped.
• All pending status and data for that logical unit for all initiators are cleared.
• No status or ending message is sent.
Terminate I/O Process
Terminates current I/O processes without corrupting the medium (e.g., hard disk).
Simple Queue Tag
Specifies that the I/O process be placed in the disk drive’s I/O process queue for execution. The
order of execution can be arranged by the disk drive in accordance with an algorithm. This
message is also sent by the target when it reconnects to the initiator.
Head of Queue Tag
Specifies that the I/O process be placed first in the identified logical unit’s queue for execution.
In-progress I/O is not preempted. A subsequent I/O process received with this message goes to
the head of the queue for execution in last-in, first-out (LIFO) order.
Ordered Queue Tag
Specifies that the I/O process be placed in the disk drive’s I/O process queue for execution in
the order received, with respect to other commands with this message.
NOTE: Processes with Head of Queue Tag messages jump ahead of processes with Ordered
Queue Tag messages.
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Table 34. SCSI messages (Continued)
Message
Description
Ignore Wide Residue
Used with wide (16-bit) data transfer. Indicates that the number of valid bytes sent during the
last REQ/ACK handshake of a Data-In phase is less than the negotiated transfer width. The
“ignore” field indicates the number of invalid data bytes transferred.
Identify
Sent by either the initiator or target to establish the physical path connection between the two.
The Identify bits are set as follows:
• Bit 7 is always set to one to distinguish Identify from other messages.
• Bit 6 may be set only by the initiator, and indicates whether or not it supports
disconnection and reconnection. It also indicates whether Save Data Pointer, Restore
Pointer, and Disconnect messages are supported.
• Bits 2 to 0 specify a logical unit number in a target, and must be set to zero.
Identify is required in SCSI-2.
Bus Device Reset
Issued by the initiator to clear all current commands and pending operations. It forces a Bus
Free Phase for all devices on the SCSI bus, but does not affect Mode Select command
parameters. This is message initiates a Soft Reset.
For more information, see “Mode Pages” on page 164.
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Bus Phases
Communication across the SCSI bus is divided into stages known as phases.
There are eight SCSI bus phases:
•
•
•
•
•
•
•
•
NOTE
Bus Free
Arbitration
Selection
Reselection
Command
Data-In/Out
Status
Message-In/Out
See Table 35 on page 171 for a description of each bus phase.
Phases are driven by control signals and preceding phases. The SCSI bus incorporates numerous time delays to ensure all the signals are at the right voltage
level before changing phases. However, there is a maximum time delay after
which the target will “time-out” and release the bus if signals don’t arrive in
time. (The bus won’t wait forever.)
Figure 60 illustrates a bus phase sequence for a system that supports arbitration, where devices on the SCSI bus negotiate for bus access. Arbitration is
essential to multiple-host systems and is required in SCSI-2.
Bus phases 1
Figure 60. Phase sequence with arbitration
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Figure 61 illustrates a bus phase sequence on a system that does not support
arbitration.
Bus phases 2
Figure 61. Phase sequence without arbitration
The first four bus phases, Bus Free, Arbitration, Selection, and Reselection,
are controlled by the Select, Busy, and Input/Output control signals. The
sequence of these phases is limited:
• The Arbitration Phase can only be entered from the Bus Free Phase.
• The Selection or Reselection Phases can only be entered from the
Arbitration Phase.
• The Bus Free Phase, however, can be entered from any other phase.
NOTE
For more information on control signals, see “Control signals” on page 144.
The next four bus phases, Command, Data In/Out, Status, and Message In/
Out, are called the Information Transfer phases. Unlike the first four phases,
there are no restrictions on the sequence of occurrence in these phases. The
Information Transfer phases are controlled by the Message, Control/Data, and
Input/output control signals. The target drives these three signals and controls the changes between Information Transfer phases.
STOP
170
The initiator’s only way to disrupt the target’s control over the Information Transfer phases is to
create a bus condition. For more information, see “Bus Conditions” on page 158. See “SCSI
control signals and Information Transfer phases” on page 308 for a table describing the control
signals asserted during the Information Transfer Phase.
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Table 35 lists the eight bus phases and describes their related activities.
Table 35. The eight SCSI bus phases
Bus Phase
Description
Bus Free
The bus is empty; no one is riding. This is SCSI’s starting point at power-up, after a Reset, and where it
returns when a command is completed.
Both the Busy and Select signals must be negated for a defined period of time before this phase is
entered.
Arbitration
This phase is where SCSI devices, including the computer, can vie for bus access.
1. Competing devices assert their SCSI ID number and the Busy signal on the appropriate DB line.
2. The computer waits a defined period before examining the bus.
3. The device with the highest priority SCSI ID “wins” the arbitration, gains control of the bus, and
asserts the Select signal.
Arbitration is necessary for multi-host systems, and is required in SCSI-2. Following Arbitration, the
system goes to either the Selection or Reselection Phase. Non-Arbitrating systems skip Arbitration and
go directly to Selection or Reselection.
Selection
In this phase an initiator selects a target device. Once selected, the target determines:
• What information will be sent down the bus
• When it will be sent
• Whether the information is going to, or leaving, the host
(Also see the Information Transfer Phases described below.)
An initiator, usually the computer, selects a device by:
1. Asserting the device’s ID number on the appropriate DB line.
2. Negating the Busy and I/O signals.
The target:
1. Detects its asserted SCSI ID, and the negated Busy and I/O signals (I/O is negated to distinguish
this phase from Reselection. A positive I/O indicates communication flow is from the target to the
initiator.)
2. Asserts a Busy signal, causing the initiator to get off the bus.
3. Negates the Select signal.
Reselection
In Arbitrating systems, a target that has disconnected from the bus (in order to perform a timeconsuming task) gets reconnected in this phase.
A target device asserts the Reselection signal and its own ID number. It gets reconnected if the bus is
free or its SCSI ID has a priority higher than another device that is asserting the Select signal.
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Table 35. The eight SCSI bus phases (Continued)
Bus Phase
Description
Command
In this phase the target requests command information from the initiator.
1. The target device first asserts the Request signal.
2. The host, usually the Macintosh, goes to the Command Data Block (CDB) indicated by the
Command Pointer, gets the information byte, sends it along the bus, and asserts the
Acknowledge signal.
3. The target picks the command information off the bus and negates its Request signal.
4. When the host detects that Request has been negated, it gets off the bus and increments the
Command Pointer in the CDB.
This process is repeated until all bytes of information in the CDB requested by the target have been
transferred.
The target interprets the command code and locates the requested data block on the specified logical
unit number. It then enters the Data-In Phase.
Data-In/Out
The Data-In Phase allows the target to ask for data to be sent from the target to the initiator. In the
Data-Out Phase, the target asks to receive data from the initiator. Data is transferred in either the
asynchronous or synchronous data transfer mode.
In the Data-In Phase:
1. Target puts the first byte of data on the bus and asserts the Request signal.
2. Host takes the data byte, puts it in memory, and asserts the Acknowledge signal.
3. Target then negates the Request signal and puts the second byte on the bus.
4. Host increments the data pointer in the CDB.
5. Target then asserts Request and repeats the process until all bytes in the logical data block have
been transferred to the host’s memory.
Status
Here the target sends a byte of information to the initiator that indicates the success or failure of a
command (or a series of linked commands).
The Status Phase is critical for error detection and defect management. The status bytes and their
meanings are listed below:
•
•
•
•
•
00
02
04
08
10
Good
Check Condition
Condition Met
Busy
Intermediate
•
•
•
•
14
18
22
28
Intermediate-Condition Met
Reservation Conflict
Command Terminated
Queue Full
A single status byte always follows the Command Complete message. This status byte is an error code
that describes any errors that may have occurred. A Check Condition status indicates that an error has
occurred. The status byte is cleared if the command is completed successfully. If not successful (if an
error occurred), the next command can request sense data to figure out what the problem was.
1. The target enters the Status Phase by putting its Status information on the bus and asserting the
Request signal.
2. The host accepts the status and puts it where the Status Pointer indicates. Then it increments the
Status Pointer in the CDB and asserts the Acknowledge signal.
3. The target then negates the Request signal.
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Table 35. The eight SCSI bus phases (Continued)
Bus Phase
Message-In/Out
Description
In the Message-In Phase, the target may request to send a message to the initiator.
The Message-Out Phase is invoked by the target only in response to an Attention condition generated
by the initiator. It can be generated at any time.
For more information, see “Bus Conditions” on page 158.
Since not all SCSI devices support messages, this is optional. If the Identify message is not sent before
the Command Phase is entered, it is assumed that messages are not supported.
A Command Complete message is always sent at the conclusion of every SCSI command operation.
The target places the Command Complete message on the bus and asserts the Request signal. The
host accepts it and asserts the Acknowledge signal. The target then negates the Request signal, gets
off the bus, negates its Busy signal, and the phase returns to Bus Free.
SCSI, Apple-Style
Apple has included SCSI controllers in all Macintosh CPUs beginning with
the Macintosh Plus in 1986. Despite this long history, some of Apple’s SCSI
implementations—as contained in the SCSI Manager and SCSI chip—have
caused problems. The following subsections describe some of these.
Fast reset
After a SCSI bus reset, devices have only 250 milliseconds to get data on the
bus. Certain devices cannot meet this requirement and are locked out of the
bus following a reset, which causes problems with mounting volumes on
these drives.
Unit attention condition
A SCSI device enters the Unit Attention Condition whenever it or the bus has
been hit by one of the following:
• a hard reset
• a power-up reset
• a reset generated by a Bus Device Reset message
NOTE
For more information, on resets see “Bus Conditions” on page 158.
It may also occur in certain other situations that cause a shock to the device.
This feature is mandatory in SCSI-2.
A device in Unit Attention Condition wants to let a host know that it has
been traumatized and should be queried regarding its status. If the next comChapter 5: All About SCSI
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mand the target receives is either Inquiry or Request Sense, everything is fine;
the device sends a status message reflecting Unit Attention Condition and
then exits that condition. If the command is anything else, however, the target completely rejects it and sends a Check Condition status code; saying, in
effect, “Check me out before doing anything else.” The target tries this tack
only once.
If the initiator ignores the Unit Attention Condition and the target’s request—
which it can do—it will reissue the command and this time the target will
comply (so much for the target’s “intelligence”).
The Unit Attention Condition feature causes power-up problems when used
with Macintosh Plus ROMs:
STOP
ROM stands for read-only memory. It is permanently stored data in memory. It usually contains
the permanent instructions for a computer’s general housekeeping operations.
1. At power-up time, the ROM code tries to read bootstrap information
from each SCSI device on the bus. It only tries once, going on to the next
device if refused.
STOP
Bootstrap information is a sequence of instructions whose execution causes additional
instructions to be loaded and executed until a complete computer program is loaded into storage.
2. Because power-up causes the device to go into the Unit Attention Condition, the device refuses the command, asking instead that its condition be checked before responding to any command.
3. However, the Macintosh moves on to the next device.
4. The ROM code resets the bus if it can’t read from even one SCSI device,
and tries them all again.
5. Resetting the bus puts all SCSI peripherals into the Unit Attention Condition again.
6. The loop could go on forever, and the devices would not boot up.
This was fixed in the “Platinum” colored Macintosh Plus ROMs, so that only
one SCSI Reset would be sent.
Unit attention also caused problems on early Macintoshes prior to the IIci
when users tried to set a drive as the startup drive.
Some manufacturers have produced special Macintosh-specific versions of
their drives that don’t implement the Unit Attention Condition. Unit attention may also be disabled via jumpers or firmware changes, or by adjusting a
mode parameter in Mode Page 0. Other companies resorted to cutting the RST
(reset) line on the SCSI drive cable so it would never get a hard reset.
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Parity checking
Parity checking caused problems with early Macintoshes prior to SCSI
Manager 4.3. Parity typically had to be disabled on drives to enable compatibility.
STOP
The Macintosh Centris 660AV and Quadra 840AV were the first machines sold with SCSI
Manager 4.3.
Blind data transfers
In the Macintosh Plus, the 68000 microprocessor couldn’t properly handle the
SCSI Req/Ack handshaking protocol because it had no physical hardware line
for relaying handshaking or interrupts. The 68000 did not know if a drive was
responding during handshaking. Drivers had to either handshake every byte, a
very slow process called polled I/O, or had to blindly hope that data was ready.
NOTE
For more information on handshakes, see “Data Transfer Options” on page 139.
Apple’s answer was to include hardware support on the Macintosh SE and
above for the hardware handshake line. Many Macintosh models still did not
support SCSI Direct Memory Access (DMA), so if data did not show up consistently with no more than 16-microseconds between bytes, the SCSI bus
would generate a bus-error and cause the Macintosh to crash or transfer
invalid data.
Some devices, such as early Conner Peripheral hard drives, had hiccups during
data transfers when the transfers were larger than the track buffer on the
drive. Workarounds were developed that limited data transfers to smaller
sizes or had the driver define synchronization points, using transfer information blocks (TIB) when calling the SCSI Manager.
ANSI irregularities
The Macintosh SCSI port also differs from the ANSI specification in two
ways:
• The Macintosh line uses a DB-25 connector instead of the standard 50-pin
connector. An Apple SCSI System 25/50 adapter cable is available to
convert the DB-25 to the standard DB-50 connector.
• The termination and termination power are nonstandard.
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Apple exceptions to the SCSI specification
The Mac Plus does not include internal termination because it has no internal
SCSI connector. According to Apple, the devices closest to and farthest away
from the Macintosh Plus should be terminated. But if there is only one peripheral attached to the Plus, only that device needs a terminator.
The Macintosh IIfx is in a class by itself. One of its features is a SCSI chip that
provides faster data transfer rates than possible with earlier Macs. This necessitates the use of a combination of the following three termination parts.
• Apple SCSI Cable Terminator II. This is a black external terminator that
ships with the Macintosh IIfx. It includes a “glitch-eating” capacitor, as
does the filter. (See explanation below.) Only one is used on the SCSI bus.
AND either
• Internal SCSI Termination Block. This provides the internal termination
for IIfx machines without internal hard drives, and is installed by Apple
when shipped.
OR
• Internal SCSI Filter. This provides filtration of SCSI lines for Macintosh
IIfx internal drives on systems that shipped prior to March 19, 1990. It
was installed by Apple when shipped.
Macintosh IIfx termination configuration is simple. There is one terminator
installed internally in the Macintosh itself, and another externally at the end
of the SCSI chain.
The reason these parts are required is that the IIfx’s SCSI chip thinks that
glitches on the Request line are genuine signals. The internal SCSI filter is
actually a capacitor that may be thought of as a glitch-eater. The glitches
occur when a majority of data lines change their state simultaneously, which
drains the terminator power line (TPWR line) and consequently causes a
power spike on the Request line. The solution is to have the internal SCSI filter provide the TPWR line with a little extra current when needed.
STOP
A notice in the Macintosh IIfx finished goods box instructs customers to return self-terminated
SCSI devices to the service provider to have the termination disabled.
Newer Macintosh computers, starting with the Centris 660AV/Quadra 840 AV
and continuing through the Power Macintosh line, include automatic termination when no internal device is connected. When this occurs, special circuitry terminates the bus on the logic board near the external connector.
Previous Quadras and Macintosh II series computers required a motherboard
in-line terminator to be connected when a system had external drives but no
internal ones.
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The PowerBook 100 and 200 series have unusual termination requirements:
• When you attach an external drive to one of these PowerBooks, you must
connect it to the PowerBook with a cable that has a built-in terminator,
and plug an external terminator into the second SCSI port on the external
drive.
• If you daisy-chain external devices to one of these PowerBooks, you must
use a cable with a built-in terminator and terminate the last device on the
chain.
• If you use one of these PowerBooks in SCSI disk mode—where it is used
as an external drive attached to a host computer—there must be a terminator connecting the cable leading from the PowerBook to the cable leading from the host.
NOTE
See the documentation that came with you PowerBook for specifics on termination for these
computers.
PowerBook SCSI
PowerBook laptops continue Apple’s tradition of using SCSI. They did utilize
a new proprietary, high-density, 30-pin cable (HDI-30) to save space. PowerBooks starting with the PowerBook150 and PowerBook5300 started to use
IDE drives internally but continued to have SCSI ports externally.
STOP
IDE stands for Integrated Drive Electronics. It is a control system for storage devices where most
of the control is contained on the device itself.
Some PowerBook computers support SCSI disk mode, which allows the internal hard disk to be mounted and used as an external drive by another Macintosh. To operate a PowerBook computer in SCSI disk mode, the user connects
a special HDI-30 SCSI Disk Adapter cable between the external SCSI connectors on the PowerBook computer and the desktop computer and then starts
both computers. The adapter cable grounds pin-1 of the HDI-30 connector,
causing the PowerBook computer’s ROM code to bypass the normal startup
procedure and enter SCSI disk mode.
STOP
PowerBook SCSI disk mode makes even IDE drives appear as SCSI drives.
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SCSI Manager
SCSI Mgr.
Figure 62. SCSI Manager software hierarchy
The SCSI Manager provides the interface between the SCSI software (operating system drivers or application programs) and hardware (chip). The SCSI
Manager offers a well-defined set of calls that allows the software drivers or
applications for different SCSI devices to use any Macintosh SCSI port
smoothly, without worrying about the underlying hardware.
The original SCSI Manager code includes commands for all types of SCSI
operations. Table 36 lists and describes the original code.
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Table 36. Original SCSI Manager code
NOTE
Code
Description
SCSIGet
Arbitrates for control of the SCSI bus.
SCSISelect
Selects the target device.
SCSICmd
Sends the command in the Command Phase that tells the target what
operation to perform. It indicates whether the command is six or ten bytes
long (Group 0, 1, 2 and 5).
SCSIRead
Read data from device.
SCSIRBlind
Read data without handshaking every byte.
SCSIWrite
Write data to device.
SCSIWBlind
Write data without handshaking every byte.
SCSIComplete
Tells the Macintosh how long it should wait for the target to complete the
command, and returns the status and message bytes.
SCSIReset
Resets the bus.
SCSIStat
Gets bus status information.
SCSIMsgIn
Initiator receives a message from target.
SCSIMsgOut
Initiator sends message to target.
SCSISelAtn
Selects a target device and sets ATN line active.
SCSI commands are more fully described in “Commands” on page 161.
SCSI Manager 4.3, introduced with the Quadra 840AV in 1993, increased the
power of SCSI on the Macintosh by allowing it to support most of the SCSI-2
specification, including:
•
•
•
•
•
disconnect/reconnect
parity detection
synchronous transfers
multiple buses and LUNs
asynchronous operation
The new interface was much more modern. There was no need to drive all
SCSI bus phases. It also enabled full support of third-party expansion cards
and DMA, while maintaining backward compatibility with software. However, running old drivers under SCSI Manager 4.3 imposed a 20 percent perforChapter 5: All About SCSI
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mance penalty. SCSI Manager 4.3 also added more overhead on small data
transfers, causing a slight slowdown compared to the earlier SCSI Manager.
SCSI Manager 4.3 is also supported on all Macintosh Quadras running
System 7.5 or newer. (It is loaded from a System Extension.) It is present on all
PowerMacs.
SCSI Manager 4.3 is based on ANSI X3.232 Common Access Method (CAM)
but adds some Apple-specific improvements. CAM documents a layered
architecture with a transport layer on top (XPT). The single XPT vector calls
to one or more SCSI interface module (SIM) layers. Each SIM controls a single
host bus adapter (HBA).
Parameter blocks
A common SCSI Manager 4.3 data structure is the parameter block. A parameter block is the basic unit of data that is exchanged between a program and
the SCSI Manager. Each client of the SCSI Manager allocates a SCSI parameter
block and fills in the required fields before passing it to the SCSI Action function. A function-specific SCSI parameter block consists of two parts:
• The header (SCSIPBHdr), contained in all SCSI parameter blocks.
• The body, which contains information specific to the active function.
Figure 63 illustrates a typical SCSI parameter block header.
#define SCSIPBHdr \
struct SCSIHdr
*qLink;
//
(internal) Q link to next PB
short
qType;
//
(unused) Q type
ushort
scVer;
//
-> version of the PB
ushort
scPBLen;
//
-> length of the entire PB
FunctionType
scFunctionCode; //
-> function selector
OSErr
scResult;
<- returned result
DeviceIdent
scDeviceIdent; //
-> (bus + target + LUN)
CallbackProc
scCompFn;
//
-> callback on completion function
ulong
scFlags;
//
-> flags for operation
//
// end of SCSIPBHdr
Figure 63. Typical parameter block construct
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Table 37 lists and describes SCSI Manager 4.3 calls. This information would
be included in the body of the parameter block.
STOP
Calls are requests or instructions from the initiator or target.
Table 37. SCSI Manager 4.3 calls
Call
Description
SCSIAction
Performs a SCSI action.
SCSI_ExecIO
Performs an I/O Command.
SCSI_AbortCommand
Aborts a command.
SCSI_ResetBus
Resets a SCSI bus.
SCSI_ResetDevice
Resets one device.
SCSI_TerminateIO
Terminates an incomplete I/O.
SCSI_GetVirtualIDInfo
Gets information on a specific device.
SCSI_ReleaseQ
Releases a frozen queue.
SCSI_BusInquiry
Gets information on a host bus adapter (HBA).
SCSIRegisterBus
Registers a new SCSI bus.
SCSIDeregisterBus
Deregisters a SCSI bus.
SIMinit
Initializes a SIM.
SIMAction
XPT uses this to call SIM.
SCSI Manager 4.3 is present in ROM on all machines newer than the Quadra
840AV. It is also supported on all Quadra models utilizing 53C9X series SCSI
chips when running System 7.5 or newer. It was not included in any PowerBooks before 1996.
STOP
Apple’s future Mac OS, also known as Copland, is rumored to be introducing another SCSI
Manager model. It may borrow heavily from DEC VMS and Windows NT’s layered driver
architecture.
Device drivers
A device driver on the Macintosh is the software program that translates operating system requests into the SCSI commands that direct the operations of a
SCSI peripheral. It does this via the SCSI Manager and SCSI chip. Properly
written software drivers communicate with a SCSI device via the SCSI ManChapter 5: All About SCSI
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ager and SCSI chip and maintain the device at peak performance throughout
its lifetime. Among the devices that SCSI drivers control are:
•
•
•
•
•
•
•
•
hard drives
removable drives
scanners
tape drives
printers
film recorders
digital cameras
CD-ROM drives
Device drivers are required for all classes of devices. There are drivers for fixed
and removable disk drives as well as special drivers used to implement drive
spanning and data striping.
STOP
Corrupt drivers cannot usually be detected by diagnostic utilities. If you are unable to get a
particular device to work by itself, reinstalling the driver will often help.
Requests
Table 38 lists the five requests to which a driver responds.
Table 38. Driver requests
182
Request
Description
Open
This is where the driver is first called and initializes itself, allocates
memory, and generally gets ready for business. Open typically
occurs in the boot-up or mounting process.
Close
At this request, the driver shuts down, deallocates memory, and
usually turns off.
Prime
Following a Prime request, the driver will perform a task, typically
a read or write.
Status
Seldom used in SCSI.
Control
This request queries the driver for “where” data is located,
including Finder and icon data, and puts the driver in different
operational modes.
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Input
The driver is capable of receiving the following input after receiving a request:
• The number of bytes the access is offset from the start of the partition.
This is the logical address that the driver will translate into a drive’s block
address.
• The driver can determine if it was called as the result of a Read or Write
command and whether the command was made asynchronously or
synchronously.
• How many bytes of data are requested.
• The address of the I/O buffer.
If the driver was called for an asynchronous request, it will also accept the
address of a completion routine that will be called when the request has been
processed.
Output
Device drivers can return the following information to the computer’s operating system:
• the number of bytes actually transferred
• an error code (zero, if no error occurred)
Call Chain
Now that you’ve waded through the command structures, understand the
functioning of the SCSI chip and device controllers, and have an appreciation
for device drivers, the SCSI Manager, and hard drive technology, we can fit
them all together. This is a “behind the scenes” look at how information is
routed in a computer system that uses SCSI.
Host computers and devices that communicate in a structured manner do so
in a complicated sequence of continuous calls known as the call chain.
Calls are requests or instructions from the initiator or target. The call chain
represents the many layers through which calls pass and the levels at which
they are translated—in effect, readied for the next level.
The sequence is “modular”; that is, each step is distinct and self-contained.
This is the result of purposeful intent by good engineers. If an aspect of the
call chain is improved, only the layer affected needs to be changed.
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Our example is the common call chain involved in writing data to a hard disk.
1. First, the application in which you are working makes a write call to the
Macintosh’s File Manager.
STOP
The File Manager is a software library that manages I/O flow to and from files. Typically, the File
Manager is located in ROM.
2. The File Manager translates the write call into another protocol and
sends it to the Macintosh’s Device Manager, a lower-level software
library that typically also resides in ROM.
3. The Device Manager translates the call into a parameter block the
Device Driver will recognize and calls the appropriate driver for the volume to be accessed.
4. The Device Driver, residing in RAM, sends the write call, which it
translates into a sequence of calls, to the SCSI Manager:
• to gain control of the SCSI bus
• to select a particular device for I/O
• to send a command
• to write the appropriate data
• to complete the operation
5. The SCSI Manager takes these calls and turns them into a form that the
SCSI chip can use to send commands across the bus.
6. The SCSI chip sends electrical signals across the bus that correspond to
the above calls.
7. These signals are received by the SCSI chip on the SCSI peripheral. This
chip provides the device’s intelligence. It is controlled by a CPU on the
device, which is part of the peripheral’s controller board.
8. The controller CPU coordinates the specific operations of the original
call and oversees its completion by writing data to the platter.
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Data Mapping
D ri v e r M a p
D escrip t o r
A p ple M a p
P a rtiti o n iv e r
F W B Dr nents
Compo
A p ple D
A p ple _
Partition
ri v e r
ac
HFS ( M
A p ple _
O S)
Free
Figure 64. Partition map
Having described what drivers are, what the call chain is, and how drives
function, we can now describe how the Macintosh places data on the disk.
Data is physically laid out on the disk according to specifications set forth by
Inside Macintosh V and Inside Macintosh: Devices. Block zero contains the
driver descriptor map (DDM). The DDM is the first data read from the disk
and contains:
•
•
•
•
number of blocks on the device
block size
number of drivers
location and size of the first driver
The boot code in the ROM of the Macintosh reads in the driver descriptor
map, then loads the driver into memory and executes it.
The partition map is contained on a series of blocks, beginning with block
one. Each partition map entry occupies one block and contains:
•
•
•
•
•
•
name of the partition
processor type
type of partition
partition status
partition size
more
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The partition map can vary in size from a few to hundreds of blocks. Partitions can be created for:
•
•
•
•
•
•
•
HFS
A/UX
ProDOS
drivers
partition maps
scratch space
custom partitions (as with RAID configurations)
After the partition map comes each partition in sequence. Every block on the
disk must belong to a partition. Even free space is stored within a partition.
Inside the Mac HFS partition
The Macintosh HFS partition contains structures to help the Macintosh operating system boot the computer and locate information on files and folders.
This data structure is set up when the hard disk setup utility program initializes a partition. The following data structures are written to disk:
•
•
•
•
•
•
boot blocks
volume information block (VIB)
volume bitmap
extents tree
catalog tree
file data area
These data structures are described in Table 39.
Table 39. Data structures in the Macintosh HFS partition
Data Structure
Description
Boot Block
The boot blocks on each HFS partition occupy blocks zero and one. They are
filled in with data when a System Folder is copied into the partition. The boot
blocks contain the following:
•
•
•
•
186
information that allows the system to start up from the System Folder
information on the maximum number of open files
the name of the debugger
the names of system files
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Table 39. Data structures in the Macintosh HFS partition (Continued)
Data Structure
Description
Volume Information Block (VIB)
The volume information block is a master directory block that contains
important information on the volume including:
•
•
•
•
its name and type
the number of files and folders on the volume
the allocation block size
pointers to the location of files
There is a backup copy of this near the end of the disk.
Volume Bitmap
The volume bit map is a directory structure that keeps track of allocation
blocks that are in use and that are free. It is used to determine where to put
data that needs space and where to free up space when a file is deleted.
Extents and Catalog Trees
The extents and catalog trees are data structures that track the location and
statistics for all files and folders on the disk. Each file record indicates:
•
•
•
•
•
•
•
File Data Area
name
ID
type
creator
size
creation and modification date
other file information
The file data area is the space on the disk where data is stored in allocation
block-sized chunks.
Macintosh volume limits
Systems prior to 7.5 were limited in size to 2 GB per volume. System 7.5
increased this to 4 GB per volume, while the PowerMacintosh with PCI raised
the limit to 2 terabytes (TB). As a volume gets larger, its smallest unit of allocation grows because there are only 65535 block units that can be allocated to
a given volume. A 1 GB volume has an allocation block size of 16 KB. This
means that a file with only one character of data in it will nonetheless consume 16 KB of disk space. A 2 TB volume has a minimum allocation block
size of a whopping 32 MB!
STOP
A terabyte is equal to 1012, and is written out as 1,000,000,000,000.
Multiple partitions can be created on larger disks to minimize allocation
block size. Minimizing allocation block size allows more real data to be stored
on a given drive. Partitions can be used to organize data, for example to put all
applications in one partition and all data in another, and to create storage
areas for password-protected and/or encrypted data.
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Use disk utility software to create partitions on your storage media.
Macintosh SCSI performance
Macintosh SCSI performance has improved significantly from the first implementation on the Macintosh Plus. Apple has remained conservative and been
slow to adopt new technologies that improve data transfer rates (Table 40).
Table 40. Progress of data transfer rates in the Macintosh
Macintosh
Maximum SCSI Transfer Rate
Plus
0.3 MB/s
SE
0.7 MB/s
Macintosh II family
2.8 - 4.5 MB/s
Quadra family
3.8 - 4.5 MB/s
Power Macintosh family
3.8 - 4.5 MB/s
Power Macintosh with PCI family
4.5 - 10 MB/s
The original SCSI implementations in the Plus and SE were limited to under
one megabyte a second by the SCSI hardware and software implementation.
The later Macintosh II, Quadra, and Power Macintosh families increased SCSI
performance but were still limited to performing slower asynchronous data
transfers. The first members of the PowerMacintosh with PCI family finally
introduced synchronous data transfers on their internal SCSI buses, increasing
data rates to 10 MB/s.
Third parties produce SCSI accelerator add-in cards that break the SCSI bottleneck inherent in the Macintosh architecture. The data transfer rates on these
cards are often two to four times faster than the standard data transfer rates.
At a time when the Macintosh was transferring 10 MB/s, third parties were
handling 40 MB/s.
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PC SCSI Implementation
SCSI host adapters were initially difficult to install on PCs. Many contained
lots of jumper settings. Any single setting could conflict with another card
and cause problems. SCSI was also actually slower for many single-tasking
applications because it introduced a lot of overhead over the very efficient
native IDE interface.
STOP
A jumper is a pair of pins on a circuit board. A jumper block is a small plastic-covered metal
connector used to connect jumpers. Jumpers and jumper blocks are used to set device parameters,
such as the SCSI ID.
SCSI has also been slow to take off on the PC because of many conflicting
standards and software driver dependencies. In the early days of the PC, there
were as many as three separate incompatible standards being promoted:
• Microsoft originally promoted a Layered Architecture for Device Drivers
(LADDR) with the advent of OS/2.
• Adaptec presented Advanced SCSI Programming Interface (ASPI) in 1988.
• Future Domain promoted ANSI’s Common Access Method (CAM).
• IBM OS/2 promoted Adapter Device Driver (ADD).
• Novell promoted Host Adapter Module (HAM).
If you had three SCSI host adapters from different vendors, you could need to
load three separate sets of drivers even though all were talking SCSI. You
would also need a driver for each additional operating system.
By the mid-90’s things had calmed down. ASPI became the dominant standard
on the PC with virtually everyone supporting it—including Microsoft, in
Windows 95. Standardizing on ASPI allowed host adapter manufacturers to
simply produce an ASPI layer. Application and driver developers could write
ASPI-compliant software that would run on any ASPI-compatible adapter.
ASPI became the SCSI programming interface of choice on DOS, Windows,
OS/2 and Novell Netware.
NOTE
See “PC SCSI: ASPI for Win32 calls” on page 309 for a table of the calls ASPI for Win32 makes to
access SCSI devices.
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DOS
Most PC SCSI host adapters include a SCSI BIOS boot ROM on board. This
BIOS is used to initialize the SCSI chip on startup and to support the transparent operation of the adapter under DOS through the standard INT13 BIOS
interface. Because the host adapter has its own BIOS, the drives on a SCSI card
need not be set up with the computer’s Setup utility. The SCSI drive appears
to be a normal drive to DOS. You can use standard DOS utilities such as
FDISK and FORMAT to partition and high-level format the drive. Application
software runs transparently. SCSI drives co-exist with IDE drives, although
the system attempts to startup from IDE drives first.
STOP
The INT13 BIOS interface is the standard method of accessing a PC hard disk.
Versions prior to DOS 5 supported only two drives. Versions later than DOS 5
support up to seven physical drives. DOS supports up to 24 partitions (Drive
letter C: through Z:). DOS 3.x supports up to 32 MB per partition. Versions
after that expanded on this limit.
Windows®
The original Windows 3.0 communicated with SCSI drives through the standard INT13 BIOS mechanism. Bus mastering SCSI Controllers needed to support the Virtual DMA Services (VDS) standard to ensure compatibility with
memory extensions such as Expanded Memory Specification (EMS). The real
mode INT13 interface ensured compatibility but at the expense of performance. On every I/O, the computer needed to switch from 386 enhanced
mode to real mode as many as six times. This slowed I/O.
With the advent of Windows 3.1 and Windows for Workgroups 3.11, Microsoft
created a 32-bit fastdisk interface that SCSI drivers could be hooked into. Each
of these fastdisk drivers was hardware-dependent. The operating system came
with a built-in fastdisk driver for IDE drives smaller than 528 MB. These drivers allowed the computer to remain in enhanced mode, significantly improving performance. This was also known as 32-bit disk access. Windows for
Workgroups added 32-bit file access or VFAT.386 and VCACHE.386. These
drivers install 32-bit operating system components for caching and for the
disk file system. The old DOS Smartdrive was now needed only to cache
floppy and CD-ROM drives.
Vendors also had to produce proprietary Windows ASPI (WinASPI.DLL) implementation to support third-party storage software under Windows.
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Windows 95 and NT
These newer operating systems were designed from the ground up to accommodate SCSI. Microsoft and Intel even put together a Plug-and-Play specification to make adding SCSI host adapters easier. Windows 95 and NT feature
layered driver architectures. SCSI host adapter vendors simply need to supply
a small Miniport driver to support the new operating systems.
STOP
“NT” stands for new technology. See “PC Plug-and-Play” below for details about the Microsoft
and Intel developed Plug-and-Play specification.
The Miniport driver is essentially a hardware-specific translation layer that
abstracts the I/O architecture of the host adapter. It initializes and configures
the adapter, starts I/O, verifies the adapter state, and resets the SCSI bus. Hard
drive, removable, and CD-ROM drivers are built into the operating system.
ASPI support is also available in both operating systems, with NT having a
32-bit version—ASPI32—built-in. SCSI host adapters still need BIOS Boot
ROMs to facilitate booting the operating systems.
Windows NT has support for multiple requests per unit, so it can exploit
tagged command queuing. It also has built in support for RAID. The workstation version includes RAID 0 and 1; the server version supports RAID 5.
PC Plug-and-Play
The PC Plug-and-Play specification was created by Intel, Microsoft, and Compaq to make it easier to add adapter boards into PCs. It was primarily designed
to make the allocation of interrupt, DMA, and I/O port addresses automatic
when using ISA-based cards. (Previously you had to manually adjust jumpers.)
It also mandated the need for self-resetting terminator power fuses and for a
special SCSI icon on the back of the adapter. The Plug-and-Play standard
required additions to the computer BIOS to be implemented.
This specification was also extended to SCSI by including a new standard
called SCAM (SCSI Configured Auto Magically). SCAM is a protocol that
allows a host adapter to set the SCSI ID number of devices on the SCSI bus,
preventing ID conflicts, and to set termination automatically. The protocol
uses a SCAM master that isolates devices by toggling SCSI lines. It is compatible with legacy devices whose IDs are not changeable. Two levels of SCAM
compliance have been defined. The more advanced level allows multiple initiators and hot plugging. SCAM acceptance has been slow, with few SCAM
compliant peripherals available in 1995.
STOP
SCAM acceptance has been hindered by the fact that PCs can’t have SCSI IDs moved around.
Moving IDs around would cause problems with assignment of drive letters.
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PC SCSI performance
PC SCSI performance has improved significantly from the first implementations on the ISA bus. Improvements in bus technology combined with newer
SCSI chips have driven performance to new levels. Each computer model in
Table 41 was coupled with the fastest available SCSI host adapter.
Table 41. Overview of improved transfer rates in PCs
Computer and Bus
Maximum SCSI
Transfer Rate
IBM AT 80286/8 - ISA
2.5 MB/s
PC Compatible 80386/33 - ISA
3.0 MB/s
IBM PS/2 Model 80 80386/16 - MCA
7.0 MB/s
PC Compatible 80486/33 - EISA
7.5 MB/s
PC Compatible 80486/66 - VL
20 MB/s
PC Compatible P5-133 - PCI
40 MB/s
PC SCSI data transfer performance has mainly been limited by computer
expansion bus limitations. Virtually all PC SCSI implementations have handled synchronous data transfers and disconnect/reconnect since inception.
Manufacturers of PC SCSI host adapters were quick to add support for Fast/
Wide SCSI-2 and Fast-20 SCSI.
STOP
192
To help you sort out the SCSI variations, we offer this list:
•
•
•
•
Fast SCSI-2 means transfer rates at 10 MB/s on an 8-bit narrow bus.
Fast/Wide SCSI-2 means transfer rates at 20 MB/s on a 16-bit wide bus.
Ultra SCSI (same as Fast 20) means transfer rates at 20 MB/s on an 8-bit narrow bus.
Ultra Wide SCSI means transfer rates at 40 MB/s on a 16-bit wide bus.
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SCSI-2: A Transition From SCSI-1
The SCSI-2 specification has been developed by ANSI Task Group X3T9.2 on
Lower-Level Interfaces, with:
• John B. Lohmeyer, Chair
• I. Dal Allan, Vice-Chair
• Lawrence J. Lamers, Secretary
The SCSI-2 specification was approved by ANSI’s Technical Committee X3T9
on I/O interfaces, with:
• Del Shoemaker, Chair
• Robert L. Fink, Vice-Chair
ANSI began work on SCSI-2 in 1986, even before SCSI-1 Revision 17B had
been formally adopted. There was no technical rationale for doing so—the
specification for SCSI-2 was far from complete—but at some point work must
be committed to print.
SCSI-2 Revision 10c was approved in August of 1990 as an American National
Standard. However, several errors and ambiguities were discovered during the
review period, and it was returned to committee for correction and clarification. The result was Revision 10L, introduced in September 1993. It became a
published ANSI standard before the end of 1994 as X3.131-1994. It became an
ISO standard in 1996.
ANSI specified that SCSI-2 be backward compatible with SCSI-1. Devices
using the new standard will work with those that use the old standard. All
command, status, and message information will continue to be transferred in
the eight-bit asynchronous mode.
SCSI-2 overview
SCSI-2 builds on the SCSI-1 specification in the following ways:
• defines extensions to the SCSI-1 specification
• provides more complete standardization of SCSI-1 command sets
• describes the necessary mechanical, electrical, and functional qualities
that will allow for interoperability of devices meeting the new standard
To meet these demands, SCSI-2 devices require more intelligence and firmware than their SCSI-1 counterparts.
SCSI-2 furthers the goal of SCSI-1 to promote device-independence within a
class of devices. It provides for the implementation of vendor-unique fields
and codes, and sets aside fields and codes for future standardization.
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Another key objective of SCSI-2 is placing device-independent intelligence
onto the SCSI device. This requires a command set that allows an operating
system to get the necessary initialization information from the SCSI-2 device.
Parameters that are not required by the operating system for operation, initialization, or system tuning will be managed exclusively by the SCSI-2 device.
The structure of the published SCSI-2 specification, although much larger
than SCSI-1, makes it more understandable to the layman as well as the engineer. Rather than following a numeric sequence that caters to those who wish
to memorize the message and opcode tables, alphabetization and organization
are the new order of the day. The specification is organized along device class
lines, with descriptions of each model’s characteristics.
SCSI-1 defined the following types of devices:
•
•
•
•
•
•
random access
sequential access
printer
processor
WORM
read-only random access
SCSI-2 added the following types of devices:
•
•
•
•
•
CD-ROM
scanner
magneto-optical
medium changers (such as CD-ROM jukeboxes)
communication devices
Separate sections of the specification that address particular devices, many of
which received new command sets, include the following devices:
• CD-ROM (which replaces SCSI-1’s read-only random access)
• scanners
• optical memory (which provides for WORM, ROM, and erasable media)
SCSI-2 removes support for the following SCSI-1 features and options:
•
•
•
•
•
•
194
single initiator option
non-arbitrating systems option
SCSI-1 alternative 1 shielded connector
non-extended sense data option
reservation queuing option
read-only device command set
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SCSI-2 commands
All existing SCSI-1 command sets were enhanced in SCSI-2:
•
•
•
•
Device models were added.
Extended sense and Inquiry data were expanded.
Pages for Mode Select and Mode Sense were detailed for all device types.
The Change Definition, Log Select, Log Sense, and Read and Write Buffer
commands were added for all device types.
• The Copy command definition was expanded.
• The direct-access command set added cache management and greater
control over defect management.
• The sequential-access device command set received several mode pages,
some designed to support partitioned media.
• The Write-Once command set added several new commands.
SCSI-2 also caused the following:
•
•
•
•
NOTE
addition of device models
expansion of extended request sense
expansion of inquiry data
expansion of Copy to support inexact block size and image copies
See “SCSI-2: new commands” on page 309 for more SCSI-2 commands that include descriptions,
opcodes, and whether a command is mandatory or optional. See “SCSI-2 mode pages” on page 314
for a list and description of the mode pages introduced in SCSI-2.
SCSI-2 highlights
The following excerpts have been selected from the 600-plus page SCSI-2
specification, which establishes future guidelines and helps broaden the commonality that devices must support. SCSI-2 was submitted to ANSI as
X3.131-1993.
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Required features
The SCSI-2 specification mandates that SCSI-2 devices support the features
listed and described in Table 42:
Table 42. Mandated SCSI-2 supported features
Feature
Description
Common Command Set (CCS)
The Common Command Set (CCS), considered a de facto standard among SCSI hard drive
manufacturers, has become a central feature of SCSI-2. Since 1988, virtually all hard
drive manufacturers have incorporated CCS or a subset of it. ANSI CCS 4B (as CCS is
formally known today) will represent the minimum level of commonality all SCSI-2
devices must support.
SCSI-1’s goal of manufacturer compatibility was thwarted by an industry proliferation of
undocumented vendor-unique features. Differences in hardware required custom
programs and unique microcode. This self-defeating diversity provided the impetus for
creation of the CCS, which defines the following:
•
•
•
•
data structures for Mode Select and Mode Sense commands
defect management of the Format command
error recovery procedures
numerous other command functions
CCS was the beginning of SCSI-2, but it is only for disk drives. The first efforts of the
SCSI-2 technical committee focused on using the SCSI interface with tapes, optical discs,
and other devices. However, the SCSI-2 specification has gone far beyond applying CCS
to various types of devices.
Initiator Provided TPWR at 100 ohms
SCSI-2 requires that initiators provide termination power, and defines an active
terminator that lowers termination to 100 ohms. SCSI-1 specified 132-ohm termination
for the single-ended electrical specification. Unfortunately, that was mismatched with
cable impedance, which is typically below 100 ohms. The result is that the terminator
allowed for greater current than the cable could handle. The signal noise thus caused
errors at faster data transfer rates, or when many devices were attached.
Parity Checking
SCSI-2 mandates inclusion of parity checking. See pages 89, 143, and 175 for more
information on parity.
Required Messages
A basic set of messages is defined and required (most devices already support them).
•
•
•
•
•
•
•
•
Command Complete
Initiator Detected Error
Abort
Message Reject
No Operation
Message Parity Error
Bus Device Reset
Identify
For a list of SCSI messages and descriptions, see “Messages” on page 164.
Bus Arbitration
196
Bus Arbitration is now required, and the arbitration delay has been lengthened from 2.2
to 2.4 ms.
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Table 42. Mandated SCSI-2 supported features (Continued)
Feature
Description
Rotational Position Locking (RPL)
This features causes synchronization of drive spindles, so the spinning of multiple disks
can be coordinated. This is critical for faster transfer rates in disk arrays. For more
information on disk arrays, see “What is RAID?” on page 79.
Contingent Allegiance
Targets in the Contingent Allegiance state reject commands until the error status is
cleared. SCSI-1 did not deal with deferred errors in systems with buffers. However,
deferred errors will occur if anything goes wrong after the target has accepted the data
into the buffer and sent the Good status, when the target writes the buffer contents to
the media.
Extended Sense Keys and Sense Codes
Sense keys and sense codes have been formalized and extended. Specifics on the type of
error being reported help engineers (or their programs) analyze errors or failures in the
field or on the fly.
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Optional features
The SCSI specification still allows for optional features, as well the flexibility
of vendor-unique implementations. If supported, optional features must be
implemented in uniform accordance with the SCSI-2 specification. Optional
features are listed and described in Table 43.
Table 43. Optional features allowed in SCSI-2
Optional Feature
Description
Tagged Command Queuing
Tagged command queuing enhances SCSI I/O capabilities. Rather than ask the initiator
for a new command each time it finishes an operation, a “popular” peripheral can have
the initiator queue, or line up, commands, so they’re ready to go when the device is free.
Up to 256 commands may be queued, and the initiator can specify the order in which
the target is to execute them. This ability to prioritize queued commands allows the
target to operate more efficiently. For example, a hard disk drive can read from one
data block to the next according to how closely they are situated, rather than hopping
around from block to block according to the order in which the commands were received.
Devices with this feature are compatible with devices that do not support command
queuing.
Command queuing is among the more difficult SCSI-2 benefits to implement. It may
cause the vendor to run into firmware problems if attention is not paid to the types of
commands being queued prior to their being sorted. For example, if a Write and a Read
command were queued, and the Read command was executed first because doing so
was most efficient for the target, it is possible that the Write command was sent to
modify the same data block that is now being read. In this case, the Read command
would read “old” data. Queuing is really only helpful when used on multitasking
operating systems.
Optional messages have been added to support command queuing capabilities. Also, the
operating system must support multitasking, or multiple commands would never be
issued.
198
Command Linking
Command linking allows two or more SCSI commands to be linked together. If a linked
flag is set in the CDB, the completion of one command triggers the execution of the
following command, saving arbitration time.
Terminator Design and Drive Technology
Improvements in terminator design and driver technology may be required to support
fast SCSI on a single-ended cable (this is now one of SCSI-3’s goals).
Fast SCSI
Fast SCSI uses the standard 50-pin connector to double the maximum data transfer rate
to 10 MB/s in fast synchronous mode. (Up to 10 mega-transfers per second.) It does this
by cutting synchronous transfer timings in half. However, this works only with modern
SCSI chips, which were not used in the Macintosh for many years, but are available
through third-party add-ons.
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Table 43. Optional features allowed in SCSI-2 (Continued)
Optional Feature
Description
Wide SCSI
Wide SCSI is an option that increases the SCSI data path from eight to 16 or even 32 bits.
(Commands, status, messages and arbitration are still transferred as eight bits for
compatibility with SCSI-1.) It does this by using a second 68-conductor “B” cable that has
24 extra data lines. The standard 50-pin cable remains, still handling eight bits. This twocable requirement (50- and 68-pin) may be impractical for many systems because the
additional cable requires space for a second connector—space that may not exist.
SCSI-3 describes a 68-pin “P” cable that will support arbitration and data transfers at 16
bits. Most vendors have adopted this scheme. This 68-pin cable makes it difficult to
daisy-chain standard 50-pin narrow SCSI devices to the 68-pin wide devices. It is possible
to do this, with the wide devices connected first and the narrow ones later, but
terminating the upper 8-bits and sending the lower 8-bits through is very difficult.
Usually the best strategy is to connect all wide devices to the host adapter’s internal or
external port and connect narrow ones to the other port. Most host adapters allow for
partial termination of the terminators on the card.
Both Fast and Wide SCSI can exist on the same bus. Combined, rates of 40 MB/s are
possible. However, implementing Fast and/or Wide SCSI capabilities may require silicon
changes in the SCSI protocol chip. Because much information will continue to be
transferred in the eight-bit asynchronous mode, fast and wide capabilities may not
significantly impact the I/O rates of your system unless you use fast drives and CPUs.
Optional Messages
Optional messages have been added to support wide-transfer capabilities.
Smaller, High-Density Connectors
Smaller, high-density connectors (DB-50) are stipulated to fit in tighter spaces and make
room for an increased number of pins.
Asynchronous Event Notification
Asynchronous event notification (AEN) is a protocol that can be used to inform processor
devices that an asynchronous event has occurred. Instead of the initiator polling, the
target would notify the initiator.
Optional Extended Contingent Allegiance
Extended contingent allegiance is now optional. It is a sleep mode in which devices won’t
respond to other requests for access, and command queuing is suspended.
12-Byte Command Descriptor Block
A 12-byte Command Descriptor Block (CDB) has been defined, allowing for more
complex commands and access to a larger range of data. This format is supported by the
following commands:
•
•
•
•
•
•
•
•
Chapter 5: All About SCSI
Read
Search Data Equal
Search Data High
Search Data Low
Set Limits
Write
Write and Verify
Verify
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The SCSI-2 Common Access Method (CAM)
In October 1988 a group of 40 peripheral suppliers led by Adaptec, Inc. formed
the Common Access Method (CAM) Committee to establish a common software interface for attaching SCSI peripherals. Such an interface would be
independent of the host bus adapter (HBA) hardware implementation. ANSI
completed final action on CAM in early 1996. It was adopted as ANSI
X3.232:1996, SCSI-2 Common Access Method Transport and SCSI Interface
Module.
CAM reduces the number of drivers needed by an HBA and its peripherals on
various operating systems. Its modular design allows the use of only those
software drivers that belong with the SCSI peripherals. CAM also addresses
the use of a protocol chip on the motherboard and the ATA (Advanced Technology Bus Attachment) interface as a method of attaching SCSI peripherals.
Macintoshes do not require SCSI adapters, but instead offer the SCSI Manager
and SCSI chips on the motherboard. However, because the SCSI Manager
solution is vendor-specific, SCSI devices intended for use with the Macintosh
must have Macintosh-tuned device driver software. CAM’s major goal is to
provide one interface across all hardware platforms, Macintosh included, to
ease software development for every new system.
CAM has not become important in the PC world. There the profusion of manufacturers’ HBAs and software drivers has caused significant software-compatibility problems. With PCs, ASPI is the dominant standard.
The CAM committee’s proposals consider the SCSI-2 CAM Transport (or
XPT) and SCSI Interface Module (SIM).
STOP
The XPT (transport) is a layer of software that programs and peripheral drivers use to initiate the
execution of CAM functions. SIMs are modules that execute the SCSI commands.
Macintoshes were not impacted immediately by CAM development. They
benefited indirectly as CAM accelerated SCSI’s marketplace acceptance and
helped with third-party SCSI accelerator add-ons that had no such standard
(such as NuBus SCSI cards). Apple later adopted CAM as part of their SCSI
Manager 4.3 interface.
Details of CAM operation
With CAM, requests for SCSI I/O from the peripheral driver are made through
the XPT interface. The XPT receives the request as a CAM Control Block
(CCB), which is a data structure containing the action required. The XPT
function routes CCBs to a lower level SIM.
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The XPT allocates CCB resources—such as getting and releasing CCBs—independent of the operating system. It also ensures the CCBs are properly formatted and the fields needed to accomplish a request are primed.
To prevent each driver from having to scan the SCSI bus for devices at initialization, the XPT detects all installed SCSI devices and constructs an internal
table, which is accessed by drivers and programs.
The XPT function may be a separate element or incorporated with the SIM
into a single module. Either way, a logical separation between the two is
maintained to distinguish among SIMs that may be loaded.
The SIM function processes SCSI requests and manages the hardware-independent interface to the HBA.
SIM services do the following:
• monitor I/O behavior and perform error recovery if needed
• manage data transfer hardware, such as DMA circuitry
• queue multiple operations for single or multiple LUNs, “freezing” the
queuing of requests as necessary to perform queue recovery
• post results back to the initiating device driver
• manage the selection, disconnection, reconnection, and data pointers of
the SCSI HBA protocol
The CAM committee is also polishing up specifications for the ATA interface,
which specifies how a peripheral controller can emulate the original IBM AT
hard disk drive interface. This is important because a lot of software requires
that the hardware look precisely like an IBM machine in order to run. ATA
and EATA (Extended ATA) define how to create interfaces for SCSI (and other)
peripherals that the operating system will “accept” as normal.
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SCSI-3
The ANSI committee responsible for SCSI-2 realized that the only way it was
going to complete SCSI-2 was to begin planning SCSI-3. SCSI-3’s main target
is extending SCSI-2’s functionality so that it is suitable for much higher performance rates, while maintaining backward compatibility with present SCSI.
The SCSI-3 specification has numerous goals, including better cabling
schemes for Fast and Wide SCSI. In fact, some of these cabling schemes may
be standardized before the SCSI-3 specification is released.
The SCSI-3 specification consists of many files, each covering a specific area.
This allows different teams to concentrate on different areas. It also allows
the different parts of the specification to be approved as they are finalized.
Table 44 lists the files and topics covered in SCSI-3.
Table 44. SCSI-3 files and topics
File
Topic
Description
F20
SCSI-3 Fast-20 Parallel Interface
(20 MB/s on 8-bit bus or 40 MB/s on
16-bit)
Also part of the specification and a proposed X3T10/855D standard is Fast-20 or
Ultra-SCSI. This basically doubles the data rate and allows 40 MB/s transfers over
a 16-bit wide SCSI bus or 20 MB/s over an 8-bit wide SCSI bus. This performance
boost is achieved by faster synchronous negotiation speeds. It is intended to bridge
the gap before serial SCSI takes off. The first drives for this standard started to
appear in 1995. The main limiting factor was that cabling was now half the length
of the previous generation. Cabling needed to be 90 ohms and only active
termination could be used. Other cable tolerances were also more stringent.
Single-ended buses could be three meters in length when using four maximum
capacitance devices, or 1.5 meters when using five to eight devices. Differential
buses could now be 25 meters in length.
FCP
SCSI-3 Fibre Channel Protocol
(for Fibre Channel)
This part of SCSI-3 describes the Fibre Channel Protocol used in Fibre Channel
implementations of serial SCSI. Fibre Channel allows for data transfer rates of up
to 100 MB/s over small wires that can be up to 30 meters in length between
devices. FCP defines a Fibre Channel mapping layer (FC-4) that uses the services
defined by ANSI X3.230.199x (Fibre Channel - Physical and Signaling Interface
[FC-PH]) to send SCSI commands and data between serial SCSI devices. This
protocol involves sending and receiving this information using standard Fibre
Channel frame and sequence formats. FCP functions with Fibre Channel point to
point, fabrics, and arbitrated loops. It supports both the optical and electrical
connections of Fibre Channel.
For more information see Chapter 6, “All About Serial SCSI.”
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Table 44. SCSI-3 files and topics (Continued)
File
Topic
Description
GPP
SCSI-3 Generic Packetized Protocol
Technical Report
SCSI-3 GPP describes packetizing SCSI, which allows it to be transported by cabling
other than traditional parallel cable. This stretches the SCSI cable well beyond its
current limits while adding support for very high-speed transfers. Serial
implementations also use information packets for data transmission, a radical
departure from the present SCSI architecture. It basically translates SCSI protocols
into network protocols, where data is encapsulated in packets. The cable could then
be:
•
•
•
•
MMC
SCSI-3 Multi-Media Commands
(CD-ROM command set)
Fiber-optic
Copper coaxial
Twisted pair
Wireless
SCSI-3 MMC is an important new part of SCSI-3 that describes commands for
multimedia devices (i.e., CD-ROM and CD-R). This document describes the various
CD-ROM and CD audio disk formats. Some new and updated commands include:
• Read CD Recorded Capacity (25H) - Returns actual recorded capacity of CD.
• Play Audio Track/Index (48H) - Plays an audio track on the CD from an index
point.
• Seek (2BH) - Seeks the head to a place on the CD.
SCSI-3 MMC accommodates CD drives as fast as 16x drives. (2.8 MB/s) This
specification also describes CD-R commands to do writing and incremental packet
writing, and some actual examples of how to write discs.
SAM
SCSI-3 Architecture Model
(what it means to be SCSI-3)
SCSI-3 Architecture Model describes the entire SCSI-3 specification and its
functional partitioning into separate documents. It specifies a model for I/O
system and device behavior that applies to all SCSI interconnects, protocols, access
methods, and devices. This document contains lots of definitions and diagrams
outlining basic terminology and nomenclature used throughout SCSI-3 documents.
This document discusses basic SCSI events such as:
•
•
•
•
•
Chapter 5: All About SCSI
Request/responses
Resets
Request sense
Contingent allegiance
Task management
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Table 44. SCSI-3 files and topics (Continued)
File
Topic
Description
SBC
SCSI-3 Block Commands
(for direct-access devices e.g., disks)
SCSI-3 SBC is an important new part of SCSI-3 that describes commands for direct
access block logical unit class devices (i.e., hard drives, worm, optical, and
removables). Caching is documented more thoroughly than in the SCSI-2
specification.
This part of SCSI-3 includes commands to support medium changer devices such as
the Read Elements Status and Move Medium commands. Optical drives have the
following additional commands:
• Medium Scan (38H) - to scan for contiguous written or blank blocks.
• Read Generation (29H) - to return the maximum generation address of a
block.
• Read Updated Block (2DH) - to read a specific generation and block number.
• Update Block (3DH) - requests that the drive replace data on the medium with
new data.
SBP
SCSI-3 Serial Bus Protocol
(for IEEE P1394)
This part of SCSI-3 describes the serial bus protocol used in 1394 implementations
of serial SCSI. It defines a new fixed length packet that supports the various SCSI3 standards to deliver command, data and status over the 1394 bus. The SCSI-3
SBP command data structure (CDS) allows data and command to be linked
together. SCSI-3 SBP also supports 1394’s isochronous data transfers.
For more information, see Chapter 6, “All About Serial SCSI.”
SCC
SCSI-3 Controller Commands
(for RAID controllers)
SCSI-3 SCC is an important new part of SCSI-3 that describes commands for
controller devices (i.e., RAID controllers). It describes commands that are used to
control RAID controllers. RAID controllers need to be commanded to set up arrays,
configure arrays, rebuild arrays, and monitor arrays and their components. New
commands include:
• Maintenance (A3/A4H) - Performs logical operations on the RAID such as
reporting on individual components.
• Report Redundancy Group (BA/BBH) - Returns and sets information on how
drives are grouped as RAID units.
• Spare (BC/BDH) - Returns and sets information on drives used as spares.
• Volume Set (BE/BFH) - Controls the generation of check data within an
underlying redundancy group.
SCSI-3 SCC also describes in detail mappings of logical units to volumes.
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Table 44. SCSI-3 files and topics (Continued)
File
Topic
Description
SGC
SCSI-3 Graphics Commands
(for scanners and printers)
The SCSI-3 SGC standard describes command and behavior of graphical SCSI
peripherals. Graphical peripherals are capable of transferring a visual
representation of data to and from the computer. The main graphical device that is
covered is the scanner. SCSI Commands specific to scanners include:
• Get Data Buffer Status (34H) - Gets information about the data buffer in the
scanner.
• Get Window (25H) - Gets information about previously defined scan window.
• Object Position (31H) - Provides positioning functions. Allows scanner element
to be moved.
• Read (28H) - Transfers scanned data, mask, or gamma information to the
computer.
• Scan (1BH) - Requests the scanner to begin the scan operation.
• Send (2AH) - Sends data from the computer to the device.
• Set Window (24H) - Sets up a image area window to be scanned.
Mode pages supported include:
• Page 03 - Measurements Units Page - Specifies the units of measurement
used for calculating the displacement of window and for positioning an object.
SIP
SCSI-3 Interlocked Protocol
(for parallel copper SCSI e.g., SPI)
This is an important new part of SCSI-3 that describes SCSI link protocols used for
the standard SCSI parallel interface. It describes updates to SCSI protocols that are
designed to allow coexistence with SCSI-2 devices on a SCSI-3 bus. SCSI-3 SIP
covers content and sequencing of SCSI bus phases. Messages are the main item
discussed. A new Target Transfer Disable message allows arrays to increase their
performance.
SMC
SCSI-3 Medium Changer Commands
(for separate medium changer devices)
SCSI-3 SMC is an important new part of SCSI-3 that describes commands specific to
medium changer devices (i.e., jukeboxes and autoloaders). Commands used to
access medium changers include:
• Exchange Medium (A6H) - Switches one element in one address with another
element in another address.
• Initialize Element Status (07H) - Causes the medium changer to check all of its
slots for media elements.
• Move Medium (A5H) - Moves a unit of media from one slot to another.
• Position to Element (2BH) - Positions the media changer mechanism over a
specified element.
• Read Element Status (B8H) - Causes the devices to report the status of internal
media elements to the user.
• Request Volume Element Address (B5H) - Transfers the results of the Send
Volume Tag command.
• Send Volume Tag (B6H) - Transfers a volume tag template to a medium
changer element.
Mode pages specific to medium changers include:
• 1F - Device Capabilities
• 1D - Element Address Assignment
• 1E - Transport Geometry Parameters
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Table 44. SCSI-3 files and topics (Continued)
File
Topic
Description
SPC
SCSI-3 Primary Commands
(common commands for all SCSI-3
devices)
Describes common SCSI commands that could apply to all SCSI-3 devices. It
describes the commands that must be implemented by all SCSI devices:
•
•
•
•
Inquiry
Request Sense
Send Diagnostic
Test Unit Ready
SPC updates commands, such as Change Definition and Inquiry, to support new
SCSI-3 device types. It covers basic medium changer commands such as:
• Move Medium
• Report LUNs
Additional mode pages described in SPC include:
• 09 - Peripheral Device Page
• 1A - Power Condition Page
• 1C - Informational Exceptions Control (Failure Logging)
In the industry, this is also known as SMART or PFA. SMART is an acronym for
Self-Monitoring, Analysis and Reporting Technology. PFA is an acronym for
Predictive Failure Analysis. Both of these controls protect user data from
predictable degradation in drive performance by analyzing information, such
as number of recoverable errors, number of retries or slowing performance,
and predicting when the drive is likely to die.
SPI
SCSI-3 Parallel Interface
(parallel copper hardware interface for
SCSI-3)
SCSI-3 SPI describes the physical SCSI interconnection for standard parallel fast
and wide SCSI. SCSI-3 SPI includes features such as the following:
• 16-bit transfers on a single 68-pin connector
• SCSI phase descriptions
• Having up to 32 SCSI IDs per bus, which necessitates a “fairness” scheme to
allow devices with low IDs to get on the bus
• 32-bit data path on a second 68-pin cable
• More than eight devices per bus
• Longer cables
It also covers hot-plugging drives and SCSI configured auto-magically (SCAM).
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Table 44. SCSI-3 files and topics (Continued)
File
Topic
Description
SPI-2
SCSI-4 (?) SCSI Parallel Interconnect 2
The SPI-2 standard began to appear in 1995, before even SCSI-3 SPI was
approved. This standard attempted to describe a physical layer that provides the
following additional functionality over regular SCSI-3 parallel SCSI:
•
•
•
•
•
•
•
•
•
•
Minimal compatibility problems with old hardware and software
Cost parity between single-ended and differential SCSI devices
Compatibility between single-ended and differential SCSI
Fast 40/Ultra-2 data transfers up to 40 MB/s narrow and 80 MB/s wide.
Fast 80/Ultra-3 data transfers up to 80 MB/s narrow and 160 MB/s wide
Enabling of low voltage chips (3.3V)
Allow SCSI device detection and setting of bus type (single-ended/differential)
Enable the use of new smaller cabling scheme
Enable hot-plugging of devices into backplanes
Enable moving devices through the use of Bus Extenders or Bridges
SPI-2 described:
•
•
•
•
•
Physical medium
Clock rates
Hardware drivers
Connectors
Cabling
A major feature is Low voltage differential SCSI (LVD SCSI). LVD SCSI allows
differential to be implemented without the need for expensive high voltage
drivers. It’s based on a networking standard. At speeds faster than Fast-20, singleended termination is unusable and only differential is allowed. It is believed that
cable lengths up to 12.5 meters can be allowed. LVD drives will automatically
switch to normal single-ended mode for backward compatibility on older SCSI
buses. SPI-2 will probably incorporate all of the original SPI specification.
SSA
SCSI-3 Serial Storage Architecture
SCSI-3 SSA corollary to the SCSI-3 specification describes the Serial Storage
Architecture used in implementations of serial SCSI.
For more information see Chapter 6, “All About Serial SCSI.”
SSC
SCSI-3 Stream Commands
(for tapes and printers)
SSC describes an important new part of SCSI-3 that describes commands related to
streaming devices (i.e., Tape, DAT, and printers). This part of SCSI-3 covers:
•
•
•
•
Sequential access devices
Printer devices
Processor devices
Communications devices
These devices handle data in a sequential fashion and cannot randomly access
data. Positioning is limited to either moving forwards or backwards on the
medium. A new command introduced is:
• Report Density Support - This command tells the caller what type of media the
drive supports.
Other commands from SCSI-2 are simply extended to support new devices or
clarified for documentation.
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Most projects are in X3T10 (formerly X3T9.2). FC-PH is in X3T11; SSA and
SSP are in X3T10.1; 1394 is in IEEE.
SCSI-3 SPI was approved as a standard in 1995 as X3T10/855D Rev 15a.
There is no concrete agenda for publishing the SCSI-3 specification, although
1997 seems realistic. Parts of the SCSI-3 specification began appearing as early
as 1991, and parts of it were approved in 1995.
NOTE
208
For information on many other resources for facts about SCSI, see “Appendix A: Additional
Information Sources,” on page 288 and “Appendix C: The Deep End on page 294.
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6
All About Serial SCSI
Overview
A revolution is occurring in the world of peripheral interconnections, bringing
with it higher levels of performance, availability, fault tolerance, and connectivity.
It’s coming with four primary serial I/O interfaces:
•
•
•
•
NOTE
USB
1394 (FireWire)
Fibre Channel
SSA
These interfaces are discussed in greater depth starting on page 211.
Universal Serial Bus (USB) and FireWire are low to medium speed buses
intended for desktop PC implementations. Fibre Channel and SSA are targeted at higher-end workstation and server markets and offer greatly
enhanced I/O channel throughputs previously seen only in large mainframe
computers.
These innovative, high-performance serial interfaces are designed to connect
a wide variety of devices to personal computers, workstations, servers, storage
subsystems, and networks.
Devices included the following:
•
•
•
•
•
•
•
•
•
disk drives
optical drives
tape drives
CD-ROMs
host adapters
RAID controllers
printers
video cameras
VCRs
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Traditionally, computer systems are tied together with parallel backplane
buses (such as SCSI and ATA). Although this has served well in the past,
designers are considering serial alternatives for four primary reasons:
•
•
•
•
physical constraints
cost
performance
reliability
Physical constraints
Systems are getting smaller, and the space available for connectors is rapidly
decreasing. Connector technology is having trouble keeping pace with the
changes.
STOP
Cabling problems account for 50 percent of disk drive-related customer service calls.
Cost
Semiconductor technology is still on a fairly fast evolutionary path, while
connector and inter-connect technology is relatively mature. For a fixed level
of performance, the cost of an interconnect drops much faster if its implementation is semiconductor intensive instead of connector intensive.
Performance
Parallel data is reaching its limits in subsystem topologies. It is increasingly
difficult to run large amounts of data over any distance with speeds approaching 40 MB/s. Servers, networks, and video applications, to name a few, are
demanding I/O in excess of 40 MB/s. This is very difficult to do with parallel
interfaces.
Reliability
The primary point of failure in an interconnect is usually its physical connection. The fewer the signals, the simpler (and more reliable) the connector.
Now that lines are being drawn in the sand and more and more companies are
starting to seriously consider using serial interfaces, what are the facts behind
each of these technologies?
Current Serial Peripheral Interfaces range from low speed utility buses to very
high performance network architectures. The following is a brief overview of
four of these interfaces. This overview provides general information on performance and typical applications with an emphasis towards mass storage and
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RAID technologies. Each of the interfaces discussed is detailed in a standards
document that contains more in-depth technical information.
NOTE
For information on how to obtain copies of these standards documents, see “Appendix A:
Additional Information Sources” on page 288.
USB
Universal Serial Bus (USB) is a specification jointly agreed upon by a working
group consisting of Compaq, DEC, Intel, Microsoft, NEC, and Northern Telecom
USB is a 12 Mb/s interface supporting up to 63 peripheral devices. USB is
intended to support low-speed peripherals such as keyboards and pointing
devices, and is not suitable for mass storage or video applications. USB supports automatic configuration (“plug-and-play”) and hot-plugging. USB began
appearing on PC motherboards in 1996.
STOP
While USB doesn’t implement Serial SCSI, it competes with 1394.
1394 (FireWire)
Computer
Disk 1
Disk 2
Disk 3
Disk 4
Disk 5
FireWire
Computer
Controller
1394 (backplane environment)
Controller
Computer
Disk 1
Disk 2
Disk 3
Disk 4
Disk 5
1394 (cable environment)
Figure 65. 1394 (FireWire) theoretical physical topology
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1394, also known to some as FireWire, is a low-level, low-cost serial interface
that is designed for multiple consumer applications. FireWire is a joint Apple
and Texas Instruments (TI) implementation of the IEEE 1394-1995 SerialBus
standard. 1394 was approved as an IEEE standard in December 1995. It is a
high-speed (100 to 400 Mb/s) serial bus supporting up to 64 peripheral devices.
It is intended to replace the Apple Desktop Bus (ADB).
FireWire includes the following features:
• The ability to layer SCSI protocols onto the interface. This allows for SCSI
devices to be easily adapted.
• Support for automatic configuration (“plug-and-play”) and hot-plugging.
• Fully isochronous. This means that a fixed slice of bandwidth can be
dedicated to a particular peripheral, such as video. This support of
isochronous service is a complement to such LAN architectures as ISDN,
ATM and FDDI-2.
I/O Interconnects can be viewed in two ways:
• as a type of extended memory space
• as an I/O “channel” interface
When viewed as memory space or registers, the target devices are accessed by
processor type commands, such as “read” or “write.” A channel interface uses
a higher-level protocol where commands to “read logical block” or “print
page” would be used. FireWire uses the “memory space” interconnect model.
The serial bus is broken into two parts:
• The first part is the “backplane environment,” which adapts to the host
parallel bus.
• The second part is the “cable environment” and is specified in the 1394
document as supporting up to 16 physical connections (cable hops)
between any two devices. Each hop can be up to 4.5 meters in length,
yielding a total cable distance of 72 meters.
STOP
1394 also specifies a low-voltage, GameBoy-like, differential type cable. No termination is
required. It appears that 1394 will be used mainly in consumer audio/video products, set-top
(cable) boxes, scanners, and printers. 1394.2, with its 1.25 Gb/s speed, should be fully specified in
1997, with products launched in 1998. It is targeted toward workstation clustering but should
also be used for more mainstream applications.
The first peripherals with 1394 interfaces were released in 1995 and included
Digital Video camcorders from JVC, Sony, and Panasonic. Vendors supporting
1394 include Apple, TI, IBM, Sony, and Microsoft. In 1996, future versions of
the 1394 specification were announced, referred to as 1394.1 and 1394.2. They
are specified to offer throughput from 800 Mb/s to 1.2 Gb/s.
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STOP
To obtain the final version of the IEEE 1394 standard, write to the Institute of Electrical and
Electronic Engineers, Inc., Customer Service, 445 Hoes Lane, P.O.B. 1331, Piscataway, NJ 08855.
Call them (800) 678-IEEE, fax them (908) 981-9667, or e-mail them customer.service@ieee.org.
Fibre Channel
Array
Controller
100MB/s
Loop 1
Loop 1
NL_Port
NL_Port
Loop 1
Device
Fibre1Channel Device 2
NL_Port
Host
Adapter
NL_Port
NL_Port
NL_Port
NL_Port
Device 3
Device 126
NL_Port
NL_Port
Loop 2
Loop 2
NL_Port
Loop 2
100MB/s
Cache
Loop 2
Loop 2
NL_Port
NL_Port
NL_Port
NL_Port
Device 1
Device 2
Device 3
Device 126
NL_Port
NL_Port
NL_Port
NL_Port
Loop 1
Loop 1
Cache
Figure 66. Fibre channel arbitrated loop
Background
Fibre Channel Arbitrated Loop (FC-AL) is a byproduct of a Fibre Channel-like
protocol that has been around for a couple of years (used in ESCON, IBM’s
peripheral interface) and an extension of the IPI interface. It’s geared toward
connecting large host computers to storage subsystems and network hubs.
The Fibre Channel standard was initially developed by the Fibre Channel
Specification Initiative (FCSI) in February 1993. FCSI was a consortium that
included Hewlett-Packard, Sun Microsystems, and IBM. The Fibre Channel
Association (FCA) formed in August 1993, and in 1995 inherited the development and maintenance of the Fibre Channel standard from FCSI.
The Fibre Channel interface is a loop architecture, as opposed to a bus-like
standard, such as SCSI or IPI. The Fibre Channel loop (Figure 66) can have any
combination of hosts and disks up to a maximum of 126 devices.
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Fibre Channel supports multiple protocols: SCSI, IPI, TCP/IP, and ATM. All
these protocols can run at the same time over the same cabling.
Although Figure 66 suggests that the drives are connected by a cable running
from device to device, the preferred approach is to attach disk drives directly
to a backplane. To eliminate cable congestion, make hot plugging practical,
and simplify mechanical designs.
In fact, because the Fibre Channel drives use a shortened version of the SCA
40-pin connector used on parallel SCSI drives, they can easily be designed to
fit into existing SCA drive cabinets.
STOP
“SCA” stands for single connector attachment. It is a high-density connector that carries every
type of signal passed along a SCSI bus, including SCSI ID, LED, and spindle synchronization
information. Due to its timed pins, it is particularly suitable for hot-swapping—removing and
inserting removable drives while the system is powered on. Timed pins allow for removing power
first and ground last when a drive is disconnected, and attaching ground first and power last when
a drive is plugged in.
Loop structure enables the rapid exchange of data from device to device. A
Port Bypass Circuit (PBC), located on the backplane, is the logic that enables
devices to be removed or inserted without disrupting the operation of the
loop. In addition, PBC logic can take drives off-line or bring them back on-line
by sending a command to any device to remove it from loop operation or reinstall it onto the loop. Contrast the simplicity of the loop architecture used by
Fibre Channel (Figure 66) to the parallel SCSI implementation (Figure 67).
Features of Fibre Channel Arbitrated Loop are listed in Table 45.
Table 45. Features of a fibre channel arbitrated loop
Feature
Value
Maximum Number of Devices
126
Maximum Data Rate
214
• 100 MB/s (1.062 GHz using an 8B/10B code)
• 200 MB/s (dual loop)
Maximum Cable Distance
30 meters between each device using coaxial cable (longer, with other
cabling options such as optical, which can go 10 kilometers between devices)
Cable types
Twinaxial, Coaxial, Optical, Backplane
Fault tolerance
Dual Porting, Hot Plugging Drive Switches
Jumpers
None
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Channel 1
Array
Controller
20MB/s
1
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 1
Channel 2
20MB/s
Cache
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 15
SCSI Parallel
Channel 3
20MB/s
RAID
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 15
Channel 4
20MB/s
Host
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 15
Adapter
Cache
Channel 1
Array
Controller
20MB/s
2
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 15
Channel 2
20MB/s
Cache
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 15
Channel 3
20MB/s
RAID
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 15
Channel 4
20MB/s
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 15
Figure 67. SCSI parallel implementation
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Why use Fibre Channel?
SCSI has become the interface of choice for medium- and large-sized computing systems. At the same time, the computer industry’s experience with SCSI
has brought to light the need for improvements in:
•
•
•
•
•
cable distance
data rates
command overhead
array feature support
connectability
Cable distance limitations
SCSI comes in several different versions, but the majority of products shipping today are of the single-ended variety. (That is primarily because singleended SCSI costs less and is widely available.) If a SCSI bus is limited to connecting devices found within a single cabinet, and the interface cable length
does not exceed three meters, it is usually not a problem to use single-ended
SCSI. The potential for SCSI signal problems increases, however, if the bus
must link several cabinets, converting from an unshielded ribbon cable in one
cabinet to an external shielded cable, and then back again to a ribbon cable
within the second cabinet. Differential SCSI solves this cabling issue, but usually requires that a system have both single-ended and differential ports
because most non-disk peripherals use only single-ended SCSI.
STOP
You can’t attach a single-ended SCSI device to a differential bus. You can’t attach a differential
SCSI device to a single-ended bus. You can have single-ended and differential devices attached to
the same system, but currently you can’t have them attached to the same bus on the system.
Higher data rates
Magnetic hard disk areal density is increasing at about 60 percent per year. Bit
density (measured in bits per inch) is one of the two components of areal density. It increases at about 30 percent per year, and the data rate automatically
increases proportionately. Disk rotation speeds have almost tripled over the
last five years from 3,600 to 10,000 RPM, and they continue to climb. They
also contribute to higher data rates. In the next few years, drives will be introduced that can sustain transfers in excess of 20 MB/s, thanks solely to
improvements in bit density and rotational speed. Already there are disk
drives on the market that can transfer data at rates over 10 MB/s. An interface
limited to 20 MB/s can support only one drive at that data rate, making it
technically impractical for future applications of faster drives.
STOP
216
SCSI protocols run easily on Fibre Channel, making it a cabling solution that can be quickly
implemented on drivers and hard drives.
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Fibre Channel also has benefits that go beyond current disk drive interface
solutions. When compared to conventional interface technologies, Fibre
Channel provides:
• significantly higher bandwidth
• superior drive array features
• improved network storage capabilities
Bandwidth
A Fibre Channel loop supports data rates up to 100 MB/s. Slower versions are
specified at 25 to 50 MB/s, but these aren’t popular. Multiple loops of drives
can achieve even more. Among the applications growing in popularity that
demand this kind of data rate are video storage and retrieval, supercomputer
modeling, and image processing. Moreover, as file servers replace mainframe
computers, they will require ever-higher transaction rates to provide comparable levels of service. Using the FC-4 protocol, drives are attached through the
Direct Disk Attach Profile protocol (DDA).
Since most UNIX and Windows servers lack the sophisticated I/O channel
and controller structures of mainframe computers, they have not been able to
match the large number of high-performance disk drives that enterprise systems can support. Fibre Channel loops attached to such high-performance
buses as S-Bus or PCI—both of which run 70 MB/s or faster—offer I/O configurations that can sustain mainframe-like I/O rates. Performance estimates
suggest that if a system requests the relatively short I/O transfers typical of
business transaction processing (8 KB or less), more than 60 drives can be supported without saturating the loop and bogging down performance.
Comparing the single host adapter to the many channels and controllers
mainframes employ to attach the same number of drives illustrates the
remarkable economics of Fibre Channel-attached disk storage. With up to
23 GB per disk available, 63 drives on a loop would make over 1,449 GB available to a user on a single FC-AL host port. It will be possible for any workstation or system that has a single FC port or backplane slot to become a file
server. No longer will file servers be characterized by large rack-mounted systems. With FC, they can be as small and unobtrusive as any office system.
Remote Online storage
Since the Fibre Channel interface is part of (and fully compatible with) the
Fibre Channel standard, optical cabling can be used in any part of a subsystem
except the backplane. This makes it possible to have a disk subsystem quite a
distance from the computer system it is attached to. Using single-mode Fibre
optics, online disk storage could be as far as six miles away.
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Fibre Channel with optical transmission
A typical computer system could have disks within the system unit attached
via the FC loop. The loop can be extended by using an electronic-to-optical
adapter and lengths of fibre-optic cable. On the same loop running the internal disks, remote disks would appear to the system to be directly attached, in
the same manner as the local disks. These could be used in any fashion,
including mirroring the local disks.
It makes a very attractive means for having a remote online copy of critical
data, which could be used to continue operations should anything happen to
the original computer system.
STOP
For sources of information on Fibre Channel, see “Appendix A: Additional Information Sources”
on page 288.
Serial Storage Architecture (SSA)
SSA is based on a proven storage subsystem (the 1933) developed by IBM’s
European subsidiary in Havant, England. In 1993, IBM opened this system
architecture and virtually gave the design to any company that asked for it. In
1995, the 1933 division bought itself out from IBM and is now called Xyratex.
STOP
IBM still has control over the development and manufacturing of future SSA silicon.
SSA is a powerful high-speed serial interface designed to connect high-capacity data storage devices, subsystems, servers, and workstations. Designed
with the future in mind, SSA facilitates migration from current SCSI equipment and will accommodate implementation of future configurations, including the use of fibre-optic connections. Only four signal wires are required,
compared to 68 for the closest SCSI equivalent. SSA interfaces require no
address switches and no discrete terminators.
SSA architecture operates at two protocol levels. The lower level, SSA-PH (for
SSA Physical), defines the electrical characteristics, coding scheme, and
method of information transmission. The upper level maps SCSI commands
into a form suitable for serial transmission. As a result, devices now using
SCSI-2 can be upgraded to SSA with minimal code changes.
SSA offers superior performance over the interfaces commonly used today.
Each SSA link is full-duplex and frame multiplexed simultaneously, resulting
in data rates in the 20 MB/s range in each direction, with total throughput of
80 MB/s at each node (peripheral). As many as 127 devices can be connected
per loop, with “hot plugging” supported. There can be up to 20 meters
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(approximately 65 feet) between nodes (peripherals). With optical cabling,
there can be up to 680 meters—approximately 2,231 feet—between nodes.
The combination of low power, fewer components, and fault isolation makes
SSA highly reliable. Each node performs extensive error checking, and if
errors occur, SSA provides transparent frame recovery. When SSA devices are
configured in a loop (Figure 68), alternate paths to each device ensure that
there is no single point of failure.
Array
Controller
Host
Adapter
Cache
Port 1
Initiator
1
Port 2
Cache
2 x 20MB/s
SCSI Port
SCSI Port
SCSI Port
SCSI Port
DeviceSSA
1 1
Device 2
Device 3
Device 126
2 x 20MB/s
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 126
Figure 68. SSA single-domain implementation
Besides enjoying SSA’s performance benefits, users will also appreciate its
compact four-wire cables and connectors, which have an overall diameter of
less than 6 mm. SSA cables provide a welcome contrast to the cumbersome
maze of 50- and 68-wire cables that mark a typical SCSI installation. The
reduction in cabling yields a substantial cost improvement to customers. It
provides higher levels of performance, fault tolerance, data availability, and
connectivity than is possible with today's parallel interfaces and is ideally
suited for video applications, data servers, and “mission critical” data.
STOP
SCSI protocols operate on top of SSA link protocols, allowing drivers and peripherals to be easily
adapted.
Among the functions provided with SSA is a unique capability called “spatial
re-use,” which allows data to be transferred concurrently at high speeds
between many pairs of serially connected peripherals, without the need to
route the data through the processor. This leads to more effective use of the
system’s resources in environments where users may be moving data from a
CD-ROM to memory, from a disk drive to a tape backup unit, and from memory to a printer, all at the same time from the same processor.
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SSA offers excellent performance. Its fundamental building block is a single
port capable of carrying on two 20 MB/s conversations at once—one inbound
and one outbound. An SSA node consists of two ports, allowing four conversations to be carried on simultaneously, for a total interface bandwidth of
80 MB/s.
STOP
Future versions of SSA will support 40 MB/s links resulting in 160 MB/s total throughput.
Multiple domain configurations (Figure 69) further increase reliability and
performance.
Port 1
Initiator
1
Port 2
Host
Array
Controller
Domain 1
2 x 20MB/s
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 126
2 x 20MB/s
SSA 2
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 126
Adapter
Cache
Cache
Port 1
Initiator
2
Port 2
2 x 20MB/s
Domain 2
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 126
2 x 20MB/s
SCSI Port
SCSI Port
SCSI Port
SCSI Port
Device 1
Device 2
Device 3
Device 126
Figure 69. SSA dual-domain implementation
SSA’s dual-port, full-duplex architecture allows peripherals to be connected in
loop configurations designed to contain no single point of failure. SSA’s serial
interface loop significantly improves connectivity, while its auto-configuration capability allows the true potential for hot-plugging, plug-and-play, and
dynamic reconfiguring. Low cost is achieved through the use of compact
cables and connectors and by embedding all circuitry into a single CMOS
chip. SSA is fully compatible with the Fibre Channel Interface (FCI), which
allows interconnection of processors separated by large distances.
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SSA is supported by an increasing number of silicon, adapter, storage, and system manufacturers, including members of the SSA Industry Association
(SSA IA).
STOP
The SSA IA was formed in 1995 to promote the acceptance of the SSA standard within the
industry. Its membership includes more than 25 manufacturers of systems and peripheral
products.
Fibre Channel Enhanced Loop
The Fibre Channel Enhanced Loop (FCEL) standard was submitted to ANSI
for consideration in September 1996. It merges FC-AL and SSA technology.
FCEL was developed to reduce confusion and offer a more cohesive future
growth path. Its development was led by IBM, Seagate, an the Fibre Channel
Association. Look for FCEL products in the year 2000.
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7
Other Storage Interfaces
Even though SCSI dominates its niche, it’s not the only player in the interface
game. Other technologies preceded SCSI and in some marketplaces have displaced it, primarily because of cost, compatibility, and performance considerations. Chapter 7 describes the various other interfaces that dominate the PCcompatible marketplace.
ST-506/ST-412
In 1983, Al Shugart’s Seagate Technologies introduced the ST-506 as a 5.25inch, 5 MB hard drive on the IBM XT. Seagate’s interface became the most
popular for low-capacity MFM (modified frequency modulation) drives.
Seagate later introduced the ST-412, which provided 12 MB of capacity.
STOP
Rumor has it that Seagate’s name came about from Shugart’s desire to have his company name
begin with S, have a G in the middle, and T toward the end. His own name was already in use and
consequently unavailable to him.
These early Seagate models provided a low-level serial interface. They were
inexpensive to implement, not very extendable, and slow, with a one-bit-at-atime 500 to 1000 KB/s transfer rate. The initial ST-506 drive had a 625 KB/s
(5 Mb/s) internal transfer rate. Utilizing a 1:1 interleave on the controller, the
peak data transfer rate was about 500 KB/s. The controller needed to have a
track buffer to handle a 1:1 interleave. It was used only on hard drives and was
known as a dumb interface because the drives had to be told how to access
data by an external controller card. Two cables were used:
• a 34-pin drive control cable
• a 20-pin data cable
The control cable contained signals to select the drive, seek the head, and
select a cylinder, head, and sector of the drive.
The data cable transmitted data in a differential format. The controller card
was programmed through the use of “Command Descriptor Blocks.”
STOP
A Command Descriptor Block is typically a six-, ten-, or twelve-byte data structure residing in
the computer’s memory. It contains the command code and other information needed by a target
to execute a command. See “SCSI Command Descriptor Block” on page 295 for examples of
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Command Descriptor Block structures. See “ST-506 Communications” on page 317 for tables
listing ST-506 commands and data signals.
Seagate wanted ST-506 to become a standard so that the computer industry
could pattern its products on a single interface, assuring users of compatibility. Seagate placed the ST-506 interface in the public domain and provided
specifications to anyone interested. The specifications eventually became the
IEEE 412 standard.
ST-506 drives were configured for operation through the Setup program
present in the ROM of the PC. The Setup program configured the CMOS configuration memory with the number and type of drives in the system. The
types of drives supported were typically limited to a number of fixed choices.
ST-506 controllers typically had a formatter program embedded in ROM,
accessible only through DEBUG.
STOP
CMOS is an acronym for Complimentary (symmetry) Metal-Oxide Semiconductor. It is a
microprocessor (memory chip) that permits many components to be packed together in a very
small area.
With the introduction of faster, more powerful chips, manufacturers turned to
RLL (run-length limited) encoding to meet enhanced I/O capabilities and storage requirements.
STOP
RLL (run-length limited) is an encoding scheme invented at IBM that employs a set of rules to
determine the pulse pattern for each bit, based on the value of the preceding bits. RLL allows
50 percent more information than MFM (modified frequency modulation) to be recorded on a
track. This is accomplished by recording more fluxes for every byte and packing them more
tightly onto the surface of the hard disk. It is also referred to as 2,7 RLL because the recording
scheme involves bit patterns with no more than seven and no less than two successive zeros.
ST-506 did not mandate one type of encoding, so these drives could be
encoded with RLL by switching controllers. RLL encoding crammed more
data onto the disk and improved data transfer rates by up to 50 percent, necessitating a faster interface. MFM drives had 17 sectors per track, whereas RLL
drives had 26 sectors per track. A 1:1 interleaved RLL drive with 26 sectors per
track had a transfer rate of about 800 KB/s.
NOTE
For more information, see “Encoding processes” on page 45.
Later in 1984, with the advent of the IBM AT personal computer, ST-506
drives were interfaced in a similar way to the IBM XT, except that all I/O port
addresses were moved and a secondary port was set up for another controller.
The controller was made by Western Digital and was known as the WD1003WA2. This controller and the need for backward compatibility has impacted
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the drive industry ever since. In the PC world, compatibility with standards is
extremely important.
STOP
There are two types of standards: de facto and agency. De facto standards are set by an industry
over time. Agency standards have been submitted to, reviewed, and approved by, a standards
agency. ANSI and IEEE are standards agencies.
ESDI
Enhanced Small-Device Interface (ESDI) is an improved ST-506 interface that
was developed in the mid-1980s and was championed by Maxtor. ESDI doubled ST-506’s data transfer rate and allowed for selective reformatting of
tracks. Otherwise, mechanically and electronically it was virtually identical
to ST-506. It was device-specific and only used for hard disk drives, mostly
those with very large capacity. It offered a maximum capacity of about 1 GB.
These drives typically used RLL encoding to boost capacity. Two cables were
used to interface drives:
• a 20-pin data cable
• a 34-pin control cable
ESDI was slightly less expensive to implement than SCSI, but typically
allowed only two devices to be connected. It could continuously transfer data
at up to 24 Mb/s. ESDI didn’t require device drivers, which gave it an I/O
boost. ESDI drives typically used a 5.25-inch form factor. They were appropriate mainly in servers used for networking PCs.
ESDI’s main improvements were due to separate data-separator chips that
were responsible for:
• encoding data and clocks prior to recording on the disk
• decoding data and clocks after reading them off the disk
Seeks to different tracks required only a single step command, in contrast to
ST-506, which required three. One drawback was that the controller speed had
to be synchronized with the speed of the hard drive. Getting a new drive
meant getting a new controller. It also required a separate controller board. Up
to seven drives per controller could be connected, although only two ports
were provided.
Attempts were made to add tape and optical support, but no devices ever
appeared. The standard eventually became the ANSI X3.170-1990 specification.
STOP
224
Some early optical drives used an ESDI interface between the drive and the drive’s controller.
This provided a SCSI connection to the host.
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IDE
IDE stands for Intelligent Drive Electronics or Integrated Drive Electronics.
IDE incorporates controller hardware onto the disk drive, which results in significant parts reduction and cost savings. IDE combines the physical and electrical protocols of ST-506 with the speed of ESDI. IDE drives can be “directly”
plugged into a computer’s expansion bus, providing the following benefits:
•
•
•
•
A slot is saved.
The need for a host adapter is eliminated.
It’s extremely easy to use.
It’s cheaper than SCSI.
The connections are identical to the PC AT bus, minus some of its 98 lines.
Lines made obsolete by IDE include DMA, REQ and ACK, IRQs except 14,
and memory strobe lines. It was offered in 8- and 16-bit parallel interface
types.
STOP
Eight-bit drives, such as the MiniScribe 8450XT, are referred to as XT Interface Drives. Sixteenbit are referred to as Standard AT Interface Drives. They are not interchangeable.
IBM even created its own version of IDE that allowed connection to its proprietary MicroChannel Architecture (MCA) bus present in early IBM PS/2 personal computers. This was now a parallel interface, offering a significant
performance boost from the serial architecture of ST-506 and ESDI.
STOP
In a serial architecture data is transmitted one bit at a time. In a parallel architecture each byte
(eight bits) is sent simultaneously using separate lines. Parallel architecture allows for much
faster I/O than serial.
IDE is often used interchangeably with the terms AT or ATA, although ATA
specifically references an implementation of IDE designed for the AT bus
(either ISA or EISA bus). IDE denotes a drive that has a controller on board,
hence a SCSI drive could be technically called an IDE drive.
STOP
AT stands for Advanced Technology. ATA stands for Advanced Technology Attachment. ATA-2 is
an ANSI agency standard, while IDE is a de facto standard.
Origins of IDE
In the mid-1980s the initial champions of this technology included Compaq,
Conner Peripherals, Imprimis and Western Digital. Compaq wanted to reduce
the cost of the computer by eliminating host adapters. It also wanted to create
laptops that didn’t require that valuable slot real estate be taken up by host
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adapters. These and several other hard disk manufacturers produced a Common Access Method (CAM) specification that was submitted to ANSI in 1990
and became the X3.221-1994 standard. CAM defined a common method of
accessing the hard disk on a personal computer. IDE became one of the most
popular implementations of CAM.
Always gaining in popularity, IDE muscled out ST-506 because it offered more
performance and capacity and, like ESDI, it did not require software drivers
but used a standard WD1003, chip-based translation mode instead.
IDE is typically implemented using a standard, 40-pin, 80 to 120 ohm impedance, unshielded, unterminated, flat ribbon cable. It costs significantly less to
implement than SCSI:
• Single chip VLSI implementation of IDE is available for less than five
dollars, compared to fifty for SCSI.
• An IDE host adapter (nicknamed “paddle card”) can cost five dollars
where a SCSI host adapter can cost one hundred dollars.
• An IDE drive can be as much as fifty dollars cheaper than a SCSI drive.
STOP
VLSI stands for very-large-scale integration. It’s a chip with 20- to 900-thousand logic gates per
chip. Transistors form logic gates that control the flow of electrons through chips.
The cost differences result from a combination of cheaper parts, larger markets, and economies of scale. IDE became so prevalent that in 1995 new 2.5inch laptop drives completely switched over to IDE and ceased to be available
in SCSI, forcing Apple to switch as well.
IDE and SCSI drives share a lot of electronics in common, so you will often
find the same hard drive model offered with either interface. As a rule, the
drive industry produces more IDE models in the lower capacity range and
more SCSI models in the higher capacity range.
Original IDE did have some severe limitations:
• Drive capacity was limited to 528 MB (up from 40 MB of ST-506).
• Burst transfer rate was limited to a range of 4.0 to 8.33 MB/s (sustained
data transfer rate was in the 1 to 2 MB/s range).
• Connectivity was limited to only two drives.
• Compatibility was limited to only hard drives.
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IDE details
NOTE
See “IDE data signals” on page 319 for tables that list and describe IDE commands, data signals,
and registers.
Masters and slaves
With IDE, one drive is a master and the other a slave, even though they don’t
control each other.
STOP
A master ensures data transfer to one or more slave stations. There can be only one master on a
data bus at a given time. A slave is the target selected by the master station to receive data.
A jumper setting on the drive sets the drive into either mode. The master
drive performs signal decoding for the slave. Many drives support multiple
master/slave modes. These modes include:
• ISA original
• Conner
• ATA/CAM
The ATA/CAM mode is not backward-compatible with ISA or Conner. The
ISA original method has a couple of drawbacks:
• It’s not able to tell when the slave drive is ready.
• It doesn’t have an activity indicator if two drives are connected.
Conner added a method of determining when the slave drive was ready for the
host. The ATA/CAM method is similar to Conner’s method, but the polarity
of one pin is reversed.
Handling bad sectors
Defective sectors on the disk are typically marked as such at the factory and
not utilized. If a defect is encountered later, the drive will usually automatically map out the bad sector and utilize a spare sector. Some IDE drives can
also have tracks reformatted to spare out bad sectors.
IDE data transfers
IDE data transfers are handled via PIO (programmed I/O), where the processor
handles the data transfers. These transfers are “blind” transfers because there
is no handshaking to synchronize the transfer. Drives use the I/O Channel
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Ready (IORDY) signal to determine when I/Os were ready. The original PIO
transfer rates are listed in Table 46.
Table 46. IDE data transfer rates
Mode
Burst Speed
PIO Mode 0
3.33 MB/s (600 ns per word)
PIO Mode 1
5.22 MB/s (383 ns per word)
PIO Mode 2
8.33 MB/s (240 ns per word)
Singleword DMA Mode 0
2.08 MB/s (960 ns per word)
Singleword DMA Mode 1
4.17 MB/s (480 ns per word)
Singleword DMA Mode 2
8.33 MB/s (240 ns per word)
Multiword DMA Mode 0
4.17 MB/s (480 ns per word)
The typical PIO data transfer on a PC occurs in the following sequence:
1. Drive asserts IRQ14 (interrupt request) to indicate it is ready.
2. Data is transferred in 512-byte sectors.
3. Host drives IOR (I/O read) or IOW (I/O write) lines according to selected
transfer mode.
4. If drive can’t support or receive data fast enough it de-asserts IOCHRDY
to throttle transfer.
STOP
An IRQ is an interrupt request line. It is the line that carries hardware interrupt signals to the
processor on a PC.
The typical DMA data transfer occurs in the following sequence:
1. Host sets up DMA controller.
2. Read or Write DMA command issued.
3. Drive asserts DMARQ when it’s ready to transfer data.
4. DMA controller responds with DMACK when bus control is granted.
5. Host drives IOR or IOW lines according to selected transfer mode.
6. Transfer finishes with DMARQ or DMACK dropped.
7. If drive can’t support or receive data fast enough, it de-asserts DMARQ
to throttle transfer.
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IDE limitations
The most severe limitations were imposed as a consequence of the requirement for backward-compatibility with BIOS software that used the INT13
BIOS call that came with the original Western Digital WD1003 controller.
This backward-compatibility allowed for IDE’s transparency in not requiring
new drivers for DOS and Windows 3.1. Other operating systems were not
affected. The INT13 call only allowed for 1024 cylinders, 255 heads, and 63
sectors to be accessed.
STOP
8.4 GB IDE limited access to 65536 cylinders, 16 heads, and 255 sectors per track.
The IDE task file registers limited the number of heads to 16, so the total
capacity that could be accessed was 1024 × 16 × 63, which equals
1,032,192 sectors, 528,482,302 bytes, or 528 million bytes.
STOP
It is referred to as the 528 MB limit although it is really 516 MB in true mathematical terms.
Table 47 illustrates the data transfer limitations on a PC imposed by the combined limitations of BIOS and IDE.
Table 47. IDE data transfer limitations on a PC
Cylinder
Head
Sector
BIOS Limits
1024 (10 bits)
255 (8 bits)
63 (8 bits)
IDE Task File Limit
65535 (16 bits)
16 (4 bits)
255 (8 bits)
Least Common Denominator
1024
16
63
Many vendors such as AMI, Award, and Phoenix bypassed this capacity limit
by extending the allowable cylinder number by two or four bits. Other disk
utility software also allowed this limit to be circumvented.
The ability to daisy-chain two drives from different vendors often did not
work because of loopholes in the specification. IDE still did not possess the
capability to multitask multiple commands like SCSI. With IDE, once a command was issued, the operating system had to wait until the command was
complete before going on to another command. Power management was
included in the original specification.
STOP
Power management is an energy-saving feature. It is the ability of a system to recognize when it
has been inactive for a (usually) user-specified amount of time, and consequently to spin down.
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PC IDE implementation
PC IDE
Figure 70. PC PCI IDE implementation
IDE drives are configured for operation in the computer’s Setup utility. A
modern BIOS automatically finds the capacity and configuration of an IDE
drive. There is no longer a fixed amount of drive table entries in the BIOS as
there was with ST-506 drives.
NOTE
230
For information on the commands used with IDE, see “IDE Communications” on page 319.
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DOS applications use interrupts to communicate with devices. With IDE,
applications access the hard drive using the call chain illustrated in Figure 71.
INT21 call from Application
(read, write, seek, et al.)
calls
INT13 call from BIOS
(creates AT task file)
calls
Hard Disk Controller
(controls drive’s electronics)
Figure 71. IDE application call chain
STOP
An interrupt (INT) is a momentary suspension of processing caused by a deliberate instruction to
interrupt the microprocessor.
Applications can use INT25 or INT26 to access the drive directly through the
device manager.
NOTE
See “PC IDE: INT13 call codes” on page 323 for the activities available through INT13.
DOS lays out data on the disk in a specific format. This format is laid down
when a drive is high-level formatted with the DOS FDISK and FORMAT commands. FDISK is used for hard disk partitioning. It contains the specifications
for partition size and assignments of the boot partition and the drive letter—
this will always be “C,” unless there are multiple partitions. FORMAT lays
out the DOS structure on the disk and establishes how and where files can be
stored. Figure 72 illustrates a typical DOS disk layout after FDISK and FORMAT have been run.
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Master Boot Record
Location: Cylinder 0, Head 0, Sector 1
Content: • Master boot info and program
• Partition table (location, start, end, and size of partition)
Partition 1
Location: Cylinder 0, Head 1, Sector 1
Content: • Boot record (OEM ID, BIOS parm block, loader)
• File Allocation Table (FAT)
• Copy of FAT
• Root Directory of Disk
• File Area
Partition 2
Location: Cylinder X, Head X, Sector X
Content: • Same as Partition 1
Partition 3
Location: Cylinder X, Head X, Sector X
Content: • Same as Partition 1
Partition 4
Location: Cylinder X, Head X, Sector X
Content: • Same as Partition 1
Figure 72. DOS disk layout (3.0 and newer)
A primary partition is the first active partition on the disk. Boot files are
always stored on a primary partition. On any disk, there can be only four primary partitions. Older DOS versions could support primary partitions of up to
32 MB.
Extended partitions are separate logical partitions that can be up to 8 GB in
capacity. There can be many extended partitions because they include their
own partition tables.
STOP
232
Partition tables store information on the location, starting point, ending point, and size of each
partition on a hard disk.
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Table 48 lists the size limitations for partitions on various PC operating systems and the file systems each OS uses.
Table 48. Operating system drive capacity access limitations
STOP
Drive Size
Partition Size
File System
DOS 6.X
8 GB
2 GB
FAT
Windows 3.X
8 GB
2 GB
FAT
Windows 95
137 GB
2 GB
VFAT
Windows 95 w/Service Pak 2
2 TB
2 TB
FAT32
Windows NT
2 EX
2 EX
NTFS
“EX” stands for exabyte, which is 1018 or 1,000,000,000,000,000,000 bytes. FAT stands for file
allocation table. Its purpose is to keep track of the locations of blocks in which the operating
system has written data to a disk. (The “V” in VFAT stands for virtual.) NTFS stands for New
Technology File System. It is Microsoft’s proprietary system for tracking location of data on a
hard disk.
The FAT File System is also encumbered by cluster size limitations (Table 49).
Table 49. FAT cluster size limitations
Disk Size
Cluster Size
128–255 MB
4 KB
256–511 MB
8 KB
512–1023 MB
16 KB
1024–2048 MB
32 KB
Cluster size is the smallest unit of allocatable space. A file on a 1 GB drive
that contains only one character nevertheless uses up 32 KB of space, because
32 KB is the smallest cluster or allocation unit allowed on a 1 GB drive. On
Windows NT, the NTFS file system utilizes a 4 KB cluster size for all volume
sizes.
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Macintosh IDE implementation
Mac IDE
Figure 73. Mac OS IDE implementation
Several Macintosh computers have an IDE internal storage interface. Apple
began using IDE in 1994, starting with the Quadra 630 and PowerBook 150.
Apple’s version of IDE is very similar to that used by the PC industry.
The block diagram in Figure 73 shows how IDE was interfaced to the Macintosh operating system. The ATA Manager ROM-based software provides an
application programming interface between IDE drivers and the IDE interface
hardware. IDE drivers provide partition, data and error handling services to
the Macintosh operating system. These drivers make operating on IDE drives
transparent to applications that communicate through the normal File Manager interface. Power management is supported.
NOTE
234
See “IDE pin assignments” on page 324 for a table listing the pin assignments on IDE connectors.
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To match the big-endian format of Motorola’s MC68030-compatible bus, the
bytes are swapped:
• The lower byte of the IDE data bus, DD(0–7), is connected to the high byte
of the upper word of the I/O bus, IOD(24–31).
• The higher byte of the IDE data bus, DD(8–15), is connected to the low
byte of the upper word of the I/O bus, IOD(16–23).
STOP
The term “word” is used here to denote the number of bits that constitute a common unit of
information in a particular computer system. In this instance a word is 16 bits.
Special considerations
Initial limitations of IDE in the Quadra 630, PowerBook 150 and Performa
5200/6200/6300 were that these machines could physically support only one
IDE drive. There was no provision for a secondary IDE channel. The PowerBook 150 also always loaded an IDE driver from its ROM. This made it impossible to load a disk-based driver. Additionally, there was no support for ATAPI
and no provision for the advanced transfer rate capabilities offered by DMA
mode transfers. Only PIO Modes 0 to 3 were supported. LBA addressing, part
of the ATA-2 specification, was supported. Drives larger than 528 MB could be
utilized.
NOTE
For more details, consult Apple’s Developer Note on the Quadra 630 Computer, available on
many Developer CD-ROMs from Apple. See also “IDE Communications” on page 319.
Enhanced IDE
In 1994, Rich Rutledge of Western Digital proposed the Enhanced IDE specification. It was designed to supplant IDE and to eliminate its most glaring limitations. It was also popularized as Fast ATA by Seagate and Quantum, but
without some of the extensions, like large capacity and ATAPI support. Fast
ATA was designed to minimize the need to enhance system BIOSs. Enhanced
IDE was known as ATA-2 by other vendors. It became the AT Attachment
Interface with Extensions standard. A committee known as the Small Form
Factor Committee oversaw this effort and produced a document known as
X3.279-199x, which was submitted to ANSI.
Enhanced IDE allowed support of four IDE devices when two IDE channels
were present. The primary channel was designed for high-speed hard disks,
whereas the low-speed secondary channel was designed for non-hard disk
devices. On PCs, the primary channel was located on interrupt request 14
(IRQ14), allowing it to continue to be universally supported à la ST-506. The
secondary channel was located on IRQ15. Enhanced IDE also maintained
IDE’s ease of use, low cost, and backward compatibility.
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Enhanced IDE allowed support of non-hard disk devices such as CD-ROM and
tape. New interfacing modes broke the 528 MB barrier and allowed Enhanced
IDE to support drives up to 8.4 GB in capacity. Logical block addressing (LBA)
allowed for the increase in capacity potential.
LBA support caused the BIOS to create an Enhanced Drive Parameter Table
(EDPT) during initialization. The EDPT was proposed by BIOS maker Phoenix
Technologies. It contains two sets of drive parameter information. The first
set is called CHS, for cylinder, head, sector. It provides a straight translation
of BIOS cylinder, head, and sector information, without reducing the number
of heads supported from 255 to 16.
STOP
“From 255 to 16” represents the difference between the addressable capacity limits of BIOS (255head capability) and IDE (16-head capability). Because IDE was limited to 16 heads, the far greater
capability of the BIOS could not be tapped. The mechanism was limited to its lowest common
denominator: 16.
The second set translates the cylinder, head, sector information into a 28-bit
logical block address à la SCSI. This LBA addressing requires drive firmware
support. An operating system does not need to be modified to support the
larger volume capacity, but a system BIOS does need modification to support
this feature.
The logical block address is derived from the formula illustrated in Figure 74.
LBA =
(Cylinder × Max. Head No. + Selected Head) ×
Sectors
Track
+ (Sector – 1)
Figure 74. Formula for the Logical Block Address
Faster transfer rate timings allowed data transfer rates to reach 11.1 MB/s
(180 ns per word) with Mode 3 PIO transfers and 13.3 MB/s (150 ns per word)
with multiword, DMA mode 1 data transfers. PIO transfers are where the processor handles the data transfer. DMA transfers are handled by the drive itself.
Multiword DMA mode 1 transfers were typically implemented on high-performance local bus implementations. Enhanced IDE supports several DMA
transfer rates. Type B DMA operates at 4 MB/s, while Type F DMA operates at
either 6.67 MB/s or 8.33 MB/s. Type F is typically used on PCI machines. PCI
also enables scatter/gather transfers, allowing for better virtual memory performance.
STOP
236
Scatter/gather refers to reading logically contiguous blocks of data from a disk and storing them at
discontiguous host computer memory addresses (scatter) and writing data from discontiguous
host computer memory addresses to consecutive logical block addresses on a disk (gather).
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Multiple block transfers were also now possible, allowing for a 20 to
30 percent increase in transfer rate over standard ATA performance (sustained
rates in the 2 to 4 MB/s range). It was also implemented using a standard 40pin ribbon cable to maintain backward compatibility. Cables had to be shorter
than 18 inches and slew-rate control was required to control rise/fall times of
signals. This level of performance helped satisfy the disk needs of the Intel
Pentium class of machines.
With transfer rates so high, Enhanced IDE almost requires a very fast connection to the rest of the computer. It is almost always found interfacing to the
rest of the machine via a 32-bit VESA or PCI local bus connection instead of
the slower ISA bus. Older PC machines could utilize Enhanced IDE drives,
but typically need their BIOS updated to handle some of the new features of
the faster standard.
With these changes, IDE became the industry standard for mass storage interfaces in personal computers. It now offered performance on par with Fast
SCSI-2. Users also demanded storage peripheral support beyond hard drives.
An entire machine’s I/O needs could now be handled by the IDE interface.
Even Apple Computer switched to IDE drives in some of its products in 1994
with the advent of the Quadra 630 and PowerBook 150. IDE was fast and efficient for single-user computers. Best of all, it was cheap. Intel embedded
Enhanced IDE in its PCI chipsets starting with its Triton chipset in 1995, virtually ensuring total acceptance in the PC industry.
The new standard also helped establish the PCMCIA-ATA standard, allowing
flash memory cards and 1.8-inch disk drive PCMCIA drives to be accessed
through normal ATA protocols. The latest version of this is known as the PC
Card bus, a 32-bit, 33-MHz bus that marries PCI protocols to PCMCIA 3.0
electrical specifications.
NOTE
See “Enhanced IDE commands and opcodes” on page 327 for a list of the commands and
operations codes included in EIDE.
Enhanced IDE still did not possess the capability to multitask multiple commands like SCSI, so it was not very effective with high-end multitasking operating system environments such as Novell Netware, Windows NT, or UNIX.
It could not handle large numbers of simultaneous I/O transactions from multiple users or threads. There were no features similar to SCSI’s disconnect/
reconnect, tagged command queuing, or zero latency transfers. Enhanced IDE
still could not connect a large number of devices, nor could it handle external
devices.
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Fast ATA-2
A slightly improved version of Enhanced IDE or Fast ATA-2 was championed
by Seagate and Quantum. Fast ATA-2 supports data transfer rates of 16.6 MB/
s via Mode 4 PIO or Mode 2 DMA.
ATAPI
Part of Enhanced IDE is the ATAPI specification (Advanced Technology
Attachment Packet Interface). ATAPI enabled Enhanced IDE to support alternative storage devices, such as CD-ROM and tape drives, using the same
physical and electrical interface as standard ATA hard disks. This specification defines a standardized method for interfacing non-hard disk devices utilizing the existing IDE computer interface and cabling. It is compatible with
existing IDE hardware without changes or additional cables. An ATAPI device
could co-exist with a hard disk on a single channel, although it would have to
be a slave. The co-existence would also cause problems with 32-bit disk
access in Windows for Workgroups 3.11.
ATAPI basically converted IDE from a computer register based architecture to
one that was based on more modern packet-based transport.
NOTE
See “ATAPI commands and opcodes” on page 327 for a table of the commands and opcodes that
allowed implementation of ATAPI with EIDE.
The packets were logically formatted by deriving SCSI counterparts. So in the
end, ATAPI basically runs SCSI-2 commands across IDE wires. The specification also includes a few commands specific to CD-ROM drives to select
speed, play audio, or determine if the drive is ready.
The ATAPI standard was championed by many BIOS, CD-ROM, and drive
vendors, and has been forwarded to the ANSI committee as SFF 8020 Rev 1.2.
ATAPI has become so prevalent in 1995 that many CD-ROM drives are either
available only in ATAPI or available in ATAPI first.
ATAPI requires BIOS extensions to send ATAPI Identify Device commands at
startup to locate ATAPI devices. Transfers can occur in either PIO or DMA
data transfer modes.
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Basic ATAPI operation
A typical ATAPI operation integrates SCSI-like protocols with ATA protocols.
1. Initialize task file and issue Packet Command.
2. Wait for INT, write SCSI command packet bytes using PIO.
3. Wait for INTRQ, read byte count from cylinder register, transfer bytes.
4. Wait for INTRQ, read status byte, DRQ set to 0 to end command.
5. If there is an error, read error register.
NOTE
See “ATAPI registers” on page 328 for a table of descriptions and locations of ATAPI registers.
Registers are temporary hardware storage areas that assist with the speedy transfer of arithmetic/
logical operations within the CPU.
Although ATAPI allows SCSI packets to cross the bus, there is no support for
SCSI’s command queuing, multiple logical units, and disconnect/reconnect
because there’s no way to tell which device on the bus generated an interrupt.
ATAPI CD-ROM Mode Pages
The SCSI-2 specification defines additional mode pages for use by CD-ROM
devices. ATAPI adds to this list. CD-ROM Capabilities & Mechanical Status
Page (2AH) contains information on the capabilities of the CD-ROM drive
including:
•
•
•
•
speed
volume levels
modes supported
audio commands
ATAPI tape Mode Pages
The SCSI-2 specification defines additional mode pages for use by tape
devices. ATAPI adds to this list. Capabilities & Mechanical Status Page (2AH)
contains information on the capabilities of the tape drive including:
•
•
•
•
media support
speed support
transfer rates
buffer sizes
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ATA-3
Looking to improve upon the Enhanced IDE specification, PC vendors came
together again in 1995 to propose a new and improved specification known as
ATA-3 to support 32-bit operating systems such as Windows 95 and NT. Western Digital advocated a similar Enhanced IDE 95/96 specification. The ATA-3
architecture was designed to be flexible and modular, abstracting the physical
interface from the transport mechanism and command sets. Many vendors
would like ATA-3 to add support for multitasking operating systems, support
that would involve major changes to system BIOS and disk drive firmware.
These changes could limit its compatibility, however.
ATA-3 supports the following:
• drives as large as 137 GB
• data transfer rates of 16.6 MB/s via Mode 4 PIO or Mode 2 DMA
• devices such as disk, tape, CD-ROM, and CD-R
Future speeds above 16.6 MB/s will take place only with DMA. With transfer
rates so fast, cable quality and length become ever more important. Cables
must be shorter than Enhanced IDE’s 18-inch cables. Cable insulation may
need to be fabricated from Teflon®. The specification recommends source and
receiver termination à la SCSI and rise-time control to help reduce ringing on
the ATA bus at speeds of over 15 MB/s.
STOP
Teflon insulation provides a more constant impedance than standard PVC. In really fast data
transfers, that means better signal integrity, leading to fewer errors on a cable in which the
conductors are placed very closely together.
ATA-3 also adds a password security system to prevent unauthorized access
and a self-monitoring, analysis and reporting technology (SMART). SMART
helps protect user data from predictable degradation in drive performance.
ATA-3 is also designed to fix a major loophole in the ATA-2 specification that
forces the speed of each device to run only as fast as the slowest device on that
channel. If a CD-ROM was attached to a channel with a hard disk, the hard
disk would be forced to run at CD-ROM speeds. This is because the host only
sees one data port with a single set of transfer rate timings.
ATA-3 should be compatible with ATA-2 devices. It’s likely to be finalized in
1997.
NOTE
240
See “Appendix A: Additional Information Sources” on page 288 for additional sources of
information on ATA. See “ATA-3 Communications” on page 328 for the commands and
operations codes added with ATA-3.
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Ultra-ATA
In 1996, Quantum announced the Ultra-ATA interface for desktop hard
drives. It doubled ATA DMA data transfer rates to 33 MB/s while remaining
backward compatible with previous ATA standards. This protocol was
designed to use both edges of the data strobe signal to indicate data was available so clock speeds did not have to be altered. It also added Cyclic Redundancy Check (CRC) to verify data bytes.
STOP
Cyclic Redundancy Check is a data error detection method that employs one or several extra
digits or characters generated by a cyclic algorithm.
Ultra-ATA was designed to help reduce I/O bandwidth limitations brought on
by faster CPUs, such as the Pentium Pro and PowerPC 604e. It was also introduced to counter Ultra SCSI’s 40 MB/s data transfer rate.
Other Drive Interfaces
FIPS-60
FIPS-60 (Federal Information Processing Standard 60) is also known as IBM’s
370-OEM interface. It provided for very fast transfers up to 5 MB/s and cable
lengths over 100 meters. It has been extensively used in IBM’s System 370,
3080 and 3090 mainframe families.
SMD-E and SMD-H
In the late 1980s, Storage Module Device (SMD-E and SMD-H) was, until
SCSI’s rise to prominence, the preferred interface for drives used with highend server applications, such as minicomputers and workstations. It was
developed in the mid-1980s by Control Data Corporation’s Storage Module
Drives branch. Its separate control and data cables complicate daisy-chaining,
limiting the number of attachable devices to four. Unlike SCSI, SMD uses a
separate controller board for the disk controller, which contains all the intelligence. The least expensive interface for eight-inch drives, SMD-E transfers
data at about 3 MB/s. SMD drives can store more 2 GB, with a fast average
seek time. Its large form factor (8- and 14-inch) and heavy power consumption, however, make it unpopular in the PC world.
IPI
IPI (Intelligent Peripheral Interface) is the successor to the block multiplexer
channel, or OEM channel, popularized by IBM in the 1960s. It offers parallel
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16-bit data transfer, rather than OEM’s eight-bit transfer, and can be split into
two eight-bit buses to work like the OEM channel. IPI’s dual ports allow from
one to eight devices to be connected on one port, and from two to 16 in pairs
using both ports. It is largely limited to the mainframe world, typically on 8and 14-inch drives. It offers read-ahead capability and is extremely fast for
large-data transfers. IPI has bus phases, but with the host as the sole bus master. Unlike SCSI, only one control signal can change during transition from
one phase to another. IPI uses nine control lines and transfers data at 80 Mb/s
along cables that can be as long as 125 meters. It is designed for hard disk
drives only.
IPI-2 is capable of transfer rates of 36 to 72 Mb/s, and is best when transferring
very large blocks of data. Costlier than SMD, ESDI, or SCSI, IPI-2 is a devicelevel interface, lacks intelligent features, and has no error correction. It offers
fast seek times, a storage capacity of over 2 GB, and uses a single interface
cable that can be daisy-chained easily. It has emerged as the performance
interface of the 1990s for workstations.
IPI-3, also known as HPPI (High Performance Parallel Interface), is a parallel
interface enabling 800 or 1,600 Mb/s data transfers over 32 or 64 twisted-pair
copper wires, or on fiber-optic cable. Fiber cabling can be up to 10 kilometers
in length (over six miles), while the other cabling can be up to 36 meters in
length (around 118 feet). Developed out of work at the Los Alamos National
Lab, it is intended for supercomputing applications. It allows multiple block
transfers via a single command execution, but can transfer data in only one
direction at a time.
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8
All About Expansion Buses
Buses are pathways between the CPU, memory, and I/O components in computers. Most computers do not come with adequate built-in interfaces for
high-performance I/O. To run high-performance storage products on these
computers, users must typically add expansion cards. Computer expansion
cards (bus cards or host bus adapters) play an important role in the performance of storage solutions. The performance of these cards is directly related
to the robustness of the particular expansion bus. In this section, we will
examine some of the more important computer buses.
For buses to become industry standards they must include the following critical features:
•
•
•
•
•
•
open standard, designed to be built upon rather than obsoleted
expansion compatible
high performance
easy to use
flexible
inexpensive
NuBus
NuBus
Figure 75. NuBus SCSI expansion card
NuBus is the IEEE Standard for a simple, 32-bit, backplane bus (NuBus, IEEE
Std P1196-R1990). It was introduced with the Macintosh II in 1987. It was
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Apple’s expansion bus of choice for about 8 years. MIT, Apple, and Texas
Instruments helped create the bus. Apple’s version deviated from the standard
in that its cards were 4 × 13 inches instead of 11 × 14 inches. Apple also
adjusted the specification by allowing non-master request interrupts and adding a power failure warning line.
NuBus featured the following:
•
•
•
•
•
•
autoconfiguration without dip switches
32-bit architecture
10 MHz bus clock
support for multiple processors
fair arbitration
bus mastering
• block mode transfer support
STOP
A bus clock is an electronic circuit that regulates the synchronization of the flow of information
through the bus. Bus mastering allows the bus card to pass data directly into memory, freeing the
CPU to perform more useful functions. (The CPU would otherwise act as an intermediary
between the card and memory.)
Some cards were accessed through a software manager called the Slot Manager. Others could be activated transparently through the use of a configuration ROM. Each vendor of cards would register its board with Apple and in
return receive a unique board ID number.
NOTE
For more information on the Slot Manager, see Inside Macintosh Volume V.
Early Macintosh II systems had a problem with 32-bit addressing of NuBus
memory space. This prevented cards with ROMs mapped into the top 16 MB
of space from being seen. A motherboard swap fixed the problem.
NuBus promised maximum theoretical data transfer rates of 40 MB/s, but due
to chip limitations, effective data transfer actually took place at much slower
speeds. The original NuBus implementation did not support block mode data
transfers, limiting performance to under 5 MB/s.
STOP
NuBus block mode data transfers allow large blocks of data to be transferred quickly before
releasing the bus. They require only one arbitration for the bus.
The Quadra family added support for block mode data transfers from the card
to the motherboard, effectively doubling data transfer rates. NuBus data transfers can be performed between the card and main memory or from NuBus card
to NuBus card. Card to card transfers significantly benefited from block trans-
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fers, allowing data to bypass main memory. Entries in the declaration ROM of
each NuBus card told the system whether the card supported block transfers
and the maximum number of transfers it supported.
STOP
As the Macintosh evolved, its data transfer rates escalated. Table 50 illustrates the performance
gain ushered in with each new Macintosh model. Note, however, that the step between the
Quadra 840AV and the PowerMacintosh family was, at best, a step sideways.
Table 50. Evolution of Macintosh data transfer rates
Macintosh Model
Macintosh II Family
Maximum Data Transfer Rate
3.7–4.5 MB/s
Quadra Family
5.5–9 MB/s
Quadra 840AV
15–18 MB/s
PowerMac Family
12–15 MB/s
Some machines had up to six NuBus Slots, others as few as one. The maximum per the specification was 14. A wide variety of cards are available with a
NuBus interface:
•
•
•
•
•
•
video cards
digital video capture cards
DSP accelerators
SCSI accelerators
serial port cards
RAM cards
Some cards were so sophisticated that they drew more than the allotted 13.9
watts per card and pushed the computer’s power supply to the limit.
NuBus-90, introduced with the Quadra family, doubled the bus clock to
20 MHz. Data transfer rates with NuBus-90 could reach about 80 MB/s burst,
or about 20 MB/s sustained. This new protocol mainly sped up data transfers
between two NuBus-90 compliant cards. It didn’t speed up data transfers to
the motherboard. NuBus-90 was not widely supported, and only a few NuBus90 compliant cards were ever produced.
The Quadra 840AV with its MUNI NuBus chip had the fastest NuBus performance of all Macs.
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STOP
MUNI stands for Macintosh Universal NuBus Interface. This bus offered:
•
•
•
•
Support for the full range of NuBus master/slave transactions with single or block moves.
Support for the faster data transfer rates to and from the CPU bus.
Support for NuBus ‘90 data transfers between cards at a clock rate of 20 MHz.
Provision of first-in, first-out (FIFO) data buffering between the CPU bus and accessory cards.
PowerMacs were more limited in NuBus performance due to the absence of
the MOVE16 instruction on the PowerPC processor. A work-around known as
the PBBlockMove Extension was created to maximize performance of NuBus
cards in PowerMacs. NuBus cards only worked on the Macintosh, a small
market, so they were typically priced much higher than PC type cards.
STOP
The Move16 instruction is a 68000 instruction that moves 16 bytes of data very quickly.
Some machines, such as the Centris 610, Centris 660AV, and PowerMac 6100,
could accommodate only smaller, seven-inch NuBus cards.
ISA
ISA card
Figure 76. 16-bit ISA SCSI card
The ISA expansion bus has become one of the most popular computer buses.
ISA stands for Industry Standard Architecture. It was introduced with the
original IBM PC and IBM PC XT in the early 80s. Almost any type of card is
available in the 62-pin ISA form factor. The original version ran at 4.77 MHz
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and had 8-bits of data and 20-bits of addressing range. ISA supported both
DMA and PIO.
A later version, introduced with the IBM AT, ran at 6 MHz and had a 16-bit
data bus and 24-bits of addressing range. It added 36 contacts to the ISA connector. On the AT, bus mastering was supported for the first time.
The transfer rate it offered was 3 to 5 MB/s. Later IBM ATs ran the bus at
8 MHz. AT compatibles from third parties with processor speeds up to
16 MHz later ran the bus at up to 8.33 MHz, the ISA maximum. A couple of
vendors even over-clocked the bus to 10 or 12.5 MHz, causing lots of card
incompatibilities. Its biggest limitations included the need to use jumpers to
configure cards and its use of 24 bits for addressing up to 16 MB of expansion
space. With such low data transfer speeds, it made no sense to offer anything
but 5 MB/s SCSI-1 or SCSI-2.
MicroChannel
IBM designed its proprietary MicroChannel architecture (MCA) in 1980. It
was developed along with the IBM PS/2 to improve upon and replace the aging
ISA bus. IBM gave up on backward-compatibility with ISA in an effort to create a new bus designed for 32-bit processors and multitasking operating systems, such as OS/2.
MCA cards were available in 8-, 16-, and 32-bit data types, with the addressing
range fixed at the full 32 bits. The main slot was contained in a 90-contact
connector. Additional connects were used by 16- and 32-bit cards. These were
located in front of the 8-bit connector. MCA provided for configuration without jumpers. It provided 8-, 16-, or 32-bit operation at speeds up to 10 MHz,
capable of transferring 20 MB/s. Real data rates ranged in more like the 8 to
10 MB/s range. Later versions included a streaming data procedure feature
that boosted data transfer rates up to 80 MB/s with 64-bit data procedures.
MicroChannel fully supports DMA and bus mastering. In fact, several CPU
accelerators have been designed this way. However, this bus proved to be
unpopular because it was found only on IBM machines and a few clones. Limited distribution was due to high licensing costs.
This bus continues on in IBM’s RS/6000 workstation line powered by POWER
and PowerPC processors.
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EISA
EISA stands for Enhanced Industry Standard Architecture. It was designed to
supersede the ISA bus, yet retain full compatibility with ISA. PC-compatible
vendors developed EISA as an answer to IBM’s MicroChannel bus. EISA ran at
the ISA standard 8.33 MHz clock rate and allowed support for bus masters.
EISA extended ISA to 32-bit operation and sped burst data transfer rates up to
33 MB/s (these would be DMA burst type C transfers). Its sustained rate was
closer to 8 MB/s.
EISA allowed for automatic adapter configuration without DIP switches. Each
adapter came with a .cfg file. The EISA Configuration Utility used this to
automatically configure the card so that it did not conflict with other cards.
EISA was only popular in the Intel-based server arena. This was due to high
costs. It used a high-density 188-pin edge connector that was similar in size to
ISA. EISA allowed old ISA cards to plug and run. Later versions of EISA
allowed for enhanced master burst (EMB) data transfers of up to 66 or 133 MB/
s by using both edges of clock signals to issue data transfers.
VESA Local Bus (VLB)
The ISA bus became a serious limitation with the advent of Windows, multimedia, and other technologies that required megabytes of information per second. The need for high-volume, high-performance data transfer was
particularly apparent with graphical software applications. VLB, the Video
Electronic Standards Association (VESA) Local Bus, was designed to interface
the Intel 80486 high speed processor bus to a local high-speed expansion bus.
It ran at the processor speed and provided for high-speed, 32-bit data transfers.
Version 1.0 ran at either 40 or 66 MHz. Version 2.0 ran at 50 MHz. Version 1.0
provided for data transfers up to 133 MB/s, while 2.0 provided for up to
160 MB/s. Version 2.0 supported write-back caching and 64-bit addressing,
which allowed for data transfer rates up to 267 MB/s.
STOP
Write-back caching keeps track of what information in the cache has been modified by the
microprocessor. This is done by marking modified information with a “dirty bit.” When displaced
from the cache, all the information with dirty bits is written to primary memory. It’s likely that
the term “dirty bit” was coined to denote a tag that marked information that needed to be
“cleaned out” (written) to the disk or to memory.
VESA cards were 16-bit ISA cards with an additional connector to form a 124pin VESA standard edge connector. Up to three cards were supported in each
system. DMA was not supported as VLB allowed for direct transfers into main
memory. The main problem with this bus is that it was X86 CPU specific and
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operated at the CPU’s speed, making it impractical for universal applications.
VLB initially was very popular with the advent of the 80486 processor. When
PCI shipped with Pentium processors, VLB slowly died off.
NOTE
For sources of more information on VESA Local Bus, see “Appendix A: Additional Information
Sources” on page 288.
PCI
PCI card
Figure 77. PCI SCSI card
PCI (Peripheral Component Interconnect) is a system standard that defines a
high performance interconnection method between plug-in expansion cards,
integrated I/O controller chips, and a computer’s main processing and memory systems. It was originally designed by Intel in the early 1990s as an alternative to other proposed peripheral expansion buses.
STOP
PCI was patented by Intel, but they agreed to license it openly and make it royalty-free.
PCI is now governed by a consortium of industry partners known as the PCI
Special Interest Group (PCI SIG). This body of computer systems manufacturers, peripheral vendors, component suppliers, and software companies is organized for the following purposes:
• to insure compatibility and interoperability across the PCI standard
• to continue to refine the specification to increase throughput
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The PCI SIG runs PCI-compliance test workshops every quarter, ensuring
that all cards operate in all machines.
PCI logo
Figure 78. Certified PCI local bus trademark
The first release of the specification—the PCI Local Bus Specification—
became available in June 1992. A second release (2.0) was published in April
1993. Version 2.1 was published in 1995 and offered the following:
• 66 MHz clock speeds
•
•
•
•
264 MB/s data transfer rates
card bus support
new latency rules to improve performance
user-definable configuration mechanism
Other PCI-related specifications soon followed:
•
•
•
•
PCI to PCI Bridge
PCI BIOS
PCI Multimedia Design Suite
PCI Mobile Design Guide
PCI was initially difficult to implement because it required special chips with
output buffer drivers that had both minimum and maximum AC currents (as
opposed to traditional DC current). It also allowed only one load per bus slot,
making ASICs the only feasible interface to the bus. A computer typically
would have only two or three PCI slots. Adding more would require expensive
PCI bridge chips or additional PCI peer bus chips that join separate PCI buses
together logically.
STOP
ASIC stands for application-specific integrated circuit. ASICs are computer chips developed for
specific functions. These are used in a wide-range of devices, such as video machines, microwave
ovens, and security alarms.
The PCI family includes multifunction chips that provide two distinct features, Ethernet and SCSI, on a single chip.
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PCI features
PCI is loaded with high-performance features including:
•
•
•
•
STOP
32-bit bandwidth with a compatible 64-bit and/or 66 MHz upgrade path
electrical signals of 5 volts (3.3 volts later)
33 MHz bus clock rate, compared to 10 MHz for Apple’s fastest NuBus
up to 132 MB/s burst transfer rate over the 32-bit bus
Sustained rates are much lower.
• full PCI bus master and slave support
• support for different card sizes
Supports either a 6.875-inch (known as Short Card form factor) or a
12.283-inch (known as Standard Card form factor) card size.
• support for Open Firmware Boot Specification (IEEE1275-1994) for booting
cards in an OS-independent environment
• PCI Local Bus Specification, Revision 2.1 (1995)
• operation independent of any particular microprocessor design
• ISA style mechanical bracket
• edge card connection into the motherboard
• true, Macintosh plug-and-play through recommended use of plug-in card
expansion ROM
• Macintosh PCI Performance
A PCI burst transfer is defined as a single PCI bus transaction with a single
address phase followed by two or more data phases. The PCI master arbitrates
for ownership of the bus, then issues a start address and transaction type during the address phase. The target device latches this start address into its
address counter and increments the address from data phase to data phase.
On PCI, the fastest performance is achieved with data transfer commands that
transact aligned data to and from cacheable memory regions in data chunks
the size of CPU cache lines.
NOTE
See “PCI Communications” on page 329 for a table that lists and describes these commands.
Although PCI promised 132 MB/s data transfer rates, initial implementations
on chips such as the Intel Mercury and Neptune were limited to much less.
Later implementations, such as the Intel Triton and Orion, handled the faster
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70 MB/s PCI streaming bursts (Table 51). Limitations were due mainly to PCI
controller chips. Early chips could not handle PCI’s high-performance bursts.
Table 51. Maximum data transfer rates of Intel chipsets
Intel PCI Chipset
Maximum Data
Transfer Rate
Mercury, Saturn
30 MB/s
Neptune
40 MB/s
Triton
70 MB/s
Orion, Triton II, Natoma
70+ MB/s
Some chips did not support multiple-byte data transfers or did not optimally
support the cache line size or invalidation technique of the CPU. Memory
bandwidth limitations also hampered PCI performance. Data from a card
needs to be put into memory, and if the memory subsystem speed is slow, the
data transfer rate will be limited.
Read and write performance to and from the PCI bus were often different. PCI
performance was also hampered by the fact that buses were clocked at rates
slower than the 33 MHz standard. It was easier to clock the PCI bus as a divisor of the CPU’s system bus.
• Pentium systems that ran at 75 MHz had system buses running at
50 MHz and PCI buses clocked at half that rate (25 MHz).
• Pentium systems that ran at 60, 90, 120, 150, or 180 MHz had system
buses running at 60 MHz and PCI buses clocked at half that rate
(30 MHz).
• Pentium systems that ran at 66, 100, 133, 166, or 200 MHz had system
buses running at 66 MHz and PCI buses clocked at half that rate
(33 MHz).
PCI has become the bus of choice on most PCs and most recently on the
Apple Macintosh. Digital Equipment’s Alpha line of workstations has also
adopted PCI. Even their CPU has an on-board PCI interface, the first implementation of 64-bit PCI found in this line.
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Macintosh PCI
1995 will be remembered as the “Year of Confusion” for many people: Macintosh users, resellers, and even industry insiders. With the introduction of
Apple’s Power Macintosh with PCI family and the dawning of the Mac™ OS
clones—both PCI and NuBus—the Macintosh market offers consumers such a
wide array of choices that making sure your purchase delivers real value
requires significant contemplation and research.
In high-end markets where Apple has the leading position—digital video,
color publishing, digital pre-press and multimedia—the requirements for mass
storage in performance, capacity, and cost are dramatically influenced by the
implementation of PCI on the Power Macintosh.
Apple’s SCSI interface by itself yields very low performance for the Macintosh’s high-end markets. As with the NuBus machines of the past, it will be
the third-party vendors who provide high-performance, Fast and Wide SCSI-2
cards and disk arrays to meet the speed and capacity needs of digital video,
color publishing, digital pre-press, multimedia, and networking. These cards
and arrays allow users to realize the full potential of their solutions—something Apple’s on-board SCSI interface prohibits.
STOP
Higher-performing storage interfaces, such as Fast-20 (also known as Ultra SCSI), Fibre Channel,
and SSA, will offer bandwidths with up to 100 MB/s sustained data transfer rates—over 10 times
the performance of Apple’s built in SCSI interface.
Only with PCI on the Power Macintosh, and the performance gain PCI provides, will users be able to maximize the benefits of the current high-performance SCSI interface—Fast and Wide SCSI-2—and realize the benefits of the
new coming technologies for storage.
To sort out PCI’s impact on mass storage, we’ll explore the following topics:
•
•
•
•
•
NOTE
What is PCI and why did Apple adopt it?
What is the impact of PCI on Power Macintosh and how will you benefit?
What are the implications for performance on the Power Macintosh?
What are the issues of compatibility for hardware and software?
How will the entrance of many “Mac-clone” companies affect you?
This section was written under the assumption that you are a reasonably knowledgeable user of
the Macintosh or Power Macintosh platform.
Apple based its Power Macintosh with PCI family on Version 2.0 of the PCI
specification, like the majority of companies that have implemented the Intel
PC platform. The entire Power Macintosh with PCI family—the 7200, 7500,
8500, and 9500—fully conforms to this industry standard.
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The second generation of Power Macintosh computers containing PowerPC
microprocessors uses PCI buses to communicate with both internal I/O chips
and plug-in expansion cards. Both 601- and 604-based Power Macintosh systems utilize the PCI bus architecture.
Four key reasons went into Apple’s decision to replace its aging NuBus architecture with PCI:
1. Apple wants to offer a widely-adopted industry standard so that it can
market its computers in a way that promotes commonality with other
platforms while allowing Apple to market the differentiated benefits of
the Macintosh experience.
2. Since PCI is an industry standard, Apple hopes to attract a more widely
varied pool of cards for different solutions.
3. Apple hopes the cost of PCI products will be lower than comparable
cards on the NuBus as more developers offer their products for the
Power Macintosh with PCI family.
4. The addition of PCI gives the Power Macintosh platform a tremendous
boost in performance over the NuBus. This will eventually open up
additional markets to Apple for applications that need the added performance that only PCI can deliver.
In the past, Apple has used NuBus as its expansion bus for both Macintosh
and Power Macintosh machines. With the advent of the Power Macintosh
with PCI, Apple is moving from a semi-proprietary expansion bus (the NuBus)
to one with much greater industry acceptance (PCI). Apple can assure its compatibility with other computer manufacturers through it compliance with the
PCI standard. Apple plans to differentiate its robust architecture—with its
unmatched application richness—from Intel Pentiums with PCI running
Windows 3.1 or Windows 95.
PCI provides a needed standard in the computer industry. Because the PCI bus
uses the same architecture and protocols to communicate with I/O chips and
expansion cards, it reduces the cost and complexity of computer hardware. It
allows Apple, and other systems manufacturers, to provide high-speed
expandability at minimum cost.
The first implementation of PCI on the Power Macintosh with PCI utilized
the Apple-designed BANDIT chip. Its performance varied depending on the
PCI transaction.
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STOP
Table 52 and Table 53 illustrate that data transfers performed at the cache line size of 32-bytes
result in the highest performance.
Table 52. Macintosh PowerPC to PCI data transfer rate
Transaction
Write to PCI
Read from PCI
Parallel Processing
Chip Instruction
Size (bytes)
PCI Performance
4
20 MB/s
Integer Store
8
40 MB/s
FP Store
32
85 MB/s
PCI Copyback
4
11 MB/s
Integer Store
8
20 MB/s
FP Store
32
40 MB/s
PCI Copyback
Table 53. Macintosh PCI master to memory data transfer rate
Transaction
Write to Memory
Read from Memory
Size (bytes)
PCI Performance
PCI Command
4
20 MB/s
Mem Write
8
35 MB/s
Mem Write
32
80 MB/s
Mem Write & Inv
4
10 MB/s
Mem Read
8
15 MB/s
Mem Read
32
30 MB/s
Mem Read Line/M
Open Firmware
You may have heard the term “Open Firmware” used in reference to Apple’s
PCI implementation (it is also sometimes called “Open Boot”). It refers to
PCI’s independence from any particular operating system.
STOP
Open Firmware was originally developed as Open Boot by Sun Microsystems. It debuted in 1988
with the original Sun SparcStation workstation. It provides a way of booting a computer system
that is independent of both operating system and processor. Its native language is interpreted by
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the computer and is based on the programming language Forth. Open Firmware drivers are
written in a Forth variant known as FCode.
During the Open Firmware process, startup firmware in ROM:
1. Searches the PCI buses and generates a data structure that lists all available peripheral devices.
This data structure also stores the peripheral support software, such as a
device driver, provided by each PCI expansion card.
2. Finds an operating system (for example, Mac OS System 7.5.5 on the
Power Macintosh with PCI) in ROM or on a mass storage device.
3. Loads it and starts it running.
The operating system does not need to be Mac OS, though with the Power
Macintosh it is. Hence, it is possible for PCI-compatible Power Macintosh
computers to operate PCI peripheral devices using either Macintosh or thirdparty system software. In short, all Mac OS-compatible computers (Mac
clones) can be used with the PCI bus.
STOP
This is comparable to what SuperMac and Power Computing have done with NuBus.
Any operating system that is compatible with the Power PC chip used in the
Power Macintosh will be compatible with the Power Macintosh with PCI.
Open Boot provides Apple’s PCI bus with:
• auto-configuration of devices on startup
• flexibility in system software
• wide industry acceptance
Open Boot also increases the flexibility of PCI on PCs using Intel chips by not
requiring use of the system BIOS. This differs from previous expansion buses
used in the Intel world—ISA, EISA, MCA, and VLB—where BIOS code is necessary to successfully utilize cards in the bus.
STOP
This offers no advantage to the easy-to-use architecture of the Power Macintosh, and it does not
exempt the card from needing a Power Macintosh-specific device driver to be fully functional.
The establishment of the PCI bus standard offers real benefits to the Power
Macintosh and to users at large:
• additional ease-of-use
• future expansion
• overall lower cost
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These benefits allow for incredible performance gains over NuBus. They
make the Power Macintosh with PCI platform the solution for the 90s.
The performance gain PCI offers users on all Power Macintosh with PCI platforms is the most significant advantage over NuBus-based machines and PCI
machines running Windows or Windows 95. PCI delivers unmatched performance gains for digital video, color publishing, and multimedia environments, particularly when coupled with the power of the PowerPC 604 chip
(used in the Power Macintosh 9500 and 8500), increased native System software, and 604-optimized application packages.
Mass storage is one of the key beneficiaries of the increased bandwidth and
throughput of PCI. A dramatic illustration of this improvement in performance is the comparison of a Fast and Wide SCSI-2 disk array on the Power
Macintosh 9500 with the fastest NuBus machines available (Figure 79).
Benchtest
Figure 79. PCI and NuBus transfer rates using Benchtest
NOTE
In Figure 79, the bars indicate sustained data transfer rates in MB/s. Two bus cards were used. All
testing was done with the latest System software for each platform, 5 MB transfer size and FWB’s
Hard Disk ToolKit™ Benchtest 1.7.5. Longer bars indicate faster performance.
As Figure 79 illustrates, a fast and wide array on the Power Macintosh 9500
achieves a sustained data transfer rate of over 33 MB/s. This is more than
twice the performance of the fastest NuBus Power Macintosh—the 8100/80—
and almost twice the performance of Apple’s fastest NuBus Macintosh, the
Quadra 840AV.
STOP
The 840AV is the fastest of all NuBus machines, Macintosh or Power Macintosh.
With new higher-performing storage architectures—which started appearing
in the last part of 1995 and continued to develop and debut throughout 1996—
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the increased power they offer will be realized only on Power Macintosh with
PCI. NuBus machines cannot match the performance of these interfaces.
While artificial benchmarks are useful, Macintosh users want to know:
• “What can this really do for me?”
• “How will this increase my real productivity?”
Results for real-world applications, such as Adobe PhotoShop™, impressively
demonstrate that PCI performance gains are not confined to artificial benchmarks.
Real test
Figure 80. PCI and NuBus transfer rates using Adobe Photoshop™
NOTE
Figure 80 illustrates transfer rates achieved while using Adobe Photoshop™ to open a 100 MB
TIFF. All testing was done with the latest System software and Photoshop 3.04; 32 MB of RAM
was allocated to Photoshop; primary and secondary scratch disk preferences in Photoshop were
set to disk array; and Photoshop and the TIFF file were launched from disk array. Shorter bars
indicate faster performance.
PCI’s performance gain extends to real-world applications for real-world uses.
While Figure 80 focuses on color publishing, the benefit of PCI on the Power
Macintosh extends to any application and/or hardware peripheral that
involves significant PCI bus activity. For example, digital video card vendors
were limited by the throughput of NuBus to video compression of 4:1. With
PCI, these same vendors feel they can deliver 2:1 compression, and perhaps
uncompressed video—virtually impossible on NuBus. Similar performance
enhancements will be seen by users of multimedia, pre-press, and network
applications. Overall, the Power Macintosh with PCI family sets new standards for price/performance value in the desktop computer arena.
While PCI is an industry standard, Power Macintosh users should not be
lulled into the belief that all products designed to work on PCI for the Intel
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platform will work in the Power Macintosh. PCI compliance is only the first
step to successful integration of a third-party card with Power Macintosh.
For example, a card that is fully PCI compliant but does not support Open
Boot may work fine on a Pentium with PCI, but will not work on a Power
Macintosh. Even if a card fully supports Open Boot, it still requires a Macintosh device driver. In the case of storage, a PCI card must have a Macintosh
disk device driver in order to operate properly for mass storage applications.
Additionally, that device driver must be compliant with SCSI Manager 4.3.
STOP
The SCSI Manager is that part of the Apple System software that controls the interaction of SCSI
peripherals and the Mac OS.
Apple has instituted a compatibility program to let customers know which
products meet the minimum level of Power Macintosh with PCI compatibility.
• This program DOES NOT guarantee you the fastest, best engineered, or
most suitable product for your application.
• However, it DOES indicate that the product will work in the Power
Macintosh with PCI family.
Apple requires vendors who participate in the program to submit cards to
Apple for compatibility testing. If the cards are deemed compatible, the vendor is allowed to use the “Designed for Macintosh with PCI” logo in its marketing materials and advertising (Figure 81).
PCI Mac bug
Figure 81. Apple’s certified PCI trademark
STOP
A PCI compatibility rating from Apple indicates compatibility only. Apple does not check for
optimal performance, good Macintosh technical support, or synergy with Macintosh applications.
For the consumer, the Apple PCI logo is a valuable signal that the product
meets the minimum level of Power Macintosh with PCI compatibility and is
worth considering.
PCI cards introduced a new method of installation. In the NuBus era, a Eurocard connector on the bottom edge of the card tightly mated to another connector on the motherboard. For Power Macintosh with PCI, this has been
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replaced with the use of edge connectors on the card that install into a slot on
the motherboard. Although this is a PCI industry standard, it is, unfortunately, too easy for cards to seat incorrectly in the motherboard slot. To
address this problem, Apple created a system that keeps a card in its slot by
firmly locking the ISA-style fence into the machine’s backplane plastic. Additionally, all the Power Macs with PCI have a locking mechanism that gently
secures the cards from the top. Very few PCI implementations feature these
useful innovations.
The entrance of many PC companies into the Macintosh market through PCI
will create additional confusion for users already wary of the many choices
available. These companies are unproven entities on the Macintosh platform,
though several have established leadership positions on Intel platforms. Some
of these companies are serious about Macintosh and want to offer excellent
products. Others have no idea what Macintosh users expect or want in their
products and just see the advent of Power Macintosh with PCI as a means to
enter a new market with the same expansion bus interface. Only time will
tell which PC companies can successfully deliver “real” Macintosh products
on the Power Macintosh with PCI platform.
Probably the most important question you need to ask yourself is “What do I
expect from a company whose products are to be used in my Power Macintosh
with PCI?” Once you answer this, you can determine if a PC company has
products that meet your expectations. For instance, what users expect to see
in any Macintosh product is thorough documentation and excellent technical
support. Thorough documentation is often only included in the best products
on the PC side, and users have learned not always to expect high-quality technical support.
Other key questions users should ask about PC companies selling Power Macintosh PCI solutions include the following:
• What is the true level of Macintosh compliance I will receive?
• What help will I get in the integration of their PCI solution with my
Macintosh applications and the Mac OS?
• How well will they engineer their products to work cooperatively with
other Macintosh PCI cards and software products?
Many PC companies will be entering the Macintosh market via PCI. What
remains to be seen is whether they can meet the higher expectations that
Macintosh users have of technical support, documentation, integration, OS
compliance, and synergy with complementary hardware and software.
Some of the PC companies will learn the Macintosh way and offer customers
robust, quality PCI solutions. Others will just throw their current PC products at the Macintosh. If you are shopping for these products, you will need to
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be just as aware and selective as you have been in the NuBus era and not be
swayed by wild claims of incredible performance at incredibly low prices.
PCI for Power Macintosh brings to the Macintosh platform the real power
needed for today’s demanding applications. Coupled with the next generation
of PowerPC chips and System software, PCI provides high-end users the performance they demand to continue to push the envelope.
NOTE
For sources of more information on PCI, see “Appendix A: Additional Information Sources” on
page 288.
PCI Mezzanine Specification
The PC Industrial Computer Manufacturers Group (PICMG) was established
in 1994 to develop a standard for a low-end, passive-backplane CPU board that
could incorporate PCI.
STOP
A passive backplane is a board that exists merely to pass signals; no processing takes place.
This group created the PCI Mezzanine card (PMC). The standard specifies the
physical implementation of PCI as a mezzanine card. These cards generally fit
on top of the industrial CPU board. PMC allows general purpose CPU boards
to be customized for specific applications. These cards adhere to PCI 2.0 specifications. Standardization creates an industry of add-on boards that can leverage low-cost chips used for PCs.
PCMCIA
PCMCIA stands for Personal Computer Memory Card International Association. This group was founded in 1989 and consists of about 400 members from
the computer industry. This popular specification was created to standardize
expansion cards for portable computers. The cards are credit card sized,
85 × 54 mm modules.
The original 1.0 specification, released in 1990, was targeted at memory cards.
A later 2.0 specification in 1991 added support for I/O cards and introduced
three thicknesses of cards:
• Type I cards are 3.3 mm thick.
• Type II are 5 mm thick.
• Type III are 10.5 mm thick.
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PCMCIA release 2.01 allows for ATA storage cards (Flash ATA and ATA hard
drives), I/O cards (such as fax, ethernet and SCSI), and memory cards (RAM,
ROM, flash). This specification basically mapped the 40-pin IDE interface to
the 68-pin PCMCIA connector. This is a 5 V interface that supports 16-bit,
8 MHz operation through its 68-pins.
PCMCIA defines a software layer known as card and socket services. This
software handles the insertion and removal of PCMCIA cards. It sets up the
PCMCIA bus chip on the computer and configures the PCMCIA card to manage its I/O ports and interrupt usage. The biggest challenge with PCMCIA has
been software compatibility. This has caused some cards to be incompatible
with certain computers. Hot-swapping of cards has also been problematic.
Cards should be removable and insertable on the fly at any time.
The latest version of PCMCIA is known as the CardBus or PC Card 94 bus. It
specifies a new mode that allows PCI devices to be packaged like PCMCIA
cards and be plugged into similar sockets. It is a 32-bit, 33-MHz bus, marrying
PCI protocols to PCMCIA 3.0 electrical specifications. This supports 3 V
operation, bus mastering, DMA, and both 20- and 33-MHz operation. CardBus
utilizes the same Card Information Structure (CIS) as PCMCIA cards. For the
first time, cards such as video capture, 100 Mb Ethernet, and Ultra SCSI could
be created.
NOTE
262
For more information on PCMCIA, see “Appendix A: Additional Information Sources” on page
288.
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9
Storage Software
Many currently available utility software packages are designed to maximize
the usefulness and performance of storage peripherals. Chapter 9 provides an
overview of some of these products and gives some reasons why you might
find them useful.
Formatter Software
HDT
Figure 82. FWB’s Hard Disk ToolKit™
Formatter software is utility software designed to set up hard disks for use on
a computer system. These utilities can be simple or complex.
The Macintosh requires formatter software to format, partition, and install a
SCSI driver on disks before they can be used by the operating system. There
are several makers of formatter software, including Apple and FWB.
Apple’s formatter software is called Drive Setup. It’s a no-frills formatter that
works on both IDE and SCSI drives. However, it works only on Apple hard
drives. Other formatters, such as FWB’s Hard Disk ToolKit, offer all the basics
of Apple’s software, work on virtually every SCSI hard drive, and contain
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advanced features, such as password protection, encryption, Device Copy,
SCSI Configure, and extensive drive diagnostics.
Formatter software adds support for removable-media drives to the Macintosh
operating system in the form of a System Extension. The Extension loads a
driver into memory that watches and provides for media insertion and
removal.
PCs typically utilize formatter software bundled with the operating system.
IDE drives on PCs do not need to be formatted. They are partitioned by operating system utilities such as DOS’s FDISK or Windows NT’s Disk Administrator. SCSI drives are usually formatted by software included with the SCSI
host adapter. There are several formatter programs that provide additional
diagnostics and compatibility for IDE and SCSI drives. Some also provide support for multiple operating systems on a single drive.
RAID Software
RTK
Figure 83. FWB’s RAID ToolKit™
RAID software is used to create a software-based array out of several drives.
Some RAID levels increase the performance of drives, others provide fault tolerance. This software generally runs on top of any industry-standard host
adapter.
STOP
264
An array is a collection of drives, connected and configured so as to act and appear as one drive.
Software-based arrays are created with installed software packages. Hardware-based arrays are
created through logic built into a controller chip.
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Disk striping or RAID 0 is very popular on the Macintosh. Users that require
the utmost in throughput use software such as FWB’s RAID ToolKit to stripe
multiple drives together into a high performance disk array. Apple includes
with its Workgroup Servers a utility called AppleRAID that creates a more
limited RAID set.
NOTE
To learn more about RAID, see Chapter 3, “RAID Technology.”
Disk striping is slowly gaining popularity on PCs. FWB’s RAID ToolKit for
Windows provides striping for Windows for Workgroups 3.11 and
Windows 95. Windows NT has built-in RAID 0 striping. Windows NT
Advanced Server has built-in RAID 5 support.
CD-ROM Software
CDT
Figure 84. FWB’s CD-ROM ToolKit™
CD-ROM drivers are software routines that enable the operation of a CDROM drive connected to a computer. They also map industry-standard CD
formats to native operating system structures, making it possible to mount a
variety of disc formats on a wide range of operating systems.
On the Macintosh, there are several CD-ROM driver packages. Apple includes
CD-ROM drivers for its CD-ROM drives. As with its hard disk drivers,
Apple’s CD-ROM drivers only work on Apple drives. Third-party products,
such as FWB’s CD-ROM ToolKit™, provide drivers and acceleration for a wide
variety of CD-ROM drives on the Macintosh. Part of Apple’s system software
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includes translators for the various CD-ROM formats that map them to the
Macintosh operating system.
Windows 95 includes integral support and caching for most SCSI and IDE CDROM drives, so third-party drivers are not needed. Translators for several CDROM formats are also included.
Tape Backup Software
Tape Backup software is used to perform backups and restores to tape or DAT
drives. These drives are known as sequential access devices because the stored
data can be accessed only in a linear sequence—the tape has to wind or
unwind and pass across the stationary tape head to position data for reading or
the tape for writing. Most sequential access devices do not show up as
mounted volumes on the Desktop and require tape backup software to access
the media. Advanced backup packages allow for network backups to servers
through the use of backup agents.
The Macintosh operating system does not have any built in support for tape
backup. It relies on third-party tape backup software to operate tape drives.
Windows 95 includes integral tape backup software for some tape backup
drives. The selection of functions is limited as is the support list. Only a few
popular IDE and SCSI tape drives are supported. There is no support for 8 mm,
DAT or Travan. Other tape backup software can be used through the ASPI
manager interface.
Compression Software
Compression software is used to extend the capacity of drives. Compression
uses algorithms that reduce the amount of storage needed to store a given
stream of data. These algorithms exploit redundancy and disk allocation
units. Compression software buys extra space, but at the price of performance.
Data must be compressed before it is written and uncompressed before it is
read. Compression complicates the recovery of data from a malfunctioning
drive. Compression will usually buy you from 20 to 40 percent more space,
not the 100 percent increase some programs claim.
There is no compression built into the Macintosh System 7.5 operating system. However third-party utilities are available for adding compression.
Windows 95 includes support for compression of disk drives. The Windows 95
Plus Pak adds a 32-bit version of the compression program.
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Hierarchical Storage Management Software
SMT
Figure 85. FWB’s SpaceMaker ToolKit™
Hierarchical Storage Management (HSM) software automates the process of
migrating files to less expensive storage. It is designed to manage mass storage
devices and repositories.
The growth of information stored on-line over the last 10 years is unprecedented. More users are accessing complex applications, creating large data
files, and downloading more information than ever before. Over the next five
years, both users and administrators will struggle with straining storage
capacities.
You can add more disk space and archive unused files, but the costs associated
with these strategies can be staggering. Within an enterprise, installation and
maintenance of a single disk drive costs four to five times the price of the
drive. Archiving alone burdens users and administrators alike with productivity losses.
This has created the need for real storage management tools. With HSM, as a
disk becomes full or as files age, they can be moved to optical or tape media to
create free space on primary storage. HSM works continuously and is completely transparent to users. HSM delivers significant cost and time savings
while boosting data access with near limitless storage.
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Managing data storage: the hidden costs
Most computer users don’t spend much time thinking about data storage
management. Nevertheless, it is a significant cost center for computer-integrated businesses. Managing two gigabytes of data over a one-year period costs
the average business about $7,000. This is the time spent administering and
servicing the device, restoring or recovering lost data, and all the other activities that go into maintaining an MIS—it does not include the cost of the disk
drive. Data management can also be quite expensive for home users. The
objective of HSM is to save money by lowering storage hardware and support
costs.
There are currently three primary techniques for addressing soaring storage
requirements (Table 54).
Table 54. Three common techniques for storage management
Solution
Comments
Buy more hard drive storage
• A 1 GB GB disk drive can cost from $.25 per MB to $2 per MB for RAID storage.
• System down-time, parts, and labor can double the cost of adding a disk drive.
• Increasing capacity increases backup, restore, troubleshooting, repair, and
administration costs.
Manually delete files
• Manually grooming a disk can cost $10-$100 per hour for high paid end-users.
• Many users will delete important data or application files.
• It’s inefficient.
Archive files off-line
• Like manual deletion, manual archiving is costly to perform.
• Finding and restoring archived data is labor intensive and can become a
tremendous burden as archived data grows.
Users often choose to add disk drives to address capacity problems. But, even
as storage prices fall, the exponential growth in the number and size of application, image, and data files continues to outpace cost-effective addition of
storage capacity. Much of this problem could be eliminated if “inactive” data
could be stored elsewhere but still be accessible. A large portion of information stored on a typical server could be moved to secondary storage. HSM software manages this process intelligently.
On-line, off-line, and near-line storage
Storage for applications and data can be broken down into three categories:
• on-line
• off-line
• near-line
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Table 55 explores these storage options.
Table 55. On-line, off-line, and near-line storage
Application
Description
On-line
On-line storage provides virtually immediate access to your needed applications and
data. Hard disks and system memory are the most common types of on-line storage.
• RAM … $15–$50 MB
• Array … $2–$5 MB
• Disk … $.50 MB
Off-line
• Tape … $.10–$.50 MB
• CD-ROM … $.01–$.15 MB
Near-line
• Optical Cartridge … $.40–$1 MB
• Optical Jukebox … $.20–$2 MB
Backup tapes are a good example of Off-line storage. This data cannot be accessed
without a user physically placing the media in the computer or drive.
Near-line storage manages the middle ground: faster than off-line but more
economical than on-line. The category comprises technologies such as magnetooptical disks and network servers.
Archiving, backup, and HSM
There is significant confusion over the role of Archiving and Backup in an
HSM system. These activities are an essential part of any storage management program and should be performed hand-in-hand with HSM.
Archiving
Archiving is the transfer of infrequently used data files from primary storage
to less expensive media—usually tape or removable disks of some sort. The
files are preserved for future use, but are essentially off-line. The expensive
primary storage space made available by the removal of the files can then be
used for more pressing matters.
Judicious archiving can save a considerable amount of money by reducing the
requirements for hard-disk capacity. On the Macintosh, archiving can be done
manually by dragging files to secondary storage and erasing files in the Finder.
However, for many users it will be more efficient to partially automate the
process with archival software.
While archiving is good at minimizing the hardware costs of data storage, it is
of no help in the case of drive failures, viruses, theft, or such disasters as fire,
flood, and earthquake. This is because archiving alone only maintains a single
copy of any data file.
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Backup
Backup is a process that creates duplicate copies of data files. If the original
files are lost or damaged in some way, they can be restored from the duplicates. Regular backup is an essential part of a comprehensive data security
strategy. Several software packages currently offer robust backup features.
HSM
Archiving and backup are important processes in data storage management.
However, they don’t provide a complete solution. To achieve that, HSM software should be included in the mix.
HSM software provides the following storage-management benefits:
• prevents out-of-disk space conditions
• lowers data management costs
• improves data accessibility
Hierarchical storage management software automatically moves infrequently
used data to less expensive storage devices. Unlike archiving, however, it
moves the data to storage that is maintained on-line and leaves a trail as it
does so, such as an alias file in the original file’s location. You can use this
trail to retrieve data automatically when you need it. Typically, the entire process is transparent to the user. Accessing files on the primary hard drive is virtually identical to accessing files that have been moved by HSM.
HSM solutions typically employ three levels of hierarchy:
• Primary storage typically consists of fast Winchester drives or disk arrays.
• Secondary storage typically consists of slower, higher-capacity drives or
removable cartridge devices.
• Tertiary storage typically consists of a much slower tape device.
How HSM software works
HSM software is designed to automatically move (migrate) and retrieve (demigrate) files based on a set of rules and filters defined by the user. These userdefined rules allow the software to compile a list of “migration candidates.”
This list contains the files that will be moved off-line to secondary storage.
Rules determine where, when, and which files are migrated or demigrated.
Filters can prevent identified types of files from migrating.
Once a user determines what rules and filters should be in effect, the software
acts automatically, based on those determinations. The user can intervene at
any time to change the rules and filters.
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• The Watermark Rule governs how much empty space should always be
available on the primary storage device (the source). For example, if the
“high watermark” is set to 80 percent and the “low watermark” is set to
70 percent, whenever the source becomes more than 80 percent full, data
will be migrated until the used capacity on the source is below the low
watermark.
• The Cobweb Rule monitors when a file was last accessed (not just
modified). Last-access date is a good indicator of whether or not a file
needs to be kept on primary storage. Files not accessed for 90 days are less
likely to be needed in the future.
NOTE
You can use both the Watermark and Cobweb rules together. When you do, HSM software first
selects unused files based on the aging criteria, then checks to see if any additional files need to be
migrated, based on the defined watermark threshold.
• Filters prevent certain files or folders from becoming candidates for
migration. For example, you wouldn’t want your System Folder migrated
to another drive. A filter would prevent that. Other filters might keep
applications, certain file types, or specific files from being migrated.
Migration movement of candidate files takes place in several steps:
1. Once a file has been selected for migration and the user-defined migration time is at hand, HSM copies the file to a secondary storage device
that has been designated as the destination.
2. HSM verifies that the copy is identical to the original file.
3. After a successful verification, HSM deletes the original file and replaces
it with an alias or “pointer” bearing the same name as the original file.
When you access a migrated file through its alias or pointer, it may be demigrated immediately or “tagged” for retrieval at the next scheduled migration.
If immediate demigration is selected, there may be a slight delay as HSM
moves and opens the file.
The best HSM solutions are easy to configure and transparent to the end-user.
Nevertheless, this software always informs the user if a recall is in progress
and allows cancellation of the process if system response time is impaired.
File compression during migration is a growing trend in HSM applications.
The higher capacity offered by compression is well worth the additional delay
experienced when accessing infrequently used files.
Back-up software and other utilities that scrutinize files on disk can unknowingly force simultaneous demigration of files from the hierarchy. It is important that you select an HSM application that can differentiate between true
file access and access by another probing application.
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The benefits of HSM software
HSM software reduces the cost of data storage by automatically and transparently moving infrequently used files to less expensive storage media. However, the files remain online. Furthermore, aliases are left behind with the
exact names and locations of the moved files. This means that, as far as the
user experience goes, it’s as if the files had never been moved. HSM provides
similar cost savings to archiving, with better data access time.
Archiving, backup, and HSM are related, but not identical. They are all components of a well-planned data storage management system. Used together,
they can ensure data integrity, reduce storage costs, and maintain easy access
to data. The best solutions are easily configured and operate transparently to
the end-user. With a little planning, HSM can be implemented to solve the
storage dilemma in an intelligent and cost-effective way.
Hierarchical Storage Management software, such as FWB’s SpaceMaker ToolKit, is available from third-party vendors of Macintosh software.
Several vendors on the PC offer HSM software. Versions are available for single users as well as network file servers.
CD-R Mastering
CD-R mastering software is used to create compact discs. The software formats data into forms that adhere to industry standards for data interchange.
Data sources could be files, disks, or even other CD-ROMs. A user has the
capability to produce data discs, hybrid data discs, audio discs, video-CDs, or
even CD-Extra discs. The main task of CD-R mastering software is to ensure
that there is a continuous stream of data from the source drive. Any hiccups
in data will create a flawed disc.
CD-R mastering software is built into neither the Macintosh nor the PC operating systems. Third-party solutions for CD-R mastering are available for both
platforms.
Disk Benchmarks
Disk benchmark utilities are used to measure disk performance. There are
several types of benchmarks, each measuring its own set of criteria. These
utilities are useful in measuring performance gains when adding new drives or
software.
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The leading benchmark utilities for the Macintosh are FWB’s BenchTest and
Ziff-Davis’ MacBench. BenchTest is a low-level disk benchmark program,
measuring many important low-level characteristics of drives. MacBench is a
file-system level benchmark program. It’s good at measuring real-world performance of drives used for office-oriented applications.
The most popular benchmarks for the PC are the Ziff-Davis Benchmark suite.
Ziff’s PCBench, WinBench, and Winstone measure real-world and artificial
performance of disk subsystems and the computer. They are good at mirroring
typical office application performance.
Other popular benchmarks include CoreTest for measuring burst transfer and
sustained data rates in DOS, and Drive Rocket for measuring many different
transfer rates under DOS. DiskPerf and VidTest, on Microsoft’s Development
Platform CDs, are good benchmarks for Windows 95 and NT operating systems. They perform benchmarks for large data transfers, mirroring high-end
video, and color publishing applications.
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10
Troubleshooting
Many things can happen to create errors and crashes on your storage device,
including:
• power surges or outages
• viruses
• system errors
These errors can corrupt data the computer uses to keep track of the contents
of the drive. Depending on what data has been corrupted and how badly it has
been corrupted, the result can be barely noticeable or could render your hard
drive unusable.
What follows are lists of common problems, probable causes, and recommended actions. If the proposed solutions fail to correct the problem, contact
the technical support department of your peripheral’s vendor.
General Troubleshooting
Problem: Formatting Problems
Symptoms
Can’t format a drive.
Chapter 10: Troubleshooting
Possible Causes
• Incorrect cabling.
• Improper termination.
Recommended Actions
• Make sure only the first and last
device are terminated.
• Make sure that total cable length
doesn’t exceed the SCSI maximum (3
meters for SCSI-2).
Incorrect interleave.
Make sure you format the drive with an
interleave that is supported. (Optical discs
come preformatted with a 1:1
interleave—tracks are etched into the
platter and can’t be reinterleaved. If you
choose to reformat an MO disc with FWB’s
Hard Disk ToolKit, select the default.)
Format software times out.
Get an updated formatter that can format
bigger drives.
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Problem: Drive Not Seen on SCSI bus
Symptoms
Possible Causes
Recommended Actions
Can’t see a drive on-line.
No power to drive.
Check to be sure the drive is plugged into
a “live” power source.
With internal and external drives
connected to a SCSI card, none of the
drives will mount or show up on the card’s
SCSI bus.
Incorrect cabling; improper termination.
• Make sure only the first and last
device are terminated.
• Make sure that total cable length
doesn’t exceed the SCSI maximum (3
meters for SCSI-2).
• Make sure the cables are plugged in
securely.
• Make sure all cables are securely
mated together.
• Make sure Pin 1 is correct on all
internal ribbon cabling.
Drive has hardware problem.
• Use a utility to scan the bus. If you
can’t see the drive, it may have a
hardware problem.
• Make sure the drive was spun up
when the computer was turned on.
• Check the drive’s power cord and
power supply.
SCSI ID problems.
• Make sure no two devices are set to
the same ID
• Make sure that no devices are set to
ID 7.
SCSI host adapter problem.
Make sure the SCSI host adapter is
properly seated in its slot. Make sure that
all settings on the SCSI host adapter are
accurate.
Card termination not removed.
When both internal and external drives
are connected to a SCSI card, terminators
should be removed or disabled from the
card.
Please refer to your SCSI card’s manual for
details.
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Problem: Spinup Problems
Symptoms
Possible Causes
The drive doesn’t spin up and the LEDs
don’t light up.
No power or a blown fuse.
Recommended Actions
• Make sure the power supply can
produce enough power for the drive.
• Check and replace the power cable.
• Make sure the outlet is active.
• Check the drive’s fuse. If it is burnt,
replace it with a fuse having the same
specifications. Make sure the fuse is
the fast-blow type.
If the problem continues or the fuses
repeatedly blow, you may have a power
supply problem; contact technical support
of your drive’s manufacturer.
Drive doesn’t spin up to a “ready” state.
Controller card is bad.
Scan with a SCSI utility, if the drive is not
ready, it may need repair or replacement.
With a new drive installed, the drive will
not spin up, or does not spin up reliably.
Hard drive draws more power than the
computer’s power supply can give.
Certain internal hard drives tend to use
the power that is reserved for cards in
order to spin up. In addition, some cards
require power beyond the limit for a single
card, and end up tapping into the power
reserved for other cards. The only
solutions are:
• Get a drive that will work within your
computer’s power limits.
• Remove the offending card (or cards)
that exceeds the power limitation.
OR
• Install the peripheral into a computer
with a more robust power supply.
Jumper settings are wrong.
276
Check your settings against the drive
documentation.
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Macintosh Troubleshooting
Mac boot process
The Macintosh has a complicated process for starting up the machine. Knowing more about the process will allow users to troubleshoot problems they
have booting in the future and understand better how the Macintosh works.
NOTE
For information on the PC boot process, see “PC boot process” on page 285.
1. When the Macintosh is powered on, it performs a test of RAM and other
motherboard components. Motherboard video is initialized.
2. The Mac ROM initializes all NuBus or PCI cards, causing drivers to be
loaded from these cards. Video card drivers are loaded first, then drivers
from other cards.
3. At boot time:
• If you press Command-Option-C, the Mac will attempt to boot off
its CD-ROM drive. (On Macintoshes introduced from 1995.)
• If you press Command-Option-Tab, the Mac will attempt to boot off
its floppy drive.
• If you press Shift-Command-Option-Delete, the Mac will look for a
bootable drive other than the one specified in the Startup Disk control panel.
• If you press Shift-Command-Option-E and a number from 1 through
9, your system will start up from the partition that corresponds to
the number you pressed. You will need to know the order of all your
HFS partitions to do this successfully.
4. It checks for bootable floppies in the floppy drive.
5. The Mac looks at its parameter RAM (PRAM) to see if a device has been
chosen in the Startup Device control panel. Once drivers for all block
devices that can be found are loaded, the value stored in PRAM is compared with those of each drive in the drive queue. When the correct
drive is found, it is mounted and the chosen folder (if any) is sought.
6. The Mac then looks at the Startup Device’s partition map and loads a
SCSI driver for the Startup Device from its Apple_Driver partition.
7. If no Startup Device is set or if the Startup Device is missing, the Mac
scans the first SCSI bus it encounters and looks for bootable drives from
the highest SCSI ID to the lowest.
8. The SCSI driver is called to mount volumes of type Apple_HFS found in
the drive’s partition map and to read boot blocks in from the first volume.
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9. All other SCSI devices have their drivers loaded and are mounted.
10. Compatibility of the current System file and the computer is checked.
11. The SCSI driver is called to read the System in.
12. Startup screens are displayed if present.
13. Welcome to Macintosh is displayed.
14. If the Mac requires a System Enabler, the Enabler is read in.
15. MacsBug, if present in the System Folder, is loaded.
16. The Mac loads all System Extensions, Chooser Extensions, and Control
Panels that have Extensions embedded in them.
17. The Macintosh launches any Startup applications and loads the Finder
to bring up the Desktop.
Macintosh problems and recommended actions
If the following recommended actions fail to correct your system problems,
reformatting the drive and reinstalling a clean System Folder may do the
trick.
!
Remember that formatting will erase all data on the disk.
Make sure that the replacement System Folder is not corrupted and has no
special System Extensions in it. If you cannot solve the problems, call the
technical support department of your peripheral’s vendor for assistance.
Happy Mac problems
Problem: Happy Mac Problems
Symptoms
Possible Causes
Recommended Actions
The Happy Mac appears briefly, then
disappears, and a floppy disk icon appears
with a question mark in the middle of it.
No power to boot drive.
Check to be sure your boot drive is
connected to a “live” power source.
• Bad boot blocks.
• Corrupted System files.
OR
• Run a disk recovery program.
• Reinstall the Apple System Software.
• Update the driver on the hard drive.
• Corrupted disk data structures.
Happy Mac flashes on and off, and the
drive does not boot.
278
The boot blocks or System files have
become corrupted.
• Run a recovery program
• Reinstall the Apple System Software.
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Problem: Happy Mac Problems (Continued)
Symptoms
Possible Causes
Recommended Actions
Happy Mac appears but the drive seeks
repeatedly before booting.
A “dirty” shutdown, due to a bomb or
power outage. All data structures were not
properly updated before shutdown. The
Mac sees this upon start-up and reverifies
these structures.
None needed, once the Macintosh has
verified the structures. You may want to
run Apple’s Disk First Aid to verify that the
disk is in good condition. Use Restart to
reboot the computer and Shutdown to turn
off the computer instead of simply turning
off the machine.
Symptoms
Possible Causes
Recommended Actions
The Bomb System Error Dialog box
appears.
This can be caused by many software
problems including conflicts with an
extension, too little system heap, or
another application.
Attempt to restart the system. If the bomb
continues, check for free system heap
space, disable System Extensions, and
reinstall the software.
Problem: System Bomb
Heap space is a memory pool that is used
by the Macintosh operating system to
store system data.
NOTE: System error code descriptions are
available in public domain programs or in
Inside Macintosh books from Apple.
Sad Mac Problems
The sad Mac appears when the Macintosh fails one of its diagnostic tests on
start-up. The characters below the sad Mac indicate what has gone wrong. For
the Macintosh Plus and earlier, if the first two characters are “0F,” there is a
software problem. Any other two characters indicate a hardware failure. For
the Macintosh SE and later, if the first four characters are “000F,” the problem
is software, and the last four characters indicate the specific problem. Typically, this is caused by damaged Apple System Software.
NOTE
The error codes can be found in Inside Macintosh or in some public domain programs.
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Problem: Sad Mac Problems
Symptoms
Possible Causes
Recommended Actions
The Macintosh crashes with a SCSI
accelerator installed.
Possible card conflict.
Try reconnecting the card’s devices to the
native Macintosh’s SCSI port. If the
problem remains when no drives are
connected to the card, contact your card
vendor.
Parameter RAM corrupted.
Zap PRAM. PRAM is parameter RAM that is
backed up by a battery on the Macintosh’s
motherboard.
• If you’re using System 7 or newer,
restart and hold down the Command,
Option, “P” and “R” keys until you
hear the startup sound again.
• If you’re using System 6, hold down
the Command, Option, and Shift keys
while selecting Control Panel from the
Apple menu, and click OK in the
resulting dialog box.
This will clear the parameter RAM, and
should allow normal operation. You may
need a utility such as TechTool to clear all
PRAM.
Corrupted driver.
Reinstall the driver.
Symptoms
Possible Causes
Recommended Actions
Multiple icons appear on the Desktop for
the same drive. If you have partitioned
your drive, there should be a separate
icon for each partition that is mounted, but
no more.
SCSI ID conflict.
Problem: Drive Problems
Failed SCSI ID switch.
280
• Make sure that no two SCSI devices
share the same SCSI ID, and that the
ID numbers range between zero and
six (seven is reserved for the
Macintosh).
• Make sure cabling and termination
are OK.
Have an authorized service provider
replace the switch.
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Problem: Drive Problems (Continued)
Symptoms
Possible Causes
Recommended Actions
The drive does not mount.
There are many software or hardware
possibilities.
• Make sure your drive is properly
connected to the Macintosh, properly
terminated, and has a “live” power
source.
• Check the SCSI ID of the drive as well
as any other devices on the SCSI bus.
• Use your formatter software to test
the media and update the driver.
• Make sure any SCSI cards are seated
securely in their slots.
Parameter RAM corrupted.
• If you’re using System 7 or newer,
restart and hold down the Command,
Option, “P” and “R” keys until you
hear the startup sound again.
• If you’re using System 6, hold down
the Command, Option, and Shift keys
while selecting Control Panel from the
Apple menu, and click OK on the
resulting dialog box.
This will clear the parameter RAM, which is
RAM that is backed up by a battery on the
Macintosh’s motherboard.
Why can’t I see my drive in my formatter?
Multiple SCSI buses exist.
If you are using a Quadra 900/950 with a
PowerMac Upgrade Card or running SCSI
Manager 4.3 extension (part of System
7.5 and newer) on a PowerMac 8100,
PowerMac 8150, or PowerMac 9150, you
have 2 (two) SCSI buses. The internal drive
is usually on Bus 0 (internal). The internal
CD-ROM drive, any drives in the second
drive bay, and all external devices are
usually on Bus 1 (internal/external).
Chapter 10: Troubleshooting
To see these devices you need to select
another SCSI bus.
SCSI ID conflict.
Check the SCSI IDs of all attached devices.
Make sure that no devices on a given bus
have the same SCSI ID.
Bad drive (dead controller).
Repair drive.
Bad cable leading to drive.
Replace cable.
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Problem: Drive Problems (Continued)
Symptoms
Possible Causes
A “Disk is full” message appears when the
disk is not full.
• You may have large invisible or
temporary files.
• The directory may be corrupted.
• You have attempted to access a large
volume on a machine that does not
support large volumes.
The drive mounts but cannot be used as a
start-up disk.
• Bad boot blocks on the disk.
• System Software on the drive.
• A hardware problem with the drive.
Recommended Actions
Run a recovery program or Disk First Aid.
• Make sure drive is marked as start-up
device in the control panel.
• Check the hardware connections.
• Reinstall the System software.
NOTE: The Macintosh Plus will only start
up off the device with the highest SCSI ID
number.
A dialog box appears saying “This disk
needs minor repairs. Do you want to
repair it?”
The invisible Desktop file has become
corrupted due to abrupt shutdown.
Click OK. The Macintosh will rebuild the
Desktop file.
A dialog box appears saying “This disk is
unreadable. Do you want to initialize it?”
Corrupted data structure on the disk.
Do not click on OK; this will erase the
data on the disk. Run a disk recovery
program.
Blind data transfers are not supported.
You should turn blind transfers off under
the following circumstances:
In Blind Data Transfer mode, the CPU
allows the SCSI chip to oversee transfers,
freeing the CPU for other tasks. The CPU
checks in only once before a block of data
is transferred, requiring constant timing of
the computer rather than a polling
method, where the CPU would have to
check for a Request/Acknowledge
handshake with every byte transferred.
The polling method requires more CPU
time, so blind transfers complete much
faster.
282
• The device is not fast enough to keep
up.
• The device has irregular timing with
the SCSI chip.
• When using Daystar™ or other third
party CPU accelerators that are
incompatible with blind SCSI
transfers.
FWB’s Guide to Storage
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Problem: Drive Problems (Continued)
Symptoms
Possible Causes
Recommended Actions
The drive starts to mount, but then
crashes.
Damaged System File.
Boot off another disk by holding down
Command-Option-Shift-Delete at startup.
The hard disk’s System Folder or the SCSI
drivers may have been corrupted by a
system crash or virus.
Replace the System Folder on the hard
disk first by booting initially off a floppy
disk and reinstalling using the Installer.
This could have many causes.
If the problem persists after reinstalling
the System Folder, use your formatter
software to update the drivers on the hard
disk.
Multiple System Folders on the drive.
Make sure you have only one System and
Finder pair on your drive. Find any extra
ones using Find File, and delete them. If
you try to remove an active System/
Finder, you will get an error indicating that
it is in use. Remove all inactive System and
Finder files.
The Desktop file may be corrupt.
Rebuild the invisible Desktop file:
1. Hold down the Command (Open
Apple key) and Option keys while the
Macintosh is starting-up.
2. A dialog box will ask you if you want
to rebuild it. Click OK.
This does not damage your data, but does
remove any file comments.
The Desktop file may be too big.
With System 6.X, disks with Desktop files
larger than about 275 KB would cause
resource manager problems.
The Finder may not have enough memory.
Under System 6.X MultiFinder, the Finder
is normally allocated 160 KB. You can see
how much memory it is using by bringing
up “About The Finder” from the Apple
menu.
Too little system heap space. Under System
6.X, especially when many INITs or System
Extensions are loaded, you may need to
increase heap space.
Chapter 10: Troubleshooting
• Upgrade to System 7.
OR
• Partition the drive.
• Increase the Finder’s memory
partition by doing a Get Info on the
Finder. Try allocating 256 KB or
more.
OR
• Upgrade to System 7.
Use a utility program, such as Heapfixer,
to increase this system setting.
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Problem: Drive Problems (Continued)
Symptoms
Possible Causes
Recommended Actions
The directory on your hard disk may be
damaged.
Repair the directory with Apple’s Disk First
Aid. Use a recovery program or restore the
data from previous backups. Use a lowlevel bad block scan program to search for
bad blocks.
A virus may have infected your hard disk.
Check your hard disk with a virus
detection and eradication program.
Problem: Removable Media Problems
Symptoms
Possible Causes
The drive does not mount.
Incorrect SCSI ID, faulty cabling, no
termination, or an inappropriate cartridge.
Light is flashing in the front of drive without
media inserted.
284
Recommended Actions
• Make sure the cabling and SCSI ID of
the drive are correct and secure.
• Make sure the bus is properly
terminated.
• Make sure the cartridge is formatted
for the Macintosh and contains valid
data.
• If you have not installed your
formatter’s extension in your boot
drive’s System Folder, and you booted
up without a cartridge in the drive,
you need to mount the cartridge by
running your formatting software, or
from within a Control Panel based
mount utility.
Bad or missing driver.
Reload the device driver that came with
your removable media drive.
Damaged cartridge.
The media may be damaged (particularly
if dropped). Do not insert a damaged disc
into the drive because it could damage the
drive heads.
Hardware problem.
If termination and cabling are correct, the
drive may have a hardware problem.
Contact the drive manufacturer.
FWB’s Guide to Storage
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PC Troubleshooting
PC boot process
The PC has a complicated process for starting up the machine. Knowing more
about the process will allow users to troubleshoot future boot problems and to
develop a better understanding of how the PC works.
NOTE
For information on the Macintosh boot process, see “Mac boot process” on page 277.
1. The PC is powered on and the BIOS performs a test of RAM and other
motherboard components.
2. Video cards are initialized and the default video mode is enabled.
3. The machine looks at its CMOS setting to determine if any system
drives (IDE, ESDI, ST-506) are in the computer.
STOP
CMOS stands for Complementary (symmetry) metal-oxide semiconductor. It is a memory chip
that permits many components to be packed together in a very small area. The main
characteristics of CMOS chips are low power consumption, high noise immunity, and slow speed.
4. The BIOS initializes all cards, causing drivers to be loaded from these
cards and banners to be displayed.
If SCSI cards are encountered, each SCSI card’s BIOS will scan its bus
and allocate INT13 numbers for any usable drives.
5. The machine checks for bootable floppies in the floppy drive.
6. If a system drive is found, the PC will attempt to boot from it.
If no system drives are in the system, the PC will attempt to boot from
the first SCSI host adapter card it encounters.
7. Using the INT13 interface, the startup drive will be examined to see if it
has a valid boot sector.
STOP
A valid boot sector consists of two components: boot loader code and a partition information
table.
8. If the startup drive has a valid boot sector, the BIOS will execute the
boot loader code contained in the boot sector.
9. The boot loader examines the partition table for an active, or bootable,
partition. The first sector of that partition contains operating systemspecific boot code and operating system-specific partition information.
Chapter 10: Troubleshooting
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10. The partition boot code is executed and proceeds to load in whatever
operating system is present on that partition (for DOS, this would be the
hidden files IO.SYS and MSDOS.SYS).
PC problems and recommended actions
If the following recommended actions fail to correct your system problems,
reformatting the drive may do the trick.
!
Remember that formatting will erase all data on the disk.
If you cannot solve the problems, call the technical support department of
your peripheral’s vendor for assistance.
Problem: PC Problems
Symptoms
Possible Causes
Cannot see a new drive.
Drive is not set up correctly.
• IDE drives must be set up using the
ROM based Setup program. SCSI
drives usually do not need to be
allocated in the Setup program.
• Make sure cables are plugged in the
right way.
Drive is set to wrong SCSI ID.
Some SCSI Host Adapters require drives to
be set to SCSI ID 0, the next drive to 1,
and subsequent drives in sequence.
Damaged operating system.
Boot off emergency boot disk and update
operating system on drive using the SYS
command.
Invalid CMOS settings.
Use your BIOS Setup utility to reconfigure
your drive. You should write all these
CMOS settings down so that you can reenter them if they get corrupted.
Damaged Master Boot Record or partition
table.
Run a disk recovery program.
DOS boot record is missing.
Run FDISK to make the partition active.
Missing Operating System
Damaged boot block.
Run a recovery program to repair the
block.
General failure reading drive C:
Defect on disk.
Run a disk test program such as SCANDISK
to map out bad blocks.
•
•
•
•
•
286
Disk boot failure
Cannot find system files
Invalid partition table
Invalid drive specification
Error loading operating system
Recommended Actions
FWB’s Guide to Storage
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Problem: PC Problems (Continued)
Symptoms
Possible Causes
Recommended Actions
Drive is running very slow
File fragmentation
Run a disk optimizer program to clean up
your disk.
Lost Clusters Found on drive.
Entries in the file allocation table (FAT)
cannot be found on any FAT chains.
Use CHKDSK or SCANDISK to convert them
to files and fix the directories.
Cannot boot off a drive.
Incorrect BIOS settings
Make sure no drives are configured in
your BIOS. These system drives boot
before any drives are present on host
adapters. Devices present on host adapters
are booted from in order of their Device
Number on the PCI bus. Place devices that
are bootable in lower number slots.
Drive connected but not ready.
Drive was not fully spun up when it
needed to be.
Turn on the drive before turning on the
computer. Set a delay in your host
adapter’s scanning routine.
SCSI Bus scan hangs.
A device does not support synchronous
data transfers or SCSI parity checking.
Disable synchronous transfers and/or
parity for the device in question.
Inserting a new card causes the computer
to hang during initialization.
Card is incompatible with motherboard or
other cards.
Make sure that there are no conflicting
IRQ, DMA, memory address, or I/O port
settings. See if an updated BIOS is
available for card or other cards.
Operating system does not see removable
media changes.
Removable media drivers not loaded.
Disable BIOS support for removable drives
and use driver based removable support.
Chapter 10: Troubleshooting
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Appendix A: Additional Information Sources
Web Sites
Industry or Organization
Web Site Address
1394 Trade Association
http://www.firewire.org/
Fibre Channel Association
http://www.Amdahl.com/ext/CARP/FCA/FCA.html
HP Tachyon
http//tachyon.rose.hp.com
PCI Special Interest Group
http://www.pcisig.com/
SCSI Trade Association
http://www.scsita.com/
SCSI-2 Rev 10L on-line in HTML
http://abekas.com:8080/SCSI2/
SSA Industry Association
http://www.ssaia.org/
VESA Local Bus
http://www.vesa.org /
X3 Standards Committee home page
http://www.x3.org/
X3T10 Home Page
http://www.symbios.com/x3t10/
FTP Sites
Industry or Organization
FTP Site
ATA FTP Site
fission.dt.wdc.com
Fibre Channel documents FTP Site
ftp.network.com
FCSI Fibre Channel Profiles FTP Site
playground.sun.com
SCSI Anonymous FTP Site
ftp.symbios.com
Appendix A: Additional Information Sources
Directory: /pub/standards/ata/
Directory: /pub/standards/io/
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I/O Interface Reflectors (mailing lists):
The following table lists information about several electronic mailing lists
that you can “join.” To join a list:
1. Write to the address in the To Subscribe or Unsubscribe column in Table
56. (Use the same address to request removal from a list).
2. Type the information in the Subject column in the e-mail subject line.
3. In the body of a subscription message put:
[subscribe or unsubscribe] [keyword] [your e-mail address]
All of the majordomo and listserv reflectors are automatic. Be sure to use the
format listed in Step 3.
NOTE
On the reflectors using majordomo, your e-mail address is optional. If you include it and it
doesn't match the address in the e-mail headers, there will be a delay while humans verify your email address.
To post a message, send it to the address in the To Post a Message column.
STOP
These lists are often called Reflectors or Exploders because mail sent to one address gets sent to
everyone on the list (causing a small explosion of mail activity).
Table 56. I/O interface reflectors
Subject
To Subscribe or Unsubscribe …
To Post a Message …
Keyword
ATA
majordomo@dt.wdc.com
ata@dt.wdc.com
ata
ATAPI
majordomo@dt.wdc.com
atapi@dt.wdc.com
atapi
CD-Recordable
majordomo@dt.wdc.com
cdr@dt.wdc.com
cdr
Disk Attach
majordomo@dt.wdc.com
disk_attach@dt.wdc.com
disk_attach
FC Class 4
majordomo@northyork.hp.com
fc-class4@northyork.com
fc-class4
FC IP Prot.
majordomo@think.com
fc-ip-ext@think.com
fc-ip-ext
FC Isoch.
majordomo@northyork.hp.com
fc-isoch@northyork.hp.com
fc-isoch
Fibre Chan.
majordomo@think.com
fibre-channel-ext@think.com
fibre-channel-ext
HIPPI
majordomo@think.com
hippi-ext@think.com
hippi-ext
IDETAPE
majordomo@dt.wdc.com
idetape@dt.wdc.com
idetape
IEEE P1394
bob.snively@eng.sun.com
p1394@sun.com
n/a (human)
IPI
majordomo@think.com
ipi-ext@think.com
ipi-ext
MultiMedia
majordomo@dt.wdc.com
mmc@dt.wdc.com
mmc
Appendix A: Additional Information Sources
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Table 56. I/O interface reflectors (Continued)
Subject
To Subscribe or Unsubscribe …
To Post a Message …
Keyword
PCMCIA
listserv@cirrus.com
pcmcia-gen@cirrus.com
pcmcia-gen
SCSI
majordomo@symbios.com
scsi@symbios.com
scsi
SFF
bob.snively@eng.sun.com
sff_reflector@sun.com
n/a (human)
SSA
majordomo@dt.wdc.com
ssa@dt.wdc.com
ssa
STA
SCSI Trade Assn.
majordomo@symbios.com
System Issues
majordomo@dt.wdc.com
si@dt.wdc.com
si
10bit
majordomo@dt.wdc.com
10bit@dt.wdc.com
10bit
star
American National Standards Institute (ANSI)
If you wish to contact ANSI, write or call:
ANSI
1430 Broadway
New York, NY 10018
Phone: (212) 642-4900
ANSI documents are published by
11 West 42nd Street
13th Floor
New York, NY 10036
Sales Department: (212) 642-4900
The SCSI Trade Association (STA)
STA was formed in 1995 to pursue the following ends for parallel SCSI:
• promote increased use
• promote better understanding
• shape the evolution
For more information about SCSI or the STA contact:
Joe Molina, Technology Forums LTD
13 Marie Lane
St. Peter, MN 56082-9423
290
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Phone: (507) 931-0967
Fax: (507) 931-0976
e-mail: tforums@ic.mankato.mn.us
Source for Additional Fibre Channel Information
The Fibre Channel Loop Community (FCLC)
P.O. Box 2161
Saratoga, CA 95070
Phone: (408) 867-1385
Fax: (408) 741-1600
Source for Additional PC Card Standard Information (PCMCIA)
The PC Card Standard is published by
Personal Computer Memory Card International Association
2635 North First Street, Suite 209
San Jose, California 95131
Phone: 408-433-CARD (2273)
Fax: 408-433-9558
BBS: 408-433-2270
Source for Additional PCI Information
PCI Special Interest Group
PO Box 14070
Portland, Oregon 97214
Phone (US): 800-433-5177
Phone (international): 503-797-4207
Fax (all): 503-234-6762
Source for Additional SFF Information
SFF documents are published by
SFF
14426 Black Walnut Court
Saratoga, California 95070
Fax: 408-741-1600
Source for Additional X3T10 Information
X3T10 Bulletin Board System: 719-574-0424
Appendix A: Additional Information Sources
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Appendix B: Suggested Additional Reading
This section contains a list of storage-related publications that provide more
information on particular topics.
A Case for Redundant Array of Inexpensive Drives (RAID), Patterson, David A; Gibson, Garth;
Katz, Randy H.1987.
ANSI Standards
ST506 X3.170-1990
ATA CAM X3.221-1994
ATA X3.279-199x
ATA-2 Draft Specification
ATA-3 Draft Specification
SCSI X3.131-1986
SCSI-2 X3.131-1993
American National Standards
Institute.
Basics of SCSI, Ancot Corp1993.
Computer Technology Review, West
World Productions Publication,
ISSN 0278-9647, 1990–1995.
Conner CFA810A/CFA1080A Product Manual Revision A, Conner
Peripherals, May 1994.
Designing Cards and Drivers for the
Macintosh Family, 3rd Edition,
Addison Wesley Publishing
Company.
Appendix B: Suggested Additional Reading
Designing PCI Cards and Drivers
For Power Macintosh Computers, Apple Computer, 1995.
Electronic Engineering Times, CMP
Publications, 1993–1995.
Fast Track to SCSI, Prentice Hall,
ISBN 0-13-307000-X.
Hard Disk Secrets, Goodman, John
M., IDG Books, ISBN 1-87805864-9.
Inside Macintosh: Devices, Addison
Wesley Publishing Company,
ISBN 0-201-62271-8, 1995.
Introduction to Redundant Array of
Inexpensive Drives (RAID),
Patterson, David A; Gibson
Garth; Katz, Randy H., 1987.
Quadra 630 Computer Developer
Note, Apple Computer, 1993.
SCSI Bench Reference; SCSI Tutor,
ENDL Publications, 1991, ISBN
1-879936-11-9.
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SCSI Interconnection Guide Book,
AMP, 1993.
SCSI-3 Documents, Global Engineering Documents, 1995.
Small Form Factor documents, SFF,
1993.
The Book of SCSI: A Guide for
Adventurers, No Starch Press,
ISBN 1-886411-02-6.
The Hard Drive Encyclopedia, Alting-Mess, Adrian, Annabooks,
ISBN 0-929392-10-8.
Appendix B: Suggested Additional Reading
The RAIDbook, A Source Book for
Disk Array Technology, RAID
Advisory Board, 1994.
The SCSI Bus and IDE Interface,
Schmidt, Friedhelm, AddisonWesley Publishing Company,
ISBN 0-201-42284-0, 1995.
The SCSI Encyclopedia, Endl Publications, 1991.
What is Fibre Channel, Ancot Corp,
1994.
Glossary–293
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Appendix C: The Deep End
In your exploration of storage and the technology surrounding it, you may
want to dive more deeply into some of the commands, bus phases, and conditions that initiate, allow, and specify actions taken by a system and a device as
they communicate with each other. Appendix C provides tables that offer a
deeper level of information about the communications that pass between a
system and the peripheral devices attached to it. References throughout these
subsections point to places in text where these protocols are more fully
explained.
SCSI Communications
SCSI signals and bus phases
Table 57 illustrates which control signals are asserted on which device (initiator or target) during the various Bus Phases. The Data Bus column (DB) lists
the devices that are asserting their IDs during each phase. The Reset control
signal is not represented because it clears the bus rather than asserts itself
during any of the Bus Phases.
NOTE
For more information about Bus Phases, see “Bus Phases” on page 169.
Table 57. Signals asserted during different bus phases
Bus Phase
BSY
SEL
C/D, I/O,
MSG, REQ
ACK/ATN
DB (7–0, P)
Bus Free
None
None
None
None
None
All
Winner
None
None
All Active
Select
INIT/TGT
INIT
None
INIT
INIT
Reselect
INIT/TGT
TGT
TGT
INIT
TGT
TGT
None
TGT
INIT
INIT
Arbitration
Command
INIT = Initiator
TGT = Target
Appendix C: The Deep End
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Table 57. Signals asserted during different bus phases (Continued)
Bus Phase
BSY
SEL
C/D, I/O,
MSG, REQ
ACK/ATN
DB (7–0, P)
Data-In
TGT
None
TGT
INIT
TGT
Data-Out
TGT
None
TGT
INIT
INIT
Status
TGT
None
TGT
INIT
TGT
MSG-In
TGT
None
TGT
INIT
TGT
MSG-out
TGT
None
TGT
INIT
INIT
INIT = Initiator
TGT = Target
SCSI Command Descriptor Block
The Command Descriptor Block (CDB) is a six-, ten-, or twelve-byte data
structure residing in the computer’s memory that contains the command code
and other information needed by a target to execute a command.
NOTE
For more information about the CDB, see “Command Descriptor Block” on page 159.
Table 58 lists the information in a Group 0 Command Descriptor Block.
Table 58. Group 0 Command Descriptor Block
Byte 00
Byte 01
Bit 7
Bit 6
Bit 5
0
0
0
Bit 4
Bit 3
Bit 2
Bit 0
Command Opcode
Logical Unit Number (LUN)
Logical Address (NS6)
Byte 02
Logical Address
Byte 03
Logical Address (LS6)
Byte 04
Transfer Length (Number of Blocks)
Byte 05
Control Byte
Appendix C: The Deep End
Bit 1
295
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Table 59 lists the information in a Group 1 Command Descriptor Block.
Table 59. Group 1 Command Descriptor Block
Byte 00
Byte 01
Bit 7
Bit 6
Bit 5
0
0
1
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Command Opcode
Logical Unit Number (LUN)
Reserved
Byte 02
Logical Address (MSR)
Byte 03
Logical Address
Byte 04
Logical Address
Byte 05
Logical Access (LS6)
Byte 06
Reserved (CO)
Byte 07
Transfer Length (Number of Blocks) (MSB)
Byte 08
Transfer Length (Number of Blocks) (LSB)
Byte 09
Control Byte
Reserved fields are always set to zeros.
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Table 60 lists the information in a Group 5 Command Descriptor Block.
Table 60. Group 5 Command Descriptor Block
Byte 00
Byte 01
Bit 7
Bit 6
Bit 5
0
0
1
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Command Opcode
Logical Unit Number (LUN)
Reserved
Byte 02
Logical Address (MSR)
Byte 03
Logical Address
Byte 04
Logical Address
Byte 05
Logical Access (LS6)
Byte 06
Transfer Length (Number of Blocks) (MSB)
Byte 07
Transfer Length (Number of Blocks)
Byte 08
Transfer Length (Number of Blocks)
Byte 09
Transfer Length (Number of Blocks) (LSB)
Byte 10
Reserved (CO)
Byte 11
Control Byte
Reserved fields are always set to zeros.
Appendix C: The Deep End
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SCSI-1 commands and opcodes
Table 61 lists and describes general device commands and their operations
codes. Commands marked with a plus sign (+) are accomplished without disk
access. A command followed by (NON) is not part of the CCS.
NOTE
For more information about SCSI commands, see “Commands” on page 161.
Table 61. General device commands and opcodes
Command
Opcode
Description
Test Unit Ready
00H
Checks if the drive is prepared to accept commands requiring disk access. If it
is, the completion status byte indicates Good and the sense key is set to No
Sense. If it is not ready, the status byte will indicate Check Condition, with the
sense key set to Not Ready.
Rezero Unit
01H
Requests that the drive actuator be moved to the cylinder zero and head zero
position.
Request Sense +
03H
Requests that sense data be sent to the initiator. Sense data is detailed error
information generated by the previous command from the same initiator. It is
saved until it is retrieved by that initiator or until another command is received
from that initiator. There is an Extended Sense format and a Non-extended
Sense format for devices that do not support Extended Sense. The Sense Key
field helps determine why the error occurred.
Table 62 on page 301 lists and describes the Sense Key fields.
298
Format Unit
04H
Assigns logical blocks to physical sectors, optimizing sequential access and
avoiding areas known or found to be defective. Formatting options vary with
different drives. Loss of data typically occurs when this command executes. We
recommend backing up data first.
Reassign Blocks
07H
Requests the drive to reassign defective logical block(s) to areas of the platter
reserved for this purpose. The drive keeps and updates a known defect list.
Defective logical blocks that have already been reassigned by the automatic
reallocation feature of Mode Select will be reassigned again.
Read
08H
Requests that the target transfer data to the initiator.
Write
0AH
Requests that the data transferred to the target be written to the medium. It
specifies the logical block address at which to begin, and the number of
contiguous logical blocks of data to be transferred.
Seek
0BH
Requests that the target position its head actuator at a specified logical block
address.
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Table 61. General device commands and opcodes (Continued)
Command
Opcode
Description
Inquiry +
12H
Requests that target identification information—manufacturer, model, and
parameter specifications—be sent to the initiator. This includes the device’s
model number and various revision levels, as well as whether it contains
removable media and supports the Common Command Set or SCSI-2.
Mode Select +
15H
No disk access required if the Save Parameters bit is not set. Allows the initiator
to specify the operating parameters of the drive, and optionally to save them
for future commands.
A Mode Select command overrides previous parameters. Executing Mode Select
in a multi-initiator system creates a Unit Attention condition for all other
initiators.
Reserve
16H
Requests that a logical unit be reserved for an initiator or some other specified
SCSI device.
The initiator will have exclusive use of the drive until one of the following
conditions exists: the same initiator issues another valid Reservation command
superseding the last command; a Reset message from any initiator is received;
or a hard reset occurs.
No error will occur if an initiator attempts to reserve a logical unit it has
already reserved. Drives already reserved will return a Reservation Conflict
status; however, if disconnection is supported, the drive will queue the Reserve
request and disconnect. If some other initiator issues a Release command, the
reserved drive will ignore it.
Release
17H
Requests that a drive’s reservation be canceled.
The Release message is only valid when issued by the initiator that reserved
the drive. No error will occur if an initiator attempts to release a logical unit
that is not reserved.
Copy
18H
Provides the means to copy data from one logical unit to another or the same
logical unit. The logical unit may reside on the same or different SCSI device.
This command requires that the device support disconnect.
Mode Sense +
1AH
No disk access is required if Save Parameters are not read. Enables the target
to report its operating parameters to the initiator in either its current, saved,
changeable, or default categories. This command complements Mode Select.
Start/Stop Unit
1BH
Requests that the drive be spun up or spun down. Can cause removable media
to be ejected.
Receive Diagnostic Result + (NON)
1CH
Requests that the results of the self-test be sent to the initiator.
Send Diagnostic +
1DH
Requests that the drive to perform diagnostic tests on itself. This is usually
followed by a Receive Diagnostic Result command.
Prevent/Allow Media Removal +
1EH
Enables or disables the ejection of removable media.
Appendix C: The Deep End
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Table 61. General device commands and opcodes (Continued)
Command
NOTE
300
Opcode
Description
Read Capacity (NON)
25H
Requests the target send information regarding the capacity and block size of
the drive.
Read Extended (NON)
2AH
Same as Read command, but can access more blocks.
Write Extended (NON)
2BH
Same as Write command, but can access more blocks.
Seek Extended (NON)
2EH
Same as Seek command, but can access more blocks.
Write and Verify
2FH
Requests that the drive write the data it has received from the initiator, and to
verify that it has been correctly written to the medium. Useful for optical
drives.
Verify (NON)
2FH
Requests the drive to verify that it has correctly written data on the medium.
Read Defect Data (NON)
37H
Requests that the drive transfer data to the initiator regarding defects on the
medium.
Write Buffer (NON)
3BH
A diagnostic command for testing the target’s memory and the SCSI bus
integrity. It is used in conjunction with the Read Buffer command. Neither
command affects data storage.
Read Buffer
3CH
See Write Buffer command.
Read Long (NON)
3EH
Requests that the drive transfer a sector of data, followed by error-correcting
code (ECC) data, to the initiator. This command is intended for diagnostic
purposes and is usually followed by the Write Long command.
Write Long (NON)
3FH
Requests that the data usually transferred by the Read Long command be
written to the specified logical block address. The number and order of bytes
should match those in the Read Long command.
The SCSI-2 specification also introduces new commands. See “SCSI-2 commands” on page 195
and “SCSI-2: new commands” on page 309 for more information.
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SCSI-1 Sense Keys and Sense Data
Sense Data is detailed error information generated by the previous command
from the same initiator. It is saved until it is retrieved by that initiator or until
another command is received from that initiator. There is an Extended Sense
format and a Non-extended Sense format for devices that do not support
Extended Sense. The Sense Key field helps determine why the error occurred.
Table 62 and Table 63 list valid values and descriptions of Sense Keys.
NOTE
For more information on Sense Keys and Sense Data, see “Commands” on page 161.
Table 62. Sense Keys
Appendix C: The Deep End
Sense Key
Description
0H
No Sense
No error occurred.
1H
Recovered Error
Command successful but recover action was needed.
2H
Not Ready
Device not ready to be accessed.
3H
Medium Error
Non-recoverable error caused by media flaw.
4H
Hardware Error
Non-recoverable error caused by hardware problem.
5H
Illegal Request
The command or data contained illegal parameter(s).
6H
Unit Attention
Bus reset or medium changed.
7H
Data Protect
Write protect error.
8H
Blank Check
Blank media encountered.
AH
Copy Aborted
SCSI Copy operation canceled.
BH
Aborted Command
Target aborted command.
CH
Equal
Search Data Command successful.
DH
Volume Overflow
End of media encountered with data left to be written.
EH
Miscompare
Source data doesn’t match data from disk.
FH
Reserved
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The ASC (Byte 12) and ASCQ (Byte 13) fields within sense data provide more
information on exactly what the error was. Table 63 lists common sense keys
and ASC and ASCQ decoding.
Table 63. Common Sense Keys with ASC and ASCQ decoding
Sense Key
302
ASC
ASCQ
Description
01
Recovered Error
18
8X
Recovered ECC.
01
Recovered Error
19
00
Defect List Error.
01
Recovered Error
1C
00
Defect List not found.
02
Not Ready
04
00
LUN not ready, unknown cause.
02
Not Ready
04
01
LUN not ready, becoming ready.
02
Not Ready
04
02
LUN not ready, formatting required.
02
Not Ready
04
04
LUN not ready, formatting.
02
Not Ready
31
01
Formatting failed.
02
Not Ready
40
80
RAM Failure.
02
Not Ready
40
83
RAM Failure.
02
Not Ready
40
84
RAM Failure.
03
Medium Error
11
00
Unrecovered Read Error.
03
Medium Error
11
03
Multiple Read Errors.
03
Medium Error
18
8X
Recovered ECC.
03
Medium Error
27
00
Write Protected.
04
Hardware Error
00
00
No Additional data.
04
Hardware Error
40
XX
Diagnostic Failure on component XX.
04
Hardware Error
41
00
Data Path Failure.
05
Illegal Request
1A
00
Parm List Length Error.
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Table 63. Common Sense Keys with ASC and ASCQ decoding (Continued)
Sense Key
Appendix C: The Deep End
ASC
ASCQ
Description
05
Illegal Request
20
00
Invalid Command Operation Code.
05
Illegal Request
21
00
LBA out of range.
05
Illegal Request
24
00
Invalid field in CDB.
05
Illegal Request
25
00
LUN not supported.
05
Illegal Request
26
00
Invalid field in Parm List.
05
Illegal Request
26
20
Parm Value invalid.
05
Illegal Request
2C
00
Command Sequence Error.
05
Illegal Request
3D
00
Invalid Bits in Identify Message.
06
Unit Attention
29
00
Power On, Reset, or Bus Device Reset.
06
Unit Attention
2F
00
Command Cleared by Another Initiator.
06
Unit Attention
2A
01
Mode Parameters Changed.
07
Write Protect
27
00
Write Protect Error.
0B
Aborted Command
00
00
No Additional Sense.
0B
Aborted Command
25
00
LUN not supported.
0B
Aborted Command
41
00
Data path Failure.
0B
Aborted Command
45
00
Select or Reselect Failure.
0B
Aborted Command
47
00
SCSI Parity Error.
0B
Aborted Command
48
00
Initiator Detected Error Message.
0B
Aborted Command
49
00
Invalid Message Error.
0B
Aborted Command
4E
00
Overlapped Command Attempted.
0E
Miscompare
00
00
No Additional Sense.
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SCSI-1 messages
The messages in Table 64 are defined by the SCSI specification. In the In/Out
column:
• In designates a message that moves from target to initiator.
• Out designates a message that moves from initiator to target.
NOTE
For more information on SCSI-1 messages, see “Messages” on page 164. For more information on
changes introduced in SCSI-2, see “SCSI-2 highlights” on page 195.
Table 64. SCSI messages
Message
Command Complete
Hex
In/Out
00
In
Description
Sent by the target to the initiator to indicate:
• It has executed the command (or a series of linked commands)
• A valid status has been sent to the initiator.
Command Complete is neutral. It is up to the status byte to indicate whether or
not the command was completed successfully. All SCSI devices must support the
Command Complete message.
A Command Complete or Disconnect message must precede a target’s releasing
the Busy signal. If the initiator detects that this has happened—as indicated
by the resulting Bus Free Phase—it should consider that an error has
occurred.
For more information, see “Control signals” on page 144 and “Bus Phases” on
page 169.
In = Target to Initiator
Out = Initiator to Target
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Table 64. SCSI messages (Continued)
Message
Extended Messages
Hex
In/Out
01
I/O
Description
Messages that require more than two bytes to send the necessary information.
The messages supported by the disk drive are:
• Modify Data Pointers (In)
Requests adding signed argument to the value of the current data pointer.
• Synchronous Data Transfer (I/O)
Sent to establish synchronous data transfer and its parameters, including:
– The transfer period, which determines transfer rate.
– The Request/Acknowledge offset, which sets the maximum
number of Request signals that may be sent before a
corresponding Acknowledge signal is received. Zero means the
devices are using the asynchronous mode. Other than zero
indicates a synchronous transfer mode has been agreed upon.
Macintoshes previous to the PCI versions do not support synchronous data
transfers. Virtually all Macintosh and PC SCSI host adapters support
synchronous data transfers.
• Wide Data Transfer (I/O)
Establishes an agreement between two SCSI devices on the width of the
data path to be used for Data Phase transfers between two devices.
Initiated whenever it is appropriate to negotiate a new transfer width
agreement, or whenever a previously agreed transfer width agreement
may have become invalid. For example, after:
– a Hard Reset condition
– a Bus Device Reset message
– a power cycle
For more information, see “Data Transfer Options” on page 139.
Save Data Pointer
02
In
Sent from the target to tell the initiator to save the present active data pointer
for that target.
For more information, see “Executing Commands” on page 159.
Restore Pointers
03
In
Directs the initiator to restore saved pointers to the active state.
Disconnect
04
In
Sent by the target to the initiator in Arbitrating systems to indicate that it is
going to release the Busy signal and thus disconnect, and that it will be
necessary to reconnect later to complete the operation.
Disconnect or Command Complete must precede a target’s releasing the Busy
signal. If the initiator detects that this has not happened—as indicated by the
resulting Bus Free Phase—it should consider that an error has occurred.
Initiator Detected Error
05
Out
Sent by the initiator if it detects an error. The initiator retries the operation. The
target responds with a Check Condition status, which terminates the operation.
In = Target to Initiator
Out = Initiator to Target
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Table 64. SCSI messages (Continued)
Message
Hex
In/Out
Description
Abort
06
Out
Clears the operation. All pending data and status for the initiator issuing the
Abort message are cleared, and initiator and target enter the Bus Free Phase.
No status or ending message will be sent for the operation. In multi-initiator
systems, the pending data and status for other initiators will be left intact.
Message Reject
07
Out
Sent by either the initiator or target to indicate that the last message was
either deemed inappropriate or was for some other reason not implemented.
For the initiator to send this message:
• It asserts the Attention signal.
• Then it sends its Acknowledge for the Request/Acknowledge handshake of
the message that it is rejecting.
For the target to send this message:
• It goes to the Message In Phase and sends it.
• Then it requests additional message bytes from the initiator.
It does this so the initiator knows which message is being rejected.
For more information, see “Bus Phases” on page 169.
No Operation
08
Out
The initiator’s response to a target’s request for a message, if the initiator has
no message to send.
Message Parity Error
09
Out
What the initiator tells the target if one or more bytes in the last message
received had a parity error.
For the initiator to send this message:
• It asserts the Attention signal.
• Then it sends its Acknowledge for the Request/Acknowledge handshake of
the message that it is rejecting.
It does this so the target knows which message has the parity error.
Linked Command
Complete
OA
In
Sent by the target to tell the initiator that a linked command has been
executed and a status byte has been sent. The initiator sets its pointers to the
initial state for the next linked command.
For more information, see “Executing Commands” on page 159.
Linked Command
Complete (With Flag)
0B
In
Sent by the target to tell the initiator that a linked command (with the flag bit
set) has been executed and a status byte has been sent. The initiator sets its
pointers to the initial state for the next linked command.
In = Target to Initiator
Out = Initiator to Target
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Table 64. SCSI messages (Continued)
Message
Abort Tag
Hex
In/Out
0D
Out
Description
When one or more initiators have multiple I/O processes to be queued by a
target, each I/O process must have its own queue tag. In such circumstances,
the Abort Tag message is active (usable). Otherwise, it is unavailable.
• The initiator sends the Abort Tag message to the target and, following
successful receipt, creates a Bus Free phase.
• The target clears out the identified I/O process whether it has started or is
waiting.
• All other pending status, data and commands for other queued or
executing processes are not affected.
Clear Queue
0E
Out
When one or more initiators have multiple I/O processes to be queued by a
target, each I/O process must have its own queue tag. In such circumstances,
the Clear Queue message is active (usable). Otherwise, it may or may not be
usable.
• The initiator sends the Clear Queue message to the target and, following
successful receipt, creates a Bus Free phase.
• All I/O processes from all initiators in the queue for the specified logical
unit are cleared from the queue.
• All similarly identified executing processes are stopped.
• All pending status and data for that logical unit for all initiators are
cleared.
• No status or ending message is sent.
Terminate I/O Process
11
Out
Terminates current I/O processes without corrupting media (e.g., hard disk).
Simple Queue Tag
20
I/O
Specifies that the I/O process be placed in the disk drive’s I/O process queue
for execution. The order of execution can be arranged by the disk drive in
accordance with an algorithm. This message is also sent by the target when it
reconnects to the initiator.
Head of Queue Tag
21
Out
Specifies that the I/O process be placed first in the identified logical unit’s
queue for execution. In-progress I/O is not preempted. A subsequent I/O
process received with this message goes to the head of the queue for execution
in last-in, first-out (LIFO) order.
Ordered Queue Tag
22
Out
Specifies that the I/O process be placed in the disk drive’s I/O process queue
for execution in the order received, with respect to other commands with this
message.
NOTE: Processes with Head of Queue Tag messages jump ahead of processes
with Ordered Queue Tag messages.
Ignore Wide Residue
23
Out
Used with wide (16-bit) data transfer. Indicates that number of valid bytes sent
during last REQ/ACK handshake of Data-In phase is less than negotiated
transfer width. “Ignore” field indicates no. of invalid data bytes transferred.
In = Target to Initiator
Out = Initiator to Target
Appendix C: The Deep End
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Table 64. SCSI messages (Continued)
Message
Identify
Hex
In/Out
80 to FF
I/O
Description
Sent by initiator or target to establish physical path connection between them.
The Identify bits are set as follows:
• Bit 7 is always set to one to distinguish Identify from other messages.
• Bit 6 may be set only by the initiator, and indicates whether or not it
supports disconnection and reconnection. It also indicates whether Save
Data Pointer, Restore Pointer, and Disconnect messages are supported.
• Bits 2 to 0 specify a LUN in a target and must be set to zero.
Identify is required in SCSI-2.
Bus Device Reset
D0
I/O
Issued by the initiator to clear all current commands and pending operations. It
forces a Bus Free Phase for all devices on the SCSI bus, but does not affect
Mode Select command parameters. This is message initiates a Soft Reset.
For more information, see “Mode Pages” on page 164.
In = Target to Initiator
Out = Initiator to Target
SCSI control signals and Information Transfer phases
Table 65 illustrates which of the Information Transfer phases is active when
the Message, Control/Data, and Input/Output control signals are either
asserted (1) or negated (0).
NOTE
For more information on the Information Transfer phases, see “Bus Phases” on page 169.
Table 65. Control signals and Information Transfer phases
Control Signals
308
MSG
C/D
I/O
Phase
Direction
0
0
0
Data-Out
Out
0
0
1
Data-In
In
0
1
0
Command
Out
0
1
1
Status
In
1
1
0
Message-Out
Out
1
1
1
Message-In
In
0 = Negated
In = Target to Initiator
1 = Asserted
Out = Initiator to Target
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PC SCSI: ASPI for Win32 calls
ASPI stands for Advanced SCSI Programming Interface. It is the dominant
standard on the PC for SCSI host adapter cards. Table 66 lists and describes
the calls ASPI for Win32 makes to access SCSI devices.
STOP
Calls are requests or instructions from the initiator or target.
Table 66. ASPI for Win32 calls
Call
Description
GetASPI32SupportInfo
Gets information on host adapters installed.
SendASPI32Command
Sends a command. The command codes include:
SC_HA_INQUIRY
SC_GET_DEV_TYPE
SC_EXEC_SCSI_CMD
SC_ABORT_SRB
SC_RESET_DEV
SC_GET_DISK_INFO
SC_RESCAN_SCSI_BUS
Host adapter inquiry
Gets information on a SCSI device
Executes SCSI I/O Command
Aborts SCSI I/O Command
Resets SCSI Device
Gets SCSI Disk Information
Rescans SCSI Bus
ASPI for Win32 is fully re-entrant, permitting overlapped asynchronous I/O.
NOTE
For more information on the implementation of SCSI on the PC platform, see “PC SCSI
Implementation” on page 189.
SCSI-2: new commands
SCSI-2 general device commands and opcodes
NOTE
The commands listed in Table 67 are optional. No mandatory general device commands beyond
the Common Command Set commands were introduced with the SCSI-2 standard. For more
information on SCSI-2, see “SCSI-2: A Transition From SCSI-1” on page 193.
Table 67. General device commands introduced in SCSI-2
Command
Change Definition
Appendix C: The Deep End
Opcode
40H
Description
Modifies the operating definition of the selected logical unit or target with
respect to commands from the initiator. It is used to implement switching
between SCSI-1 and SCSI-2 mode and to tell which mode is currently active.
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Table 67. General device commands introduced in SCSI-2 (Continued)
Command
Opcode
Description
Compare
39H
Provides the means to compare data from one logical unit to another or to the
same logical unit. It does this in a manner similar to the Copy command, except
that with Compare, the source data is compared with the destination data on a
byte-by-byte basis.
Copy and Verify
3AH
The same as the Copy command, except that a verification of the data written
to the destination logical unit occurs after the copy is completed.
Lock Unlock Cache
36H
Requests that the target either disallow or allow logical blocks (within the range
specified) to be removed from the cache memory by the target’s cache
replacement algorithm. Locked logical blocks may be written to the media
when modified, but a copy of the modified logical block remains in the cache.
This is good for locking data or applications into the cache.
Log Select
4CH
Lets the initiator manage statistical information that the SCSI device maintains
about itself or its logical units. The log can be reset. Targets that implement Log
Select must also support Log Sense.
Log Sense
4DH
Lets the initiator retrieve statistical information that the SCSI device maintains
about itself or its logical units, typically cache information or error logs. It is
complementary to the Log Select command.
Pre-Fetch
34H
Requests that the target transfer the specified logical blocks to the cache
memory on the drive, and determines when the status will be sent. No data is
transferred to the initiator.
Prevent/Allow Medium Removal
1EH
Requests that the target either prevent or allow the removal of the medium in
the logical unit.
Read Defect Data
37H
Requests that the target transfer data regarding defects in the medium to the
initiator. Data on both factory and grown defects can be requested. If the target
cannot comply, it will send the Check Condition status.
Receive Diagnostics
1CH
Requests that analysis data be sent to the initiator after completion of a Send
Diagnostics command.
For more information, see “Commands” on page 161.
Search Data
30H
31H
Looks through one or more logical blocks for equality or inequality to a data
pattern. There are Search Data Equal, Search Data High, and Search Data Low
commands.
32H
Set Limits
310
33H
Defines the range within which subsequent linked commands may operate.
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Table 67. General device commands introduced in SCSI-2 (Continued)
Command
Opcode
Description
Synchronize Cache
35H
Ensures that logical blocks in the cache memory, within the specified range,
have their most recent data value recorded in the medium. This command will
update the medium if necessary.
Write and Verify
2EH
Asks the target to:
1. Take the data transferred from the initiator.
2. Write it to the medium.
3. Verify that it is correctly written
Write Same
41H
Requests that the target write the single block of data transferred by the
initiator to the medium multiple times.
SCSI-2 Opcodes reserved for vendor-unique commands (V)
Figure 86 lists the opcodes that are reserved for the unique commands developed and used by manufacturers of SCSI peripherals.
02H
0DH
13H
22H
29H
05H
0EH
14H
23H
2CH
06H
0FH
19H
24H
2DH
09H
10H
20H
26H
C0H
0CH
11H
21H
27H
FFH
Figure 86. Opcodes reserved for vendor-unique commands
Appendix C: The Deep End
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SCSI-2 special CD-ROM commands
In the SCSI-2 specification, CD-ROM devices utilize special commands to
perform operations. Commands marked with an “O” are optional; commands
marked with an “M” are mandatory.
Table 68. Commands and opcodes for CD-ROM devices
Command
O/M
Opcode
Description
Audio Scan
O
BAH
Performs fast forward/fast reverse audio
scanning.
Pause/Resume
O
4BH
Pauses audio/resumes playing audio.
Play Audio (10)
O
45H
Plays audio.
Play Audio (12)
O
A5H
Plays audio on larger discs.
Play Audio MSF
O
47H
Plays audio in minutes, seconds format.
Play Track Relative (10)
O
49H
Plays a track relative to the current position.
Play Track Relative (12)
O
A9H
Plays a track relative to the current position on
larger discs.
Play CD-ROM XA (12)
O
BDH
Plays an XA formatted track.
NOTE: XA stands for extended architecture.
Read CD
M
BEH
Reads data from CD.
Read CD MSF
M
B9H
Reads data from CD in MSF format.
Read Header
M
44H
Reads header data from CD.
Read Sub-Channel
M
42H
Reads CD Sub-Channel data.
Read TOC
M
43H
Returns list of recorded tracks.
Send CD-ROM XA ADPCM DATA
O
BCH
Plays XA formatted audio.
NOTE: XA stands for extended architecture.
312
Set CD-ROM Speed
O
B8H
Sets the spin rate of the CD-ROM.
Stop Play/Scan
M
4EH
Stops audio playing.
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SCSI-2 special tape device commands
In the SCSI-2 specification, tape devices utilize special commands to perform
operations. Commands marked with an “O” are optional; commands marked
with an “M” are mandatory.
Table 69. Commands and opcodes for tape devices
Command
O/M
Opcode
Description
Erase
M
19H
Erases the tape.
Locate
O
2BH
Seeks to a block on the tape.
Load/Unload
O
1BH
Loads the tape into the drive or unloads it.
Read Block Limits
M
05H
Returns possible block lengths.
Read Position
O
34H
Returns the current position of the head.
Rewind
M
01H
Performs fast reverse rewinding of the tape.
Space
M
11H
Performs fast forward/fast reverse tape
seeking.
Write Filemarks
M
11H
Writes a tape marker point to the current
position.
SCSI-2 special communications device commands
In the SCSI-2 specification, communication devices utilize a special command to perform erase operations. The Erase command is mandatory.
Table 70. Commands and opcodes for communication devices
Command
Erase
Appendix C: The Deep End
O/M
Opcode
Description
M
19H
Erases the tape.
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SCSI-2 mode pages
SCSI-2 general device mode pages
Mode pages are categories of command parameters that determine a drive’s
behavior. SCSI-2 adds mode pages to those defined in the original specification. In addition to defined mode pages, there are reserved and vendor-specific
mode pages. Defined pages are listed and described in Table 71.
NOTE
For more information, see “Mode Pages” on page 164.
Table 71. SCSI-2 defined mode pages
Mode Page
314
Opcode
Description
Caching Parameters
08H
Defines the parameters affecting use of the cache, including:
• Sending the Good status
• The length of the pre-fetch logical blocks
• Whether the target can take Read data from the medium but not the cache
• Disable or enable write cache
Control Mode
0AH
Controls several SCSI-2 features that are applicable to all device types, such as:
• Tagged queuing
• Extended contingent allegiance
• Asynchronous event notification
• Error logging
Disconnect Reconnect
02H
Lets the initiator tune the performance of the SCSI bus, including:
• Maximum time the target can assert Busy without a Request/Acknowledge
handshake
• Maximum time target waits after releasing bus before attempting reselect.
Flexible Disk
05H
Controls and reports flexible drive parameters, including:
• Number of read/write heads used
• Number of sectors per revolution per head
• Number of data bytes per sector that an initiator can read or write
Format Device
03H
Specifies the medium format, including the number of:
• Tracks
• Alternate tracks
• Alternate sectors per zone
• Alternate tracks per logical unit
• Sectors per track
• Data bytes per physical sector
Medium Types Supported
0BH
Contains a list of the medium types implemented by the target for logical units.
FWB’s Guide to Storage
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Table 71. SCSI-2 defined mode pages (Continued)
Mode Page
Opcode
Description
Notch and Partition
0CH
Applies to direct-access devices that implement a variable number of blocks per
cylinder, and indicates such things as the maximum number of notches allowed
by the logical unit and their boundaries.
Read-Write Error Recovery
01H
Specifies the numerous error recovery parameters the target uses during any
command that performs a read or write operation to the media.
Rigid Disk Geometry
04H
Specifies parameters for direct-access devices that use a rigid disk drive,
including:
• The number of heads used for data storage
• Where the landing zone is located
• Rotational position locking
Verify Error Recovery
07H
Specifies the error recovery parameters the target uses during the Verify, Write
and Verify, and Copy and Verify commands, including:
• The number of times the target will attempt its recovery algorithm
• The largest burst data error for which data error correction may be
attempted
Peripheral Device
09H
Passes vendor-specific information between an initiator and a peripheral
interface “below” the target (that is, between the target and the peripheral
device), including SCSI, ESDI, IPI.
Appendix C: The Deep End
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SCSI-2 CD-ROM mode pages
The SCSI-2 specification defines additional mode pages for use by CD-ROM
devices. In addition to specifically defined mode pages, there are reserved and
vendor-specific mode pages. The defined pages are listed below along with
their codes.
Table 72. SCSI-2 defined CD-ROM mode pages
Mode Page
Opcode
Description
CD-ROM
0DH
Defines the parameters affecting use of CD-ROMs.
CD-ROM Audio Control
0EH
Defines the parameters affecting the playing of audio on CD-ROM discs.
SCSI-2 tape mode pages
The SCSI-2 specification defines additional mode pages for use by tape
devices. In addition to specifically defined mode pages, there are reserved and
vendor-specific mode pages. The defined pages are listed below along with
their codes.
Table 73. SCSI-2 defined tape mode pages
Mode Page
Device Configuration
Medium Partition
316
Opcode
10H
11H–14EH
Description
Allows programs to read and customize the mode that the tape drive is
operating in.
Allows programs to read how the tape is partitioned.
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ST-506 Communications
ST-506 commands
Table 74 lists the commands in the ST-506 standard. Notice that many of
these commands mirror ones later found in the SCSI standard.
NOTE
For a look at SCSI commands, see “Commands” on page 161. For more information about ST-506,
see “ST-506/ST-412” on page 222.
Table 74. ST-506 commands and opcodes
Command
Appendix C: The Deep End
Opcode
Description
Test Drive Ready
00H
Test to see if drive is ready.
Recalibrate
01H
Recalibrate the head.
Request Sense
03H
Return status information on drive.
Format Drive
04H
Format entire drive.
Read Verify
05H
Read data and verify its ECC (error correction
code).
Format Track
06H
Format a single track.
Format Bad Track
07H
Format a single track and set bad block flag.
Read
08H
Read data from disk.
Write
0AH
Write data to disk.
Initialize Drive Char
0CH
Set up drive characteristics.
Read ECC Burst
0DH
Read ECC data.
Read Sector Buffer
0EH
Read from RAM on drive.
Write Sector Buffer
0FH
Write to RAM on drive.
Assign Alt Track
11H
Reallocate bad sectors.
Controller RAM Test
E0H
Test controller’s RAM.
Drive Diagnostics
E3H
Test the controller and drive.
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Table 74. ST-506 commands and opcodes (Continued)
Command
Opcode
Description
Controller Diag
E4H
Test the controller.
Read Long
E5H
Read data along with ECC.
Write Long
E6H
Write data along with ECC.
Return Drive Parms
FBH
Get drive configuration information.
Set Step Rate
FCH
Set how fast drive can step.
ST-506 data signals
Table 75 lists the data signals on the 34-pin ST-506 connector. The ST-506
interface’s differential cabling scheme determines the signal:
• on at greater than 2.0 volts
• off at from 0 to 0.7 volts
STOP
318
All signals are TTL compatible. TTL stands for transistor–transistor logic; it’s a type of chip.
Table 75. 34-Pin ST-506 data signals
Data Signal
Description
Direction
Which direction head is moving
Drive Select0-3
Specifies which of the 4 drives to access
Head Select0-2
Specifies which of the 8 heads to access
Index
Signals when index mark has passed
Ready
Signals when the drive is ready to be accessed
Seek Complete
Signals when the head has completed a move
Step
Triggers the drive to seek
Track0
Signals when head is moving to beginning
Write Gate
Determines if operation is read or write
Write Fault
Shows if an error occurred
FWB’s Guide to Storage
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Table 76 lists the signals on the 20-pin ST-506 connector. All signals are TTL
compatible. The ST-506 interface’s differential cabling scheme determines the
signal:
• on at greater than 2.0 volts
• off at from 0 to 0.7 volts
Table 76. 20-Pin ST-506 data signals
Data Signal
Description
Drive Selected
Set if the drive is at the desired address.
Write Data
Data to be written.
Read Data
Data to be read.
Because the ST-506 drive must be accessed with such low-level protocol, it is
very difficult to create drives with variable zoned densities.
NOTE
For more information on variable zone densities, see “Variable zone recording” on page 45.
IDE Communications
IDE data signals
NOTE
For more information on IDE, see “IDE” on page 225.
Table 77 lists the signals present on the 40-pin IDE connector. All signals are
TTL compatible. The IDE interface’s differential cabling scheme determines
the signal:
• on at greater than 2.0 volts.
• off at from 0 to 0.7 volts.
Table 77. 40-Pin IDE data signals
Appendix C: The Deep End
Data Signal
Description
RESET
Resets the drive during host power-up.
DD0-DD15
Carries 16-bits of data.
GND
Grounds the signal.
DIOW
Host I/O Write Strobe.
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Table 77. 40-Pin IDE data signals (Continued)
Data Signal
Description
DIOR
Host I/O Read Strobe.
IOCHRDY
I/O Channel Ready used to throttle transfers.
DMACK
Host DMA Acknowledge.
DMARQ
Host DMA Request handshake.
INTRQ
Host Interrupt line when drive is selected.
DA0-2
Address line to select registers in task file.
CS0-1
Host Chip Select to select registers.
DASP
Host Slave Active to drive LED (ISA Mode).
Drive active slave present (ATA/CAM Mode).
320
KEY
Key to allow cable to mate one way.
SPINSYNC or CBLSEL
Spindle synchronization or Cable Select.
IOCS16
Host I/O 16-bit enabled.
PDIAG
Host Passed Diagnostic to detect Slave.
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IDE registers
Table 78 lists the registers that are used for accessing IDE drives. They are all
part of the Task File Register through which all communications with the
CPU take place. This register is at memory location 1F0 on PCs. These registers are identical to the AT Fixed Disk Register Set.
STOP
A register is a temporary hardware storage area that assists with the speedy transfer of arithmetic/
logical operations within the CPU.
Table 78. IDE registers for accessing drives
Register
Memory Location
Details
Data Register
1F0
All data passes through this register.
Error Register
1F1
Contains status from last command.
Features Register
1F1
Formerly Write Precomp Register.
Sector Count
1F2
Number of sectors of data to read or write.
Sector Number
1F3
Starting sector number for I/O.
Cylinder Low
1F4
Low 8-bits of cylinder number (CHS mode).
Middle 8-bits of block address (LBA mode).
Cylinder High
1F5
High 8-bits of cylinder number (CHS mode).
High 8-bits of block address (LBA mode).
Appendix C: The Deep End
SDH Register
1F6
Drive and head number.
Status
1F7
Drive/Controller status.
Alternate Status
3F6
Same as Status except on Interrupt Acks.
Digital Out Reg
3F6
Drive Reset and Interrupt set.
Drive Adr Reg
3F7
Loopback of drive select and head select addresses.
Command Reg
1F7
Command to execute.
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IDE commands and opcodes
Table 79 lists commands that IDE drives perform. They are similar in nature
to SCSI commands but are much more basic in their functionality.
Table 79. IDE commands and opcodes
322
Command
Opcode
Description
Recalibrate
10H
Move read/write head to cylinder 0.
Read Sector(s)
2xH
Read data from the drive, with or without retry and ECC bytes.
Write Sector(s)
3xH
Write data to the drive, with or without retry and ECC bytes.
Read Verify Sectors
4xH
Verifies the sectors are error free.
Format Track
50H
Format a track to spare out bad sectors. Few drives have a full format command.
Seek
70H
Seek to the selected track.
Execute Drive Diagnostic
90H
Perform internal diagnostic tests on the drive.
Initialize Drive Parameters
91H
Setup head switch and cylinder increment points for multi sector ops.
Vendor Specific
9AH
These are specific to Conner:
Get Drive Feature
00
Reads what features are enabled on the drive.
Read Drv Switches
02
Returns what drive switches are set to.
Power Lock
08
Prevent drive from spinning down.
Power Unlock
09
Allow it to respond to Power Command.
Read Multiple
C4H
Read multiple sectors with one command.
Write Multiple
C5H
Write multiple sectors with one command.
Set Multiple Mode
C6H
Enables the drive to perform multi-sector reads and writes. (Multiple of 2)
Read DMA
C8H
Read data and enable controller to do Type B DMA Mode transfer
Write DMA
CAH, CBH
Write data and enable controller to do Type B DMA Mode transfer
Read Sector Buffer
E4H
Read contents of drive’s buffer
Power
E5H
Set a drive to sleep, idle, or standby mode.
Write Sector Buffer
E8H
Write data to the drive’s buffer
Identify Drive
ECH
Displays information on the drive including size of drive, features of drive, make, model,
revision, transfer types and speeds.
Set Features
EFH
Enable features on the drive. Read or write cache, ECC, retries, and transfer mode.
Translate
F1H
Translate head, cylinder, and sector to physical location.
Physical Seek
F2H
Seek to a given cylinder and head.
Retry Count
F4H
Returns number of retries attempted after Read, Read Verify, or Read Multiple command.
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PC IDE: INT13 call codes
Table 80 lists the function call codes available through INT13. INT13 is the
interrupt command that controls the read/write activities on a PC IDE drive.
The INT13 call is limited to a 10-bit address of data.
Table 80. Call codes available through INT13
Call Code
Function
Call Code
Function
00H
Reset Diskette(s) and/or head.
0DH
Alternate Hard Drive Reset.
01H
Read Hard Drive Status.
0EH
Read Test Buffer.
02H
Read Sectors.
0FH
Write Test Buffer.
03H
Write Sectors.
10H
Test for Drive Ready.
04H
Verify Sectors.
11H
Recalibrate Drive.
05H
Format Cylinder.
12H
XT Controller RAM Diagnostic.
06H
Format Bad Track on XT drive.
13H
XT Controller Drive Diagnostic.
07H
Format XT-type drive at cylinder.
14H
Controller Internal Diagnostic.
08H
Read Drive Parameters.
15H
Get Hard Drive AT Type.
09H
Initialize Drive Parameters.
16H
Change of AT Disk Status.
0AH
Read Long Sectors.
17H
Set AT Disk Type.
0BH
Write Long Sectors.
19H
Park Drive Heads.
0CH
Seek to Cylinder.
1AH
Format ESDI Drive.
Appendix C: The Deep End
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IDE pin assignments
Pin assignments on the IDE hard disk connector are similar to the IDE standard. Table 81 lists the IDE hard disk connector pin assignments.
Table 81. IDE hard disk connector pin assignments
324
Pin Number
Signal Name
Pin Number
Signal Name
Pin Number
Signal Name
1
/RESET
15
DD1
28
Reserved
2
GROUND
16
DD14
29
Reserved
3
DD7
17
DD0
30
GROUND
4
DD8
18
DD15
31
INTRQ
5
DD6
19
GROUND
32
/IOCS16
6
DD9
20
KEY
33
DA1
7
DD5
21
Reserved
34
Reserved
8
DD10
22
GROUND
35
DA0
9
DD4
23
DIOW
36
DA2
10
DD11
24
GROUND
37
/CS0
11
DD3
25
DIOR
38
/CS1
12
DD12
26
GROUND
39
Reserved
13
DD2
27
IORDY
40
GROUND
14
DD13
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Apple ATA Manager calls
NOTE
The terms ATA and IDE are interchangeable. For more information on ATA/IDE, see “IDE” on
page 225.
Calls to the Apple ATA Manager are made with the ATA parameter block
structure. This structure is very similar to that used by the SCSI Manager.
The elements of a typical ATA Manager parameter block header are listed in
Table 82.
Table 82. ATA Manager parameter block header structure
Appendix C: The Deep End
Category
Description
Ptr ataLink;
Reserved
short ataQType;
Type byte
uchar ataPBVers;
-->Parameter block version number
uchar hdrReserved;
Reserved
Ptr hdrReserved2;
Reserved
ProcPtr ataCompletion;
Completion routine
short ataResult;
<--Returned result
uchar MgrFCode;
-->Manager function code
uchar ataIOSpeed;
-->I/O timing class
ushort ataFlags;
-->Control options
short hdrReserved3;
Reserved
long deviceID;
-->Device ID
ulong TimeOut;
-->Transaction timeout value
Ptr ataPtr1;
Client storage Ptr 1
Ptr ataPtr2;
Client storage Ptr 2
ushort ataState;
Reserved, init to 0
short hdrReserved4;
Reserved
long hdrReserved5;
Reserved
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Functions that the ATA Manager supports are listed in Table 83.
Table 83. Functions supported by the ATA Manager
Function name
326
Code
Description
ATA_NOP
$00
No operation
ATA_ExecIO
$01
Execute ATA I/O
ATA_BusInquiry
$03
Bus inquiry
ATA_QRelease
$04
I/O queue release
ATA_Abort
$10
Terminate command
ATA_ResetBus
$11
Reset IDE bus
ATA_RegAccess
$12
ATA device register access
ATA_Identify
$13
Get the drive identification data
ATA_DrvrRegister
$85
Register the drive reference number
ATA_FindRefNum
$86
Look up driver reference number
ATA_DrvrDeregister
$87
Deregister the driver reference number
ATA_MgrInquiry
$90
ATA Manager inquiry
ATA_MgrInit
$91
Initialize ATA Manager
ATA_MgrShutDown
$92
Shut down ATA Manager
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Enhanced IDE commands and opcodes
NOTE
For more information, see “Enhanced IDE” on page 235.
Table 84. Enhanced IDE commands and opcodes
Command
Opcode
Description
NOP
00H
No operation.
Download Microcode
92H
Update firmware on drive.
Acknowledge Media Change
DBH
For acknowledging media has changed.
Post Boot
DCH
For removable media.
Pre Boot
DDH
For removable media.
Door Lock
DEH
For locking media in.
Door Unlock
DFH
For unlocking media.
Write Same
E9H
Write same data again and again.
Media Eject
EDH
For ejecting media.
ATAPI Communications
ATAPI commands and opcodes
NOTE
For more detailed information on ATAPI, see “ATAPI” on page 238.
Three new commands were added to EIDE for the ATAPI protocol (Table 85).
Table 85. ATAPI-inspired enhanced IDE commands and opcodes
Command
STOP
Opcode
Description
ATAPI Soft Reset
08H
Perform a soft reset on the device.
Packet Command
A0H
Perform a SCSI command.
ATAPI Identify Device
A1H
Locate ATAPI devices.
A soft reset is a software-based reboot, without interruption to power. A soft reset can be locked
out; that is, a system can be crashed so badly that it will not respond to a soft reset request. In this
case, a user might escalate to a hard reset, where the computer is turned off, then on.
Appendix C: The Deep End
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ATAPI registers
Table 86 lists the changes to IDE Task File registers that are used for accessing
ATAPI drives. They are all part of the Task File Register through which all
communications take place. This register is at memory location 1F0 on PCs.
STOP
Registers are temporary hardware storage areas that assist with the speedy transfer of arithmetic/
logical operations within the CPU.
Table 86. ATAPI registers
Register
Location
Details
Data Register
1F0
Exchange data between host and device.
Error Register
1F1
Contains Media Change and End of Media flags.
Interrupt Reason
1F2
Why interrupt was generated. Indicates the emulated SCSI phase.
Reserved
1F3
Reserved.
Byte Count Reg
1F4
Transfer size in bytes.
Byte Count Reg
1F5
Transfer size in bytes.
Drive Select
1F6
LUN to select.
ATAPI Status ATA Cmd
1F7
Drive/Controller status.
Dev Control Reg
3F6
Same as ATA.
Features Register
3F7
Indicates DMA mode for data transfers.
ATA-3 Communications
Table 87 lists the commands added to the IDE standard in ATA-3.
Table 87. ATA-3 new commands and opcodes
Command
328
Opcode
Description
Smart
B0H
Self monitoring, analysis and reporting.
Security Set Password
F1H
Set a security password.
Security Unlock
F2H
Remove security lock.
Security Erase Prepare
F3H
Prepare to erase drive.
Security Erase Unit
F4H
Erase drive thoroughly.
Security Freeze
F5H
Freeze drive operation.
Security Disable Password
F6H
Disable security password.
FWB’s Guide to Storage
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NOTE
For more information about ATA-3, see “ATA-3” on page 240.
PCI Communications
NOTE
For more information about PCI, see “PCI” on page 249.
Table 88 lists and describes the commands that pass between the CPU and the
PCI interface and indicates which entity initiates the command.
Table 88. PCI commands and initiators
PCI Command
Initiator
Description
I/O Read, I/O Write
CPU
Used to transfer data between the CPU and the Target’s I/O
memory space.
Configuration Read, Configuration Write
CPU
Used to transfer data between the CPU and the PCI target's
Configuration registers during computer initialization.
Memory Read, Memory Write
CPU or PCI Master
Used to transfer data between the PCI Master and the Target’s
memory space.
Memory Read Line
CPU or PCI Master
Used by the PCI Master to transfer a cache line of data from
the PCI Target’s memory space. Much faster than Memory
Read.
Memory Read Multiple
CPU or PCI Master
Used by the PCI Master to transfer more than one cache line of
data from the PCI Target’s memory space. This is the mode to
use when transferring lots of data.
Memory Write and Invalidate
CPU or PCI Master
Used by the PCI Master to transfer one or more complete cache
lines of data to the PCI Target’s memory space. The faster type
of Memory Write command.
Appendix C: The Deep End
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Glossary
1394
FireWire. A low- to medium-speed
bus intended for desktop PC
implementations. It appears that 1394
will be used mainly in consumer
audio/video products, set-top (cable)
boxes, scanners and printers.
2,7 RLL
A version of Run-Length Limited
recording that allows from two to
seven consecutive zero bits. This
results in a 50 percent increase in
drive capacity over standard MFM.
See also Run-Length Limited and
MFM.
3,9 RLL
A version of Run-Length Limited
recording that allows the run length
of the zero string to range from three
to nine. This method, also called
Advanced Run-Length Limited
(ARLL), permits a 100 percent
increase in drive capacity over
standard MFM.
See also Run-Length Limited and
MFM.
Ablative WORM
A method of writing to a CD that
changes the physical properties of the
media in a way that cannot be
undone. The “ablative” in “ablative
WORM” refers to the creation of
permanent pits in the media surface.
WORM stands for write-once readmany.
Glossary
Absolute Filter
Filters that prevent particles bigger
than a certain size from
contaminating a hard disk. They
usually have a filtering ability of
several tenths of a micron. At the
same time, they allow outside air into
the sealed area of the drive to equalize
pressures and temperatures in
accordance with changes in the
external environment.
Access Time
The time period from issuance of a
command to access a single sector to
the time when the disk drive’s head
reaches the sector. Access time can
represented by the following formula:
Seek Time + Latency + Time to Read a Sector = Access Time
ActiveAudio™
A type of Enhanced CD. ActiveAudio
is one of the approaches developers
have taken to solve the problems that
occur when you combine digital and
audio data on one CD-ROM.
ActiveAudio information is organized
in this way: digital data occupies the
silence preceding track 1 (so-called
track 0). Audio data occupies track 1
and up.
Active Backplane
A backplane that provides multiple
drives with power and data
connections even when some drives
are pulled out or plugged in on the fly.
This is necessary for a hot-swappable
drive. Without an active backplane,
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plugging in a drive while the system
is operating could kill the SCSI bus
and hang the computer.
Active High Signal
Signals along the bus that are asserted
when the signal strength is above a
certain voltage, which is typically
over 2.5 volts (plus or minus a five
percent margin).
Active Low Signal
Signals on the bus that are asserted
when the voltage falls below a stated
level, which is typically under 0.4
volts.
Active Negation
Circuitry that drives the signal line
high when it is negated. Driving the
lines high ensures that Request and
Acknowledge signals are clearly
recognized.
Active Termination
Termination that attempts to address
impedance mismatches by
compensating for voltage. With active
termination, a voltage regulator
provides about 24 milliAMPs of
current into each asserted SCSI line
over a considerable range of
terminator power values.
Actuator
See Head Actuator.
Address
The ID number of a device on the
SCSI bus, or of a block of data in
storage.
Advanced RLL
The 3,9 RLL technique, where the run
length of the zero string can range
from three to nine. This method
permits a 100 percent increase in
drive capacity over standard MFM.
See also Run-Length Limited and
MFM.
AEN
Asynchronous Event Notification. A
protocol that can be used to inform
processor devices that an
asynchronous event has occurred.
Instead of the initiator polling, the
target would notify the initiator.
AIFF
Audio Interchange File Format. It is a
full-featured audio file specification
that allows many programs on
multiple platforms to share standards
for audio storage. Electronic Arts
published the AIFF specification in
1985. It started as a digital music
instrument specification. Over the
years it has been enhanced to provide
compressed digital sound (AIFC).
Algorithm
A step-by-step problem-solving
procedure that has only one starting
point and one finishing point. In
computing, it is the process of
defining a set of instructions so that a
program can be written to find the
solution to a given problem.
The word comes from the name of an
early proponent, Muhammad ibnMusa al-Khwarizmi.
Alias
A small “marker” file that points the
Mac OS to the real file. Aliases are
commonly used to organize the
Glossary
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Macintosh Desktop. On larger
volumes, where larger allocation
blocks are used, an alias can be as big
as 4 KB, 8 KB, 16 KB, or more.
Allocation Block Size
The allowable size of units of
information in which data is
organized and stored on a disk by the
operating system. Blocks are of fixed
size, with the most common being
512 bytes. They are stored
contiguously, with space between
adjacent blocks on the same track.
Analog
Information that is highly variable
and has a direct, proportional
relationship to the thing it describes.
For example, information that
describes current wind velocity or
temperature.
ensures that qualified organizations
develop those standards, and
coordinates standards approval. If you
wish to contact ANSI, write or call:
ANSI, 1430 Broadway, New York, NY
10018; (212) 642-4900.
Arbitration
An optional phase required for
multiple-host systems. If supported,
it is a protocol by which SCSI devices
gain control of the bus.
Areal Density
A measure of disk capacity in bits per
square inch. It is calculated by taking
the number of bits per inch that can
be written to and read from each
track, and multiplying that by the
number of tracks per inch that can be
packed onto the disk.
Armature
ANSI
The American National Standards
Institute. ANSI is a private, nonprofit
membership organization that
performs two functions: it
coordinates the United States’
voluntary consensus standards
system, and it approves American
National Standards. ANSI ensures
that a single set of non-conflicting
American National Standards are
developed by ANSI-accredited
standards-developers, and that all
interests concerned have the
opportunity to participate in the
development process. These
requirements for due process have
resulted in a high level of confidence
and credibility and broad acceptance
for American National Standards. But
it is important to remember that
ANSI does not develop standards.
Rather, it provides the means for
determining the need for standards,
332
The supporting framework for a drive
head.
Array
A linked group of small, independent
drives used to replace larger, single
drive systems.
ARLL
Advanced Run-Length Limited. See
3,9 RLL.
ASIC
Application-specific integrated
circuit. ASICs are computer chips
developed for specific functions.
These are used in a wide-range of
devices, such as video machines,
microwave ovens and security
alarms.
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ASPI
Advanced SCSI Programming
Interface managers are software
programs that enable the storage
software, SCSI device drivers, your
host adapter, and your SCSI devices to
communicate with each other. ASPI
managers are written for a specific
operating system, such as DOS, and a
specific family of host adapters.
Assert
Usually assert means to send a
control signal. However, for two
control signals, Control/Data (C/D)
and Input/Output (I/O), it means
selecting one or the other.
Asynchronous
A method of data transfer that
requires a Request/Acknowledge
handshake for the movement of every
information byte.
The asynchronous mode is the data
transfer default. It is in effect at
power-up, after a Bus Device Reset
message, and after a hard reset
condition.
AT
Advanced Technology. A PC
introduced by IBM in 1984. It was
based on the Intel 80286
microprocessor and a 16-bit data bus.
It was up to 75 percent faster than its
predecessor, the XT.
ATA
Advanced Technology Attachment.
Sometimes used to mean IDE,
Integrated Drive Electronics.,
although ATA specifically references
an implementation of IDE designed
for the AT bus (either ISA or EISA).
Glossary
ATA-2
Advanced Technology Attachment-2.
Another name for Enhanced IDE.
ATA-3
Advanced Technology Attachment-3.
Designed to support 32-bit operating
systems such as Windows 95 and NT.
The ATA-3 architecture was designed
to be flexible and modular,
abstracting the physical interface
from the transport mechanism and
command sets. ATA-3 supports:
• drives as large as 137 GB
• data transfer rates of 16.6 MB/s via
Mode 4 PIO or Mode 2 DMA
• devices such as disk, tape, CDROM and CDR
ATA Manager
Provides the interface between
the ATA software (operating
system drivers or application
programs) and hardware (chip).
ATAPI
Advanced Technology Attachment
Packet Interface. Part of Enhanced
IDE, this specification defines a
standardized method for interfacing
non-hard disk devices utilizing the
existing IDE computer interface and
cabling.
Attention Condition
A condition on the SCSI bus that
results when the initiator signals the
target that it has a message ready.
Auto Compensator
A mechanism within a drive that
regularly conducts seek tests and
adjusts the movements of the head
actuators accordingly.
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Autolocking
Bandwidth
Autolocking physically locks a
parked drive head over a designated
landing zone on the disk, preventing
any head movement.
The range of frequencies that can pass
along a cable or communications
link.
Benchmark
Autoparking
A standard used to compare computer
systems or programs, usually in
relation to speed, reliability and
accuracy.
Upon power loss, the drive heads are
automatically pulled by a spring to a
safe position on the disk—the landing
or parking zone.
Bernoulli Drive
AV
A marketing designation given to
computers that are equipped with
special hardware and software to
facilitate audio, video and related
types of production.
A drive with a 5.25” cartridge
containing a flexible disk that spins
within a cushion of filtered air.
Bernoulli Effect
An effect that is observed when the
velocity of a fluid over a surface is
increased and the pressure of that
fluid on the surface decreases. In the
instance of Bernoulli drives, the fluid
involved is air. The reduced air
pressure draws the disk toward the
read/write head.
Average Seek Time
The time in milliseconds to do all
possible seeks on the drive divided by
the number of seeks possible.
Backplane
The section of a computer system
board into which other boards are
plugged.
Back up, Backup
Binary Data
Data that is represented as 0 or 1.
BIOS (Basic Input/Output System)
v. To make duplicates of files on a
separate medium; n. the duplicated
data.
Software coded into computer chips
for various purposes. The BIOS on the
motherboard of a computer is the
special program used to boot and
control the computer. Most host
adapters include an on-board BIOS
that initializes the SCSI Bus, runs
bootup diagnostics, and performs
other functions.
Backward Compatibility
Design that takes into account
compatibility requirements for earlier
models and builds support for those
requirements into later generations.
The result is a new product that can
still be used on an old product
platform.
334
Bit
Bit is a contraction of “binary” and
“digit.” All computer information is
represented as a unique combination
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of the binary digits 0 and 1, which are
also called “offs” (0) and “ons” (1).
Bit Cells
Magnetic domains created through
RLL encoding. They are one-third
smaller than those used in MFM
encoding.
Bit-Based Striping with Parity
An array of disk drives that transfer
data in parallel, while one redundant
drive functions as the parity check
disk. Another term for RAID level 3.
See also Byte-Based Striping with
Parity.
BLER
Block error rate. A measurement of
how many errors are detected in the
error correction code (ECC) blocks on
the media.
Blind Data Transfer
Data transfer that forgoes request/
acknowledge handshakes and thus is
much faster than regular data
transfer. Blind transfer is used during
programmed I/O when SCSI DMA
hardware is not available.
Block
The smallest “chunk” of memory
accessed or transferred by the disk
drive. Usually 512 bytes in size, it can
be larger in multiples of 512. The
number of bytes in a block is the
same as block size.
Block Descriptor
Specifies the logical block length for
the drive, including the following:
Glossary
• density code, which is the density
of the medium—usually used only
for tape drives
• number of logical blocks on the
medium in the density code
• block length, which is the length in
bytes of each logical block
described by the block descriptor
The rest of the data consists of one or
more mode pages.
Block Mode Data Transfer
See NuBus Block Mode Data Transfer.
Block Multiplexer
A processing standard that allows
multiple peripherals to
intercommunicate at the same
time. It became so popular that
the federal government made it a
processing standard (FIPS 60).
Also called the OEM channel.
Blown Session
A CD-ROM recording session that is
disrupted such that the recorder
cannot complete the writing of a disc
or track, rendering the recording
medium—a writable compact disc—
unusable.
Blue Book
A standard that defines a way of
combining audio and data on a disc in
such a way that the audio may be
played on any consumer CD player,
while both the audio and the data can
be enjoyed on a computer with a
compatible CD-ROM drive.
Boolean Function
A switching function in which the
function and each of its independent
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variables may have only two possible
values.
Burst Data Transfer Rate
The fastest that data can be moved at
one instance under optimal
conditions, which means data
transferred from cache.
Boot Block
Part of the Macintosh HFS partition
(although other operating systems
place similar information in the first
block of the drive platter). The boot
blocks on each HFS partition occupy
blocks zero and one. They are filled in
with data when a System Folder is
copied into the partition. The boot
blocks contain:
• information that allows the system
to start up from the System Folder
• information on the maximum
number of open files
• the name of the debugger
• the names of system files
Bootstrap, or Bootstrapping
A trigger (command code or signal)
that allows a device to start-up
without outside aid.
Boot-up
The loading of the operating system
of a computer into RAM. It is
typically an automatic loading
procedure issued by the ROM chip.
Buffer
A temporary storage area for data
being transferred from one place in
the computer system to another.
When accessing a single sector, the
controller may read the entire track
and store it in a buffer.
Buffer Underrun
In CD recording, when the device
supplying the data does not get data
to the recorder fast enough.
336
Bus
A means of transferring information,
usually referring to a set of wires.
Bus Arbitration
How devices vie for control of the bus
during an optional bus phase.
Bus Clock
An electronic circuit that regulates
the synchronization of the flow of
information through the bus.
Bus Condition
A condition on a bus that affects all
attached devices and cannot be
ignored. It is a critical operation mode
and thus overrides other lines. There
are two bus conditions:
• Attention
• Reset
Bus Device Reset
A message sent to reset the operation
of a specific device.
Bus Free Phase
The bus is empty; no one is riding.
This is SCSI’s starting point at powerup, after a Reset, and where it returns
when a command is completed.
Both the Busy and Select signals must
be negated for a defined period of time
before this phase is entered.
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Bus Mastering
A feature included with some bus
cards that allows a card to pass data
directly into memory, freeing the
CPU to perform more useful
functions. (The CPU would otherwise
act as an intermediary between the
card and memory.)
Bus Phases
Butterfly Test
A diagnostic test that is useful for
finding media errors.
Byte
A group of eight bits. The basic unit
of information.
Byte-Based Striping with Parity
Communication across the SCSI bus
is divided into stages known as
phases. There are eight possible bus
phases:
An array of disk drives that transfer
data in parallel, while one redundant
drive functions as the parity check
disk. Another term for RAID level 3.
•
•
•
•
•
•
•
•
See also Bit-Based Striping with
Parity.
Bus Free
Arbitration
Selection
Reselection
Command
Data-In/Out
Status
Message-In/Out
Bus Reset
A method of clearing and
consequently stopping all action on
the SCSI bus and returning to a
known, free state.
Cable Extender
A cable that functions as an extension
cord. They can be attached to one
another (male-female), to a System
cable, and to a Peripheral Interface
Cable.
Cable Impedance Mismatch
Condition created when two or more
cables retain different impedances,
resulting in signal reflections that
adversely impact system
performance.
Bus-Based Hardware Disk Array
Arrays that are controlled by a RAID
host adapter that resides in the
computer. These are typically PCIbased boards that include multiple
SCSI channels to drives and a RISCbased processor.
Buswidth
The number of bits that can travel
simultaneously (in parallel) across a
bus.
Glossary
Cache
Similar to buffer but more
configurable. Cache can reside in
RAM, on the drive’s controller, or on
a hard disk. It is used to store and
quickly transfer recently used data.
Caddy
A plastic tray with a hinged lid and
metal shutter used as a carrier for
CDs and CD-R discs in some readers
and recorders. The caddy helps ensure
proper alignment of the disc and aids
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in keeping CD-R discs clean prior to
writing.
Capacitance
A value expressed as the ratio of a
quantity of electricity to a potential
difference.
Call
A request or instruction from the
initiator or target.
Capacitor
An electric circuit element used to
store charge temporarily, consisting
in general of two metallic plates
separated by a nonconducting
medium.
Call Chain
A complicated sequence of
continuous calls.
The call chain represents the many
layers through which calls pass and
the levels at which they are
translated—in effect, readied for the
next level.
In a call chain, each step is distinct
and self-contained. This is the result
of purposeful intent by good
engineers. If an aspect of the call
chain is improved, only the layer
affected needs to be changed.
CAM
Common Access Method. A software
interface for attaching SCSI
peripherals. CAM describes a layered
architecture with a transport layer on
top (XPT). The single XPT vector calls
to one or more SCSI interface module
(SIM) layers. Each SIM controls a
single host bus adapter (HBA).
CAM’s major goal is to provide one
interface across all hardware
platforms to ease software
development for every new system.
Canister Drive
A hard drive and its entire head
assembly in a rigid cartridge module
that plugs into a “mother” unit. The
removable part contains the platter
and read/write head. The mother unit
contains a power supply and may
contain a hard drive controller for
communicating with the computer.
338
Catalog Tree
A data structure that tracks the
location of and statistics for all files
and folders on the disk.
CCS
See Common Command Set.
CD Bridge
A compact disc format used for
Kodak Photo CDs and Video CDs.
Many readers support the format but
it has not yet become important
globally. Software support and
demand for discs in these formats has
not yet fully developed. The standard
for PhotoCD was created and is
enforced by Kodak. Standards are
proposed, but not in place.
CDB
See Command Descriptor Block.
CD-DA
Compact Disc Digital Audio. The
format used for conventional audio
CDs. It is readable on all audio CD
players and most CD-ROM readers,
as well as a wide variety of other
multimedia devices.
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CD-E
See CD-RW.
CD-Extra
CD-Extra is one of the approaches
developers have taken to solve the
problems that occur when you
combine computer and audio data on
one CD-ROM. CD-Extra takes a
multisession approach: Audio data
occupies session 1. Computer data
occupies session 2.
See also Blue Book.
CD-G (CD Graphics)
Compact disc format used for
Karaoke CDs. The format is readable
on many devices, but most of them
are consumer audio components.
While the discs may be readable on
CD-ROM drives, special software is
required to see the graphics.
CD-i (CD-interactive)
Compact disc format developed by
Philips for use in home
entertainment systems and
multimedia applications. Like CD+G,
many readers support this format, but
there are few computers or
applications that allow the discs to be
used. The “Green Book” standard
defines this format.
CD Recorder
These drives, along with specialized
mastering software, allow users to
make their own compact discs.
CD-ROM (Compact Disk-Read-Only Memory)
Data is stored as pits on the CD
platter’s surface. The pits are read by
a laser in the CD ROM drive. The
data can be read. Data cannot be
erased. New data cannot be added.
CD-ROM Drive
A read-only drive that uses a laser
beam to read large amounts of data
from compact discs.
CD-ROM XA
CD-ROM Extended Architecture.
Compact disc format that allows the
interleaving of data and audio for
smoother playback of multimedia
content. It is readable only on XA
compatible drives. Most drives sold
after 1991 are XA compatible.
CD-RW
Compact Disc-Rewritable (formerly
called CD-E). A new proposed
standard for creating a rewritable
compact disc. Unlike CD-R, CD-RW
discs can be reused many times. CDRW specifies a 650 MB drive with an
access time in the 300 ms area.
CD-R (CD-Recordable)
Compact disc technology that enables
writing to recordable compact discs.
Data is written to a specially made
disc by activating a dye embedded in
the disc. This dye changes the
reflection of the laser in the same way
a pit does on a compact disc.
Glossary
Central Processing Unit, or CPU
The brains or “central switching
station” of any computer.
Chassis
The outer protective enclosure of a
hard drive or peripheral.
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Check Disc
In CD mastering, a disc created before
the master, used to check the quality,
organization and functionality of the
recorded material before a master disc
is created.
Chip
An integrated circuit etched into a
small slab of silicon. The circuits act
as either transistors, resistors or
capacitors.
Chip-on-Board
Chip technology that reduces the
required real estate by eliminating
standard packaging (the black plastic
square you see on circuit boards) and
attaching to the circuit board directly.
The chip is often then covered with a
dense polymer that provides
protection against environmental
rigors.
Clean Room
A room kept virtually free of
contaminants, used for laboratory
work and in the production of
precision parts.
CMOS
Complementary Metal-Oxide
Semiconductor. A memory chip that
permits many components to be
packed together in a very small area.
The main characteristics of CMOS
chips are low power consumption,
high noise immunity and slow speed.
COB
See Chip-on-Board.
340
Cobalt-Nickel Alloy
A medium for data storage, used
to improve densities and provide
better reliability.
Coercivity
A measurement of a magnetic
material’s ability to withstand
demagnetizing. Materials that are
very hard, such as steel, tend to have
high coercivity.
Command Complete
The only message supported by all
SCSI devices. Command Complete is
sent by the target to the initiator to
indicate that:
• It has executed the command (or a
series of linked commands).
• A valid status has been sent to the
initiator.
Command Complete is neutral. It is
up to the status byte to indicate
whether or not the command was
completed successfully.
Command Descriptor Block
A six-, ten-, or twelve-byte data
structure residing in the computer’s
memory that contains the command
code and other information needed by
a target to execute a command.
Command Linking
A feature that allows two or more
SCSI commands to be linked
together. If a linked flag is set in the
CDB, the completion of one
command triggers the execution of
the following command, saving
arbitration time.
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Command Overhead
The time needed for a computer’s
command to be interpreted and acted
upon by the controller.
Command Phase
The bus phase in which the target
requests command information from
the initiator.
Common Access Method
See CAM.
Common Command Set
A comprehensive de facto standard
for commands. CCS defines:
• data structures for Mode Select and
Mode Sense commands
• defect management of the Format
command
• error recovery procedures
• numerous other command
functions
CCS was the beginning of SCSI-2, but
it is only for disk drives.
Compression
A procedure in which data is
transformed by the removal of
redundant information in order to
reduce the number of bits required to
represent the data. This is done by
representing strings of bytes with
code words.
Connectivity
The degree to which a system allows
for connecting different devices to it
to operate together as a whole system.
Connector
A plug with either or both male (pins)
or female (holes) connections at
Glossary
either or both ends, used to connect
devices to each other, cards to
motherboards, canisters to
backplanes, and so on, and to pass
signals into or out of these
components.
Contingent Allegiance
A state in which targets reject
commands until an error status on
the bus is cleared.
Controller, or Controller Board
Circuitry, usually built into a drive,
that interprets signals between the
host and the peripheral. It acts upon
these signals, thus providing the
device with “intelligence.”
Controller Chip
A chip that controls one or more I/O
channels.
Control Signals
Signals that direct communications
and determine bus phases. There are
nine control signals:
•
•
•
•
•
•
•
•
•
Control/Data
Input/Output
Select
Busy
Message
Request
Acknowledge
Attention
Reset
CPU
See Central Processing Unit.
CPU Cache
Cache that resides on or near the
CPU.
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Crash
When the computer fails and the
drive stops working.
CRC
See Cyclic Redundancy Check.
Cyanine
One of the two types of dyes used in
the production of CD-R discs. On a
CD-R disc, rather than a physical pit
or depression in the groove, a dye is
used to change the reflection of the
laser in the same way that a pit
would. This dye is activated by a laser
beam of a somewhat higher intensity
than that used to read data. The use of
this dye allows discs created by a
laser to be just as readable on
conventional CD readers as discs
created by a stamper.
See also Phthalocyanine.
Cyclic Redundancy Check (CRC)
A method of checking for data
transmission errors through the use
of a cyclic, mathematical algorithm.
Cylinder
In multi-platter hard disk drives,
tracks of equal radius on different
platters form a virtual cylinder.
Cylinder Switch Time
One of the influences on data access
time. It is the amount of time it takes
to move from one track to another.
Daisy Chain
Multiple devices connected to each
other, where the SCSI bus is
dependent upon each device to pass
signals along.
342
DAO
Disk-at-Once. Also known as
uninterrupted write. It is a writing
method in which the entire disc is
written in one uninterrupted session.
Data cannot be added to the disc
later.
Data Error
Any discrepancy between recorded
data and recovered data.
Data Transfer Rate
A measure of how quickly data is
supplied to the computer from the
peripheral device.
DAT Drive
Digital Audio Tape drive. A tape drive
that uses helical scan magnetic
recording tape to store data in a
fashion similar to audio tape
recorders, only the information is
usually stored in digital form.
See also Digital Data Storage.
Data Block
The smallest contiguous area that can
be allocated for storage of data. In
UNIX, this is 8 KB. In Macintosh and
DOS, this is 512 bytes or multiples of
512 bytes.
In DOS, this unit is referred to as a
cluster.
Data Bus Line
The lines on a SCSI bus dedicated to
transmitting data signals.
The SCSI bus is composed of up to 50
lines, depending on the electrical
specification used, of which 18 are
signal lines. Nine transmit data
signals, and nine transmit control
signals. The rest are connected to
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ground, except for number 25, which
is left open.
concentric circles that emanate from
the center of the platter out to the
perimeter.
Data Density
A measurement, which can be in
tracks-per-inch, of the amount of data
that can fit on a given media surface.
Data Pattern Test
A diagnostic test that is useful for
finding media errors. A pattern is
written to disk, then read back and
compared with what should be there.
The pattern is changed, and the
process is repeated. This way, it
becomes obvious if there are any disk
areas that are not converting from 1s
to 0s as they should.
Data Redundancy
Some form of ongoing data
duplication, often associated with a
RAID architecture, that provides a
fairly high level of protection against
unrecoverable loss of data.
Data Signals
Data, command, status, message and
SCSI ID information transmitted over
data signal lines.
Data Striping
RAID level 0. An array of drives
transferring data in parallel. The data
is spread out among the drives one
segment at a time. The multiple
drives appear to the user as one large,
fast drive.
Data Track
On magnetic media, the invisible
magnetic “grooves” upon which data
is physically stored. Instead of one
continuous track, as on a vinyl
record, data tracks form distinct
Glossary
Data Transfer Modes
Data transfer occurs in either one of
two modes, or protocols:
• asynchronous
• synchronous
The mode used is determined by the
SCSI chip and via messages between
the initiator and target, usually after a
bus reset.
Data Transfer Rate
The rate at which data is transferred
from a storage medium to the
computer. Two rates of transfer are
commonly measured:
• burst data transfer rate
• sustained data transfer rate
Data Underrun
See Buffer Underrun.
Data-In/Out Phase
The Data-In Phase allows the target
to ask for data to be sent from the
target to the initiator. In the DataOut Phase, the target asks to receive
data from the initiator. Data is
transferred in either the
asynchronous or synchronous data
transfer mode.
DDA
Direct Disk Attach Profile. In Fibre
Channel architecture (FC-4), a
protocol used to attach drives to the
computer.
DDM
See Driver Descriptor Map.
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DDS
See Digital Data Storage.
Decompression
Reversal of the compression software
algorithm to return data to its
original size and condition.
De Facto Standard
Standards set by users over time,
rather than by an agency.
Defragment
To reorder the files on a platter so
that all the sectors of each file are
contiguous. This results in improved
access times. (See Fragmentation.)
Device Driver
The software program that translates
commands between the computer’s
operating system and the storage I/O
interface.
last full backup (not just the changed
parts of those files).
Digital
Describes information that is stored
as binary digits (bits), either zero or
one.
Digital Audio Extraction (DAE)
The transfer of audio information
from a CD-ROM drive to a computer
system though a SCSI cable. DAE is
capable of producing an exact copy of
the digital information contained on
the CD.
Digital Data Storage
A standard format for 4 mm tapes
using helical scan recording. It was
developed by Hewlett-Packard and
Sony to replace the audio cassette. It
has subsequently found homes in
both pro-audio and computer areas.
Digital Signal Processor
Device-Independent
Operating at the systems level and
not requiring specific customization
to run.
Differential
An electrical signal configuration
where information is sent
simultaneously through two sets of
wires in a cable. The information is
interpreted by the difference in
voltage between the wires.
Differential interfaces allow cable
lengths up to 25 meters.
Compare with single-ended.
Differential Backup
A combination of an initial full
backup and then subsequent backups
of all files that have changed since the
344
A processor that speeds the
calculation of floating point
operations. Among its many
applications, floating point
arithmetic is used for graphics (in 3-D
rendering), imaging, speech synthesis
and recognition, audio mixing and
numerical processing.
Direct-Access Device
Any device that provides direct access
to non-sequential blocks of data. (See
Sequential-Access Device.)
Disc-at-Once
CD-R disc writing method in which
the entire disc is written in a single
session. Data cannot be added to the
disc later. This method produces discs
that are compatible with all CD-
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ROM drives, and is used for
electronic publishing, audio or any
other application where compatibility
is crucial.
Diode
A non-linear, two-terminal device
that is the simplest semiconductor
used in electronics. It acts as a oneway valve, letting current flow
through it in one direction while
blocking current flow in the other.
DIP Switch
Dual In-line Package Switch. DIP
switches are usually found on
internal circuit boards. They are used
to set some of a device’s operational
parameters.
Directory
A list of all the files and groups of
files stored on a disk. Usually
includes the type and name of a file,
its size and its creation or
modification date.
Disconnect Capability
A capability of most SCSI peripherals
to leave the bus, complete a timeconsuming operation, and then
return.
Disk Array Controller Board
A board with built-in logic for setting
up and maintaining a RAID array.
Disk Drive
The physical components necessary
to transfer data to and from the
recording medium.
Glossary
Disk Duplexing
Using one drive to duplicate the read/
write moves of another. This creates
an identical copy of the original. It is
accomplished through a dedicated
set-up that has two drives and two
separate drive interfaces and host
adapters, each with its own power
supply. Data is both read from and
written to both drives.
Disk Mirroring
A single disk controller is connected
to two drives. All controller activity
is performed twice—first to one drive,
then the other.
DLT Drive
Digital Linear Tape drive. Tape Drives
use magnetic recording tape to store
data in a fashion similar to audio tape
recorders, only the information is
usually stored in digital form.
DMA (Direct Memory Access)
A mechanism that allows for the
transfer of streams of data to or from a
computer’s main memory without
the use of the host microprocessor.
DMA may require setup by the host
software. After initialization, DMA
automatically sequences the required
data transfer and provides the
necessary address information.
DMA Controller
Direct Memory Access Controller. A
microprocessor dedicated to handling
Direct Memory Access. It is usually
incorporated into the device
controller and operates without
depending on the host CPU.
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Doubled Embedded Servo
Placing servo information at the
beginning and middle of a disk sector.
DSP
See Digital Signal Processor.
Dual Channel SCSI
Double-Density Recording
Another term for Modified Frequency
Modulation.
Drive Channel
A communication path to the drive,
capable of transmitting data.
Drive Controller
Circuitry that provides the hardware
interface between the disk drive and
the computer, sending and receiving
signals. It interprets the computer’s
signals and controls the operations of
the disk drive.
Driver Descriptor Map
The first data read from a Macintosh.
It contains:
•
•
•
•
the number of blocks on the device
the block size
the number of drivers
the location and size of the first
driver
DVD
Digital Versatile Disc. Also referred
to as Digital Video Disc. A new
standard that provides for up to
4.7 GB of storage on a 120-mm, CDROM-sized optical disc.
Edge Connector
A connector with flat, metal contact
points that mate with like contact
points in connection ports, such as
motherboard slots or cable ports.
They were designed to eliminate the
vulnerability to bends and breaks
inherent in pin connectors.
EDPT
Early IBM drives that were shaped
like drums, had heads that touched
the platters, and held very little data.
Enhanced Drive Parameter Table. A
parameter table used on PC
compatibles that contains two sets of
drive parameter information. The
first set is called CHS, for cylinder,
head, sector. It provides a straight
translation of BIOS cylinder, head and
sector information, without reducing
the number of heads supported from
255 to 16.
The IBM RAMAC 350, in 1956,
stored 5 MB with 20-inch disks and
had a 600 ms access time. A one- year
lease cost about $35,000.
The second set translates the
cylinder, head, sector information
into a 28-bit logical block address à la
SCSI.
Driver Partition
The disk partition that contains a
device’s driver.
Drum Storage
346
A system with two SCSI controllers,
allowing twice as many connected
devices as single channel. In regular
dual-channel SCSI, you can have up
to 16 devices on the bus. In Wide
dual-channel SCSI, you can have up
to 32.
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EISA
Enhanced Industry Standard
Architecture. It was designed to
supersede the ISA bus, yet retain full
compatibility with ISA. EISA ran at
the ISA standard 8.33 MHz clock rate
and allowed bus masters to be
supported. EISA extended ISA to 32bit operation and sped burst data
transfer rates up to 33 MB/s. Its
sustained rate was closer to 8 MB/s.
EISA allowed for automatic adapter
configuration without DIP switches.
Embedded Servo
Servo data is embedded on the platter
between the tracks in the form of
magnetic bursts. This is also known
as wedged servo.
exhausts its allowable number of
retries.
Errors, Hard Data
Data loss through physical damage to
the hardware, usually the surface of
the recording medium, which may
not be recoverable. Hard errors are
permanent and thus are likely to be
repeatable.
Errors, Soft Data
Data misreads or miswrites due to
poorly written software or viruses,
which may not be repeatable. Soft
errors can usually be recovered
through reread attempts or a recovery
program.
ESDI
Encoding
The protocol by which data is written
to the hard disk platter. Encoding
provides timing marks for the head
and a shorthand form of the data,
which increases the platter’s capacity.
Enhanced IDE
A storage interface that allows
support of four IDE devices when two
IDE channels are present. The
primary channel is designed for highspeed hard disks, whereas the lowspeed secondary channel is designed
for non-hard disk devices, such as
CD-ROM and tape drives. Allows
support of drives up to 8.4 GB in
capacity. Logical block addressing
allows for the increase in capacity
potential. Support for multiple-block
transfers is included.
Error Correcting Code (ECC)
Additional data stored on the drive to
help correct an error before the drive
Glossary
Enhanced Small-Device Interface. An
improved ST-506 interface developed
in the mid-1980s and championed by
Maxtor. ESDI doubled ST-506’s data
transfer rate and allowed for selective
reformatting of tracks. Otherwise,
mechanically and electronically it
was virtually identical to ST-506. It
was device-specific and only used for
hard disk drives, mostly those with
very large capacity. It offered a
maximum capacity of about 1 GB.
Exabyte (EX)
An exabyte equals 1018 or
1,000,000,000,000,000,000 bytes.
Exabyte is also the name of a
manufacturer of tape drives.
Exclusive Or (XOR)
A Boolean function that sets a parity
bit to 1 if and only if either of the
compared data bits is 1, but not both.
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Expansion Card
Fast SCSI is used in only the data-in/
out phase.
An add-in card that typically
enhances system features. Typically,
cards provide additional connection
ports and/or increase I/O
performance.
Performance improvements for
sustained data transfer rates increase
as follows:
• Fast SCSI-2: 10 MB/s (8-bit narrow)
• Fast/Wide SCSI-2: 20 MB/s (16-bit
wide)
• Fast-20 SCSI/Ultra: 20 MB/s (8-bit
narrow)
• Ultra Wide SCSI: 40 MB/s (16-bit
wide)
Extended Partition
Separate logical partitions that can be
up to 8 GB in capacity. There can be
lots of extended partitions because
they include their own partition
tables.
Extents Tree
Fast/Wide SCSI
Part of both the SCSI-2 and SCSI-3
specification. Fast/Wide SCSI allows
for the support of 16 devices on a
single-channel bus (32 on a dualchannel), a 20 MB/s data transfer rate,
and 16-bit wide data transfers.
Typically a 68-pin connector is used
with Fast/Wide SCSI, although a 50pin can be used.
A data structure on the Macintosh
that tracks the location of and
statistics for all files and folders on
the disk.
External Hard Drive
A hard drive in its own chassis that is
connected to a host computer with a
cable.
Fast ATA
See also Fast SCSI.
Fast-20 SCSI
Part of the SCSI-3 specification. Also
referred to as Ultra SCSI. Fast-20 SCSI
allows for the support of 8 devices on
a bus, a 20 MB/s data transfer rate,
and 8-bit data transfers. A 50-pin
connector is used with Fast-20.
Seagate and Quantum’s version of
EIDE, but without some of the
extensions, like large capacity and
ATAPI support. It was designed to
minimize the need to enhance system
BIOS.
See also Fast SCSI.
Fast SCSI
Fast SCSI uses the standard 50-pin
connector to double the maximum
data transfer rate to 80 Mb/s in fast
synchronous mode. (Up to 10 megatransfers per second.) It does this by
cutting synchronous transfer timings
in half. However, this works only
with modern SCSI chips, which were
not used in the Macintosh for many
years, but are available through thirdparty add-ons.
348
FAT
See File Allocation Table.
Fault Tolerance
The ability of a system to continue to
perform its function (possibly at
reduced performance), even when one
of its components has failed. Fault
tolerance is often provided by
equipping a system with extra
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instances of components whose
failure would cause the system to fail.
FC-AL
File Data Area
The space on disk where data is
stored in allocation block-sized
chunks.
See Fibre Channel Arbitrated Loop.
File Manager
Ferric Oxide
Used for 40 years as the primary
medium for data storage. A floppy’s
surface is covered with rust-oxidized
iron particles held in place with a
binding agent (typically plastic).
Fibre Channel Arbitrated Loop
An architecture geared towards
connecting large scale host computers
to storage subsystems and network
hubs. The loop structure enables the
rapid exchange of data from device to
device.
Fibre-Optic Cable
Cable made of plastic or glass fibre
that is designed to transmit
information at very high frequencies,
requiring infrared or visible light as
the carrier.
Field Rate of Return (FRR)
One of the calculations used to
determine Mean Time Between
Failures. FRR only relates to the
failure rate of the installed base, not
the average life span of each drive.
with FRR, an MTBF of 1,000,000
means that out of 10,000 drives, you
can expect one drive to fail every 100
hours.
File Allocation Table
A table that keeps track of the
locations of blocks in which the
operating system has written data to a
disk.
Glossary
A software library typically located in
ROM that manages I/O flow to and
from files.
FIPS-60
Federal Information Processing
Standard 60. It is also known as IBM’s
370-OEM interface. It provided for
very fast transfers up to 5 MB/s and
cable lengths over 100 meters. It has
been extensively used in IBM’s
System 370, 3080 and 3090 families.
FireWire
See 1394.
Firmware
An often-used microprogram or
instruction stored in ROM. Usually
refers to the ROM-based software
that controls a drive.
Flag
In a Mode Page, a flag is a bit (either
on or off) that tells the drive how to
respond to a situation.
Flash Memory
A type of electrically erasable,
programmable ROM (EEPROM) that
can be programmed by the computer
or the device it is connected to. It can
also refer to a nonvolatile memory
system.
Floptical Drive
A drive that marries optical and
magnetic technology. The servo data
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is perforated into the disk. The
magnetic data is located in between
optical grooves.
Forced Perfect Termination
Developed by IBM, FPT uses a
resistor/diode network to pull current
up until the diode threshold is
crossed. The diode then maintains
the voltage at the desired level.
Form Factor
The physical diameter of a drive’s
platter.
Formatting
The process of preparing a disk for
use. The drive maps the disk into
blocks, sectors, and tracks. Bad blocks
are marked and placed on a defect list.
FPT
See Forced Perfect Termination.
Fragmentation
With use over time, the sectors of a
file are written in different areas
across the platter’s surface. This
slows access time.
Frequency Modulation
Timing mechanisms correlate a
platter’s constant speed with the
distance traveled to yield a precise
calculation of the head’s position over
the platter.
Frequency Modulation was the first
timing mechanism used. It needed
every other magnetic domain to
represent a clock pulse. This method
had the disadvantage of using half of a
disk’s storage capacity just for the
timing information.
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FRR
See Field Rate of Return.
Full-Height
A drive with a housing unit that is
3.25-inches high. It houses 5.25-inch
form factor drives.
g
A measurement value. A g is
equivalent to the pull of earth’s
gravity. Device shock ratings are
measured in g’s.
Giant Magnetoresistance Head (GMR)
The heir-apparent to the MagnetoResistive head. The GMR may permit
magnetic recording in the tens of
gigabits per square inch and data rates
in the tens of megabytes per second.
This new technology has sensitivity
in the greater-than 3000 microvolt
per micron of track width range.
Gigabit
1024 Megabits or 1,073,741,824 bits.
Gigabyte (GB)
1024 Megabytes or 1,073,741,824
bytes.
Green Book
One of the “color-book” standards for
CD recording, in this instance CDinteractive (CD-i). CD-i is used for
some home entertainment systems.
Philips, which developed the
standard, has been the most vocal
proponent of this technology. Like
CD-G, many readers support the
format, but there are few computers
or applications that allow the discs to
be used.
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Green Drive
Environmentally correct drives that
support conscientious features like:
•
•
•
•
low-power sleep mode
deep sleep mode
power management
lower power requirements
The EPA estimates that a system that
powers down during inactivity could
save the user $70 to $90 a year.
Grown Defects List
Created during formatting, it is a list
of the bad areas on a disk made
available to the controller so that it
knows not to write data to them.
Half-Height
A drive with a housing unit that is
1.625-inches high. It houses 3.5- and
5.25-inch form factor drives.
Handshake
The SCSI Request/Acknowledge
handshake signifies completion of a
data transfer between the target and
host across the SCSI bus.
• drops all SCSI device reservations
(for disconnects and command
queuing)
• returns all operating modes to their
default conditions
This condition is asserted on the
Reset signal line and goes out to all
devices.
Compare with Soft Reset.
Hardware-Based Disk Array
An array where the drives are
connected to a RAID controller board
installed inside the disk array chassis.
The host computer recognizes the
RAID controller board as the SCSI
storage device, and not the drives
connected to it. Read and write
commands sent from the computer
are processed by the RAID controller
board, which in turn sends the
appropriate read and write commands
to the drives.
HBA
See Host Bus Adapter.
HDA
See Head Disk Assembly.
Hard Disk Drive
A data storage device that employs
one or more rigid disks as the storage
medium.
Hard Drive Cache
Cache that resides on the hard drive’s
controller board.
Head
See Read/Write Head.
Head Actuator
The physical device (a type of motor)
that moves the armature and thus the
read/write head(s) across the surface
of the hard disk platter.
Hard Reset
The Reset condition kicks every
device off the bus.
A Hard Reset:
• knocks out all commands in
process
Glossary
Head Disk Assembly
An assembly that includes magnetic
disks, magnetic heads and an access
mechanism, all enclosed in a
container.
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Head Drift
A diminishment of the positioning
accuracy of the drive head due to
wear-and-tear.
Header
The first data contained in the Mode
Parameter List, which is sent by the
initiator to the drive during the DataOut Phase. It contains:
• the type of storage medium, if any,
used in the drive
A setting of zero for the medium
type, which is the default, indicates
that the medium is nonremovable.
• the length in bytes of all Block
Descriptors
Head Gap
In magnetic head/disk assemblies,
most heads consist of an iron core
with 8 to 30 coil turns of wire banded
around it. There is a gap in the core
through which a magnetic field
passes. This is the head gap.
Head Parking
When a drive is turned off, the read/
write head is moved to a specified
location on the disk that is not used
for data storage - the landing or
parking zone. Most modern drives
have an autopark function.
Head Switch Time
The time, measured in
microseconds, it takes to switch from
one read/write head to another in a
read or write operation.
A Macintosh partition that contains
structures to help the Macintosh
operating system boot the computer
and locate information on files and
folders. This data structure is set up
during disk initialization. The
following data structures are written
to disk:
•
•
•
•
•
•
Boot blocks
Volume information block (VIB)
Volume bitmap
Extents tree
Catalog tree
File data area
Hierarchical File System
The file system used by the Apple
Macintosh for storing files and folders
on a hard disk.
High-Density Connector
Defined in the SCSI-3 specification,
high-density connectors pack the pins
more tightly, and are 1.44 inches wide
and a quarter-inch thick.
High-Level Formatting
Formatting performed by the
operating system to create the root
directory, file allocation tables and
other basic configurations.
High-Level Interface
A sophisticated interface level that
describes a nonphysical abstraction
(a logical perspective) that can be
manipulated with software.
Holographic Memory
HFS
See Hierarchical File System.
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HFS Partition
A variation of conventional
holography. Two laser beams are
focused in the system at different
angles and optically interfere on a
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light-sensitive film. One beam carries
the data, which can be either a
representation of the digital data
stream or a pictorial image. By using a
scanner to change the angle or
position of the other beam, multiple
views of the image are recorded.
Thousands of these holograms can be
recorded on a single piece of film.
Host
Housing Unit
The rigid enclosure that contains the
mechanism and controller. It is the
foundation to which all other parts
are connected. It needs to be rigid so
that components within the drive are
not hurt on installation or during
operation. Most housing units are
made from aluminum alloys.
HPPI
The computer—specifically, its
central processing unit (CPU) and
SCSI chip.
High Performance Parallel Interface.
This is another name for IPI-3. It is a
parallel interface enabling 800 or
1,600 Mb/s data transfers over 32 or
64 twisted-pair copper wires, or on
fibre-optic cable. Fibre cabling can be
up to 10 kilometers in length, while
the other cabling can be up to 36
meters in length. Developed out of
work at the Los Alamos National Lab,
it is intended for supercomputing
applications. It allows multiple block
transfers via a single command
execution, but can transfer data in
only one direction at a time.
Host Bus Adapter
Also known as a SCSI adapter or a
SCSI host bus adapter. Circuit, card,
or device that translates between a
computer's “native” bus architecture
and the SCSI specified bus
architecture.
Hot Spare
In some RAID subsystems, an extra
drive that is fully spun up but will be
used only if one of the other drives
fails.
HSM
Hierarchical Storage Management.
HSM is a concept that creates a
hierarchy—a layered structure—of
your storage capacity. At the top of
this hierarchy is your fastest-but
most expensive-storage device. At the
bottom, your slowest, least
expensive device.
Hot-Swappable Drive
A drive in a canister, shuttle or carrier
that can be removed and plugged in
while the system is running.
Hot-Swappable Power Supply
Hot swap capability allows you to
swap out the inoperable power supply
with an operable spare while the
system continues to run. Hot
swappable power supplies should be
used in conjunction with
uninterrupted power supplies and
power conditioners to ensure a
continuous flow of power during
swaps.
Glossary
IC
See Integrated Circuit.
IDE
Integrated Drive Electronics or
Intelligent Drive Electronics. IDE
incorporates controller hardware onto
the disk drive, which results in
significant parts reduction and cost
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savings. IDE combines the physical
and electrical protocols of ST-506
with the speed of ESDI. IDE drives
can be “directly” plugged into a
computer’s expansion bus.
IEEE
Institute of Electrical and Electronics
Engineers. An ANSI-accredited body
of scientists and engineers that
promotes standardization and
provides expertise to ANSI on
matters relating to electrical and
electronic development.
the coil of wires wrapped around the
iron core, depending on the direction
current is flowing in the coil.
Initialization
The process of laying down the
directory information that the
operating system needs to locate files
and folders. Usually destroys only
directory information, not the actual
data, so that recovery programs can
still recover files from initialized
disks.
Initiator
Impedance
The total opposition offered by a
component or circuit to the flow of an
alternating or varying current.
A SCSI device, usually the
computer—or more specifically its
SCSI chip—capable of initiating an
operation. In multiple-host systems
initiators (hosts) may also be targets.
Incremental Backup
Creating data security through
duplication. With an incremental
backup, a software utility reads
whatever information has been added
to the hard drive during that work
session and copies it to a secondary
storage medium.
Incremental Recording
A recording method in which data
can be added to a CD-R disc in
segments. There may be, but does not
have to be, interruptions in the data
flow between segments. Data can be
added using track-at-once (TAO) or
packet writing and single session or
multisession recording. These discs
are Orange Book compliant.
Inductive Head
A read/write head that consists of an
iron core with 8 to 30 coil turns of
wire banded around it. There is a gap
in the core through which a magnetic
field passes. The field is induced by
354
Input/Output (I/O)
The communication flow between
the Mac and its peripherals.
Inside Mac IV and V
Technical manuals provided by
Apple. Also used to denote
partitioning schemes.
Integrated Circuits, or Chips
An IC performs a host of electronic
functions. One IC replaces many
discrete transistors. Because it resides
on a silicon chip, these ICs are
referred to as “chips.”
Intelligent
Refers to a device capable of
processing commands on its own.
Interface
The go-between that provides a
common basis for communication
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between two otherwise incompatible
devices.
Intrasector Gap
Data on a track sector that separates
the sector ID information from the
data track. Data follows this gap.
Interleaving
The ordering of sectors on a track so
that the next sector (in the file being
read or written) arrives at the read/
write head just as the computer is
ready to access it. (See Latency.)
I/O
Input/Output. Refers to the sending
and receiving of data from the CPU to
attached peripherals, such as CDROM and external hard disk drives.
INT13 BIOS
The standard software interface
method for accessing a PC hard disk.
Intelligent Peripheral Interface
The successor to the block
multiplexer channel, or OEM
channel, popularized by IBM in the
1960s. It offers parallel 16-bit data
transfer, rather than OEM’s eight-bit
transfer, and can be split into two
eight-bit buses to work like the OEM
channel. IPI’s dual ports allow from
one to eight devices to be connected
on one port, and from two to 16 in
pairs using both ports. It is largely
limited to the mainframe world,
typically on 8- and 14-inch drives. It
offers read-ahead capability and is
extremely fast for large-data transfers.
Interoperability
Connecting different devices to
operate together as part of a system.
Interrupt
A momentary suspension of
processing caused by a deliberate
instruction to the microprocessor.
Intersector Gap
Data on a track sector that separates
the preceding sector from the next
sector.
Glossary
IPI
See Intelligent Peripheral Interface.
IRQ
Interrupt Request. Refers to the lines
in a computer that carry hardware
interrupt signals to the processor on a
personal computer.
ISA
Industry Standard Architecture. An
expansion bus introduced with the
original IBM PC and IBM PC XT in
the early 80s.
Isochronous
Having the capability of dedicating a
fixed slice of bandwidth to a
particular peripheral.
ISO 9660
ISO is an acronym for the
International Standards Organization.
ISO 9660 is an established
international standard file structure
for CD-ROM discs adopted by ISO.
Jukebox
A large chassis that contains multiple
optical cartridges, tapes, or CD-ROM
discs that get shuffled around by a
mechanical device.
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Jumper
Latency
A small plastic-covered metal
connector used to connect pairs of
pins on a circuit board. Jumpers are
used to set device parameters, such as
the SCSI ID.
The time, in milliseconds, it takes for
the spinning platter to bring around
the desired sector to where the read/
write head can access it. Does not
include head positioning time. It is
one of the factors of access time.
KB/s
Kilobytes per second.
LBA
See Logical Block Address.
Kb/s
Kilobits per second.
Level 2 Cache
Cache that is external to the CPU.
Kerr Effect
The Kerr effect states that when a
polarized light is reflected off a metal
surface in a magnetic field, the
polarity of the light is rotated
clockwise or counterclockwise,
depending on the field’s polarity. The
drive detects this rotation and
interprets it as data.
Kilobit (Kb)
One thousand bits (actually 1024
bits).
Kilobyte (KB)
One thousand bytes (actually 1024
bytes).
LIMM DOW
Light intensity modulation method
direct over-write. In optical drives,
LIMM DOW allows data to be written
in a single pass.
Logical Block
Data is grouped in standard sizes
known as logical blocks. 512-, 1,024-,
and 2,048-byte blocks are common.
Logical Unit Number (LUN)
The numerical representation of the
peripheral’s address. A SCSI device’s
address can have up to seven LUNs.
Logical Unit
Landing Zone
A data-free “safe” area on a disk, set
aside for parking the drive head.
A physical or virtual device that is
addressable as a target.
Logical
Laptop
A small, portable personal computer.
Laser Diode
A laser that generates an infrared or
visible-light beam.
356
An abstract representation of
something that physically exists.
Loop Architecture
An architecture geared towards
connecting large-scale host
computers to storage subsystems and
network hubs.
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In a loop architecture, the preferred
approach is to attach disk drives
directly to a backplane, rather than
daisy-chaining them together.
Low-Level Formatting
The electronic equivalent of a
detailed street map, written on a hard
disk. Electronic markings are made
on the disk that tell the controller the
locations of cylinder/head/sector
addresses, servo tracking information
and data start/stop points.
Low-Level Interface
Specifications for the wiring,
electrical operations and physical
connections in an interface standard.
LUN
See Logical Unit Number.
Magnetic Domain
The area on a platter that contains
one bit of data.
Magnetic Flux
The magnetic exchange between the
read/write head and the platter,
which allows the head to write and
read data.
Magneto-Optical Drive
Devices that use a high-powered laser
to write to a disc and a low-powered
laser, which does not heat the disc
significantly, to read a disc.
Magneto-Resistive Head
Thin conductive strips that detect
magnetic domain transitions by
measuring magnetic effects on the
resistive element within the head.
Glossary
This element changes its resistance
as its angle of magnetization changes.
Mainframe
A large and fast central computer.
The term “mainframe” originally
referred to the metal framework that
housed the computer circuits, but it
came to mean the entire computer.
Typically, a mainframe is defined by
function rather than by size. It
describes a computer that handles all
of an organization’s processing needs.
Master
When two devices or systems are in
such a relationship that one of them
has control over the other:
• The controlling device is the
master.
• The controlled device is the slave.
MB/s
Megabytes per second. Equal to 8
Megabits per second.
Mb/s
Megabits per second. Equal to 1/8 or
0.125 Megabytes per second.
MCA
See MicroChannel Architecture.
MCM
See Multichip-Module.
Mean Time Between Failures
A vendor-supplied rating that
indicates the longevity of a drive.
Often referred to by its initials MTBF.
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Mean Time to Repair
Amount of time it takes a trained
repairman to repair a device.
Media
In hard disks, another term for the
platter, but more specifically the
magnetic coating that covers the
platter. The surface of the platter that
holds the data.
Media is also the term used for the
recording surface of all drive
technologies, such as MO, CD-R and
Phase Change Optical.
Megabit
One thousand kilobits (actually, 1024
Kb). Abbreviated as Mb.
MicroChannel Architecture
An IBM-designed proprietary
architecture, developed in 1980 to
improve upon and replace the aging
ISA bus.
Microcontroller
A chip that controls input/output
channels.
Micron
A unit of length equal to onemillionth of a meter (10–6).
Microvolt
One-millionth of a volt (10–6).
Millisecond (ms)
Megabyte
One thousand kilobytes (actually,
1024 KB). Abbreviated as MB.
Message-In/Out Phase
A SCSI bus phase. During the
Message-In Phase, the target may
request to send a message to the
initiator.
The Message-Out Phase is invoked by
the target only in response to an
Attention condition generated by the
initiator. It can be generated at any
time.
Metallurgy
The science that deals with
extracting metals from their ores and
making useful objects from them.
MFM
See Modified Frequency Modulation.
358
One-thousandth of a second. In
computing, this measurement is
often used to specify the access time
of a hard disk drive.
Miniport Driver
A hardware-specific translation layer
that abstracts the I/O architecture of
the host adapter. It initializes and
configures the adapter, starts I/O,
verifies the adapter state, and resets
the SCSI bus.
MIPS
Million Instructions Per Second. A
measurement of the speed of
execution of a computer’s CPU.
Mirrored Cache
A method of using duplicate caches in
which a mirror image of the first
cache is stored in the second,
providing 100 percent duplication. If
the first cache fails, the second one
takes over in a way that is transparent
to the user.
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Mirroring
Writing the same data to two drives
simultaneously. (See RAID.)
Mode Page
A device’s behavior, or “mode,” is
determined by its operating
parameters. These mode parameters
are grouped into related pages that are
numbered for easy reference. A mode
page is the minimum unit that can be
specified by the Mode Select or Mode
Sense Command. A page consists of
multiple parameters, which must be
specified whenever a page is invoked.
Mode Parameter List
A list that specifies device parameters
and is sent by the initiator to the
drive during the Data Out Phase. The
Mode Parameter list contains:
• a header
• zero or more Block Descriptors
• mode pages
Modified Frequency Modulation
Timing mechanisms correlate a
platter’s constant speed with the
distance traveled to yield a precise
calculation of the head’s position over
the platter.
Modified Frequency Modulation is
where timing information is encoded
onto its own track. During the
original low-level formatting process
of an MFM drive, the synchronization
bytes—magnetic data that marks
location and time—are added. MFM
is also known as double-density
recording because it results in twice
as much data storage as the original
FM.
Glossary
Motherboard
The primary circuit board in a
computer. It contains such
components as expansion slots, the
CPU, memory and device controllers.
Mount
To appear as a drive letter on PCs or
on the Desktop on Macintoshes.
Move16
An instruction inherent in model
68000 microprocessors that moves
16 bytes very quickly.
MPEG
Motion Picture Experts Group, and
the algorithms endorsed by that
group. Both MPEG-1 and MPEG-2
provide configurable compression for
video data. They allow content
producers to control the amount of
data required by the video portion of
their media, sacrificing visual quality
for greater compression. MPEG-2
offers better quality than MPEG-1. It
allows both backward and forward
motion video, but results in larger file
sizes.
MTBF
See Mean Time Between Failures.
MTTR
See Mean Time To Repair.
Multichip Module
A small, high-speed, high-density,
multi-function, multi-layer chip
designed to provide the greatest
amount of processing in the smallest
amount of space.
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Multiple-Host System
A system that has more than one
computer connected to the SCSI bus.
Multisession Recording
A CD made with at least two
sessions, where the boundary
between the sessions also forms a
boundary between tracks. Each
session is recorded as a contiguous,
spiraling string of data, one session
following the other. The boundary
between the lead-out of one session
and the lead-in of the next is a small
bit of smooth, unmarred CD surface.
Multivolume/Multisession Recording
A format where each session on a disc
appears as a separate volume of data.
• On a PC, different volumes can be
displayed as different drive letters.
• On a Mac, different volumes can be
displayed as different icons.
This is the most common type of
multisession disc.
MUNI Chip
Macintosh Universal NuBus
Interface. This bus offered the
following features:
• support for the full range of NuBus
master/slave transactions with
single or block moves
• support for the faster data transfer
rates to and from the CPU bus
• support for NuBus ‘90 data transfers
between cards at a clock rate of
20 MHz
• provision of first-in, first-out (FIFO)
data buffering between the CPU
bus and accessory cards
Negate
To withdraw a control signal or
indicate its absence.
Network Server
A computer attached to a network
that stores applications, databases,
data, and other tools and information
used in common by people connected
to the network. A server’s multipleaccess capability is made possible
through software that contains
multitasking and multi-user support.
Nibble
A half-byte, or four bits, of data.
Noise
Reflected or distorted signals or
voltages on the bus.
Nonproprietary
Refers to a device that implements
shared or open technology and can
communicate with other devices
using the same technology. (See
Proprietary.)
Nonproprietary Standard
An open standard that is not owned
by any enterprise and is therefore not
subject to licensing fees by developers
who create products to that standard.
Nonvolatile
Content, such as data or memory,
that is retained after power is
removed.
Notch
Each section or a range of the logical
unit with a different number of
blocks per cylinder. Also known as a
zone.
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Notebook
A small, portable computer that
typically weighs six pounds or less. A
notebook is usually smaller than a
laptop.
A housing unit that is 1-inch high. It
houses 2.5- and 3.5-inch form factor
drives.
On-Going Reliability Test
ns
See nanosecond.
NuBus
A Macintosh expansion bus whose
technology was originally developed
by MIT in the 1970s and enlarged
upon by IEEE in the mid-1980s.
NuBus boards support multiple
processors and bus mastering and are
self-configuring (eliminating the
need to set board parameters with
DIP switches).
NuBus accelerates system
performance and enables you to
expand your system by adding cabling
ports.
NuBus Block Mode Data Transfer
Allows large blocks of data to be
transferred quickly before releasing
the bus, with only one arbitration for
the bus.
OEM
Original Equipment Manufacturer.
OEM applies to a product that is
manufactured by one enterprise then
sold to another, where it is bundled
and sold with the purchasing
enterprise’s own products.
Offset
The Request/Acknowledge offset
sets the maximum number of
Request signals that may be sent
before a corresponding Acknowledge
signal is received.
Glossary
One-Third Height
One of the techniques manufacturers
use to produce MTBF ratings.
Manufacturer’s randomly select
drives at the plant and test them.
ORT is useful in combination with
combined life expectancy and FRR to
determine MTBF ratings on older
drives.
Opcode
The first byte of a command. The first
three bits encode the command group
(6-byte, 10-byte, 12-byte, vendorunique and reserved), and the next
five encode the command.
Open Boot
See Open Firmware.
Open Firmware
Open Firmware was originally
developed as Open Boot by Sun
Microsystems. It debuted in 1988
with the original Sun SparcStation
workstation. It provides a way of
booting a computer system that is
independent of both operating system
and processor.
ORT
See On-Going Reliability Test.
Overhead
The incidental command processing
time that is necessary to complete a
task.
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Packet Writing
A method of writing to a compact
disc. A packet is a logical group of
data. A disc can hold up to 9,500
packets, space permitting.
parallel, while one redundant drive
functions as the parity check disk.
Parallel Port
A connector port that allows all eight
bits of a byte to be transferred at once.
Paddle Card
A nickname for an IDE host adapter
card.
Palmtop
A very small computer, conceivably
small enough to fit in the palm of a
hand. Usually their storage capacity
is minimal, and applications are built
in.
Pancake Motor
A brushless, direct-drive (no gears or
belts), direct-current (DC) electric
motor that drives the spindle in an
HDA. This model resides below the
spindle and is called a pancake motor
because of its flat shape.
Parallel Architecture
In a parallel architecture each byte
(eight bits) is sent simultaneously
using separate lines.
Parallel Backplane Bus
A backplane with connectors for
peripherals that conduct parallel, 8bit data transfers.
Parallel Data Transfer
Data bits travel simultaneously
along eight parallel wires in the cable.
(Also see Serial Data Transfer.)
Parallel Disk Array
Another way of referring to RAID
level 3. RAID level 3 is an array of
disk drives that transfer data in
362
Parameter
Configuration values for the
operation of a logical unit. Parameters
are sent across the bus in blocks
called pages. Some pages pertain to all
devices, while some are only for
specific classes.
Parameter RAM
A LUN has three copies of its
parameters: the current, the default
and the saved. The current
parameters are those with which the
device is currently functioning. These
reside in RAM. This is the parameter
RAM.
Parity
A method of checking the accuracy of
binary numbers. SCSI uses odd parity,
which means that the sum of all ones
in a number plus its parity bit will
always be odd.
Parity Bit
An extra bit, added to a number, used
for checking the accuracy of binary
numbers.
Parity Check Disk
A disk in an array used exclusively for
storing parity information.
Partial Response Maximum Likelihood
A technique used by a read/write
head in a drive mechanism for
detecting data. Instead of spacing out
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analog peaks as in peak detection,
digital filtering is used to compensate
for signal overlap. After filtering, the
scheme identifies what are the
likeliest sequence of data bits written
to the media.
Partition
A portion of a storage area allocated
to a particular use or user.
Partition Map
A map detailing the layout of the
medium, used by the operating
system and drivers to find partition
locations.
Follows the Driver Descriptor Block
(block 0) on a formatted disk and
contains the following information
about all partitions on the disk:
•
•
•
•
•
•
name of the partition
processor type
type of partition
partition status
partition size
more
Each partition’s information
occupies one block in the partition
map. The partition map can vary in
size from a few to hundreds of blocks.
After the partition map come the
partitions in sequence.
Partition Table
Part of the Master Boot Block.
Partition tables store information on
the location, starting point, ending
point and size of each partition on a
hard disk.
Passive Backplane
A board that exists merely to pass
signals; no microprocessing takes
place.
Glossary
PBC
See Port Bypass Circuit.
PCI
Peripheral Component Interconnect.
A system standard that defines a high
performance interconnection method
between plug-in expansion cards,
integrated I/O controller chips and a
computer’s main processing and
memory systems. It was originally
designed by Intel in the early 1990’s
as an alternative to other proposed
peripheral expansion buses.
PCMCIA
Personal Computer Memory Card
International Association. A joint
effort of various special interest
groups aimed at setting a standard for
memory cards used in PCs. PCMCIA
cards add improved computer
memory capacity or enhance
connectivity to external networks
and services.
Peak Detection
Data is detected in hard drives
typically with a technique known as
peak detection. It involves detecting
data at high speeds, using a dataencoding method that spaces the
signal during reads. The analog
detection circuits can then scan each
peak in series.
Peer-to-Peer Interface
An interface in which all devices on a
bus are treated as equals in terms of
their capability to communicate
directly with other devices on the
bus.They share the processing and
control of the exchange.
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Peripheral
An optional input/output device that
connects to a computer’s CPU.
Typical peripherals include massstorage devices, CD-ROM drives,
printers and modems.
laser to be just as readable on
conventional CD readers as discs
created by a stamper.
See also Cyanine.
Physical Path
The channel taken by SCSI
communications.
Peripheral Interface Cable
A cable that makes it possible to
daisy chain SCSI devices, one after
the other, up to a total of seven, not
including the computer.
This cable has two (male-male) 50-pin
connectors. It is referred to as a 50-50
cable.
PIO
See Polled In/Out.
Platter
The rigid disk that is used for storing
memory on hard disk drives.
Personal Digital Assistant (PDA)
Hand-held electronic computerized
devices that assist a user to become
better organized in terms of
administrative and
telecommunications tasks. Apple’s
first PDA was the Newton.
PFA
See Predictive Failure Analysis.
Phase Change Optical Drive
A drive that records by using a laser
to change the physical nature of the
disk from amorphous to crystalline.
Phase change drives write in one pass.
MO drives write in two.
Phthalocyanine
One of the two types of dyes used in
the production of CD-R discs. On a
CD-R disc, rather than a physical pit
or depression in the groove, a dye is
used to change the reflection of the
laser in the same way that a pit
would. This dye is activated by a laser
beam of a somewhat higher intensity
than that used to read data. The use of
this dye allows discs created by a
364
Plug-and-Play Specification
Created by Intel, Microsoft and
Compaq to make it easier to add
adapter boards into PCs. It was
primarily designed to make the
allocation of interrupt, DMA and I/O
port addresses automatic when using
ISA-based cards. (Previously you had
to manually adjust jumpers.) It also
mandated the need for self-resetting
terminator power fuses and for a
special SCSI icon on the back of the
adapter. To be implemented, the Plugand-Play standard required additions
to the computer BIOS.
Pointer
Information in the Command
Descriptor Block that tracks the
execution of command information.
There are two types of pointers to
keep track of command, status, and
data transfers:
• current (or active)
• saved
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Polled I/O (PIO)
The transfer protocol employed by
IDE, where data transfers are handled
by a processor. PIO transfers are
“blind” because there is no
synchronizing handshaking.
Polling Method
direct current (DC) that most
computers’ integrated circuits
require. Regulates and delivers
voltages at designated levels to
specific locations. The power supply
contains a variety of filters that
smooth voltage irregularities and
provide an electrically clean,
consistent power source.
See Normal Mode.
PRAM
Port
A connector on a computer or
peripheral device used for sending and
receiving data (I/O).
Port Bypass Circuit
In Fibre Channel Architecture, the
logic that enables devices to be
removed or inserted without
disrupting the operation of the loop.
In addition, the PBC logic can take
drives off-line or bring them back online by sending a command to any
device to remove it from loop
operation or reinstall it onto the loop.
POST
Power On Self-Test. The sequence of
diagnostic checks that a device runs
on itself as it starts up.
Power Management
An energy-saving feature. It is the
ability of a system to recognize when
it has been inactive for a (usually)
user-specified amount of time, and
consequently to spin down.
Power Spike
A momentary, sharp surge in voltage.
Power Supply
See Parameter RAM.
Predictive Failure Analysis
A technique for predicting when a
drive will die. PFA records and reports
suspicious levels of activity, such as
many recoverable errors, many retries
or slowing performance.
Prefetch
Similar to buffering, except
prefetching can read ahead to the next
track. These larger reads get more
data ready for the CPU’s next request,
thus speeding up access time.
Primary Partition
The first active partition on the disk.
Boot files are always stored on a
primary partition. On any disk there
can be only four primary partitions.
Older DOS versions could support
primary partitions of up to 32 MB.
PRML
See Partial Response Maximum
Likelihood.
Proprietary
Vendor-unique technology or devices
that are incompatible with other
products in the industry.
Converts alternating current (AC)
coming through the power cord into
Glossary
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Protocol
A strict set of rules that govern the
exchange of information between
computer devices.
Pseudo-DMA Mode
Pseudo-Direct Memory Access
Mode. An operating mode in which
the Macintosh SCSI chip oversees
data transfer, letting the CPU tend to
other tasks. This is also known as the
Blind Data Transfer method because
the CPU checks in just once, before a
block of data is transferred, and then
turns things over to the SCSI chip.
Blind transfers are much quicker than
the polling method because larger
chunks of information-blocks—as
opposed to a byte—are transferred at a
time.
Punch Card
A card punched with holes that, along
with the lack of holes, represented
binary data.
RAID
Redundant Array of Independent
Disks. A typical RAID unit contains a
set of disks that appear to the user as
one large storage volume. It is unlike
a single drive in that it provides
improved system performance
delivered in the form of either
dramatically increased data transfer
speeds, various levels of data security,
or both.
RAID Controller
A microprocessor that contains the
logic and command control to oversee
the creation and maintenance of a
RAID architecture.
RAM (Random Access Memory)
Temporary memory usually found on
single in-line memory modules
(SIMMs) on the motherboard of the
computer. RAM is lost when power is
turned off.
RAM Buffer
QIC Tape Drive
A quarter-inch tape drive that uses
magnetic recording tape to store data
in a fashion similar to audio tape
recorders, except the information is
stored in digital form.
RAB
RAID Advisory Board. An association
of suppliers and consumers of RAIDrelated products and other
organizations with an interest in
RAID technology. The goal of the
RAID Advisory Board is to foster an
orderly development of RAID
technology and to introduce RAIDrelated products into the
marketplace.
366
Space allocated in RAM for
temporary storage. Large-capacity
RAM buffers are useful in CD
recording to prevent data underrun.
RAM Chip
Random Access Memory chips whose
storage capacities are determined by
their number of built-in electronic
switches.
Read-Ahead Cache
A variant of the read cache, where an
entire track of data is read and stored
in cache when a single sector of that
track is requested. This allows all
further transfers within the track to
come from cache.
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Read Caching
With read caching, each time a data
request is made, a cache algorithm
checks to see if it’s in RAM cache. If
the data is in RAM cache, it is
returned right away. If the data is not
in RAM cache, it is read from disk,
copied to the cache, and transferred to
the host. Read data remains in cache
until the cache is full. When it is full,
there are different methods of
selecting which data should be
replaced with new data, including:
• a random entry
• the least recently used
• the oldest entry
Read/Write Channel
A group of circuits that transfers the
data off the platter. These circuits
read the sector header information to
ensure that the data being read or
written is the data that is desired.
They also amplify signals to ensure
they are strong enough to record
information. They must be extremely
fast to handle the transitions between
looking at sector information and
reading or writing the data into the
sector in the same rotation. Read/
write channels are often the limiting
factor in drive performance.
Read/Write Head
The disk drive’s electromagnet,
directed by the controller, that
creates and reads magnetic
information on the surface of the
platters.
Real Estate
The limited, and therefore extremely
valuable, space on a hard disk platter,
a controller board, an HBA, or inside
an external device chassis.
Glossary
Red Book
Standard for normal audio CD. Refers
to the specifications for the compact
audio disc format developed by
Philips and Sony. It is the standard
format of commercial audio CDs.
When a disc conforms to the Red
Book standard, it will usually have
“digital audio” printed beneath the
disc logo.
Reflection
A signal that reaches the end of the
line and improperly flows back
towards the device. This situation is
remedied with proper termination.
Register
A temporary hardware storage area
that assists with the speedy transfer
of arithmetic/logical operations
within the CPU.
Removable Cartridge Drive
One or more hard drive platters in a
self-contained rigid case that can be
removed from the main drive unit.
Removable Drive
A hard drive and its entire head
assembly in a rigid cartridge module
that plugs into a “mother” unit. Also
called a canister drive.
Reselection Phase
In Arbitrating systems, the phase in
which a target is reconnected to the
bus after having disconnected to
perform a time-consuming task.
A target device asserts the
Reselection signal and its own ID
number. It gets reconnected if the bus
is free or the target’s SCSI ID has a
priority higher than another device
that is asserting the Select signal.
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Reset Condition
A condition that kicks every device
off the bus. It is caused when a SCSI
device asserts the Reset control
signal. A Reset condition may occur
at any time, forcing the bus into the
Bus Free Phase. Targets may go
through a complete power on self test
(POST) sequence and clear out
pending or current operations.
Retries
When you hear drives seeking the
head hard (a hard seek usually makes
a cha-chunking noise), it’s probably
because retries are occurring. Retries
cause the head to seek back to track
zero and retry the data access. A drive
will try this multiple times. The
number of times is defined in a SCSI
drive’s Mode Page 1 settings.
RISC
Reduced instruction set computing. It
is the successor of CISC, complex
instruction set computing. A
microprocessor commonly used in
professional workstations as well as
PCs. The RISC chip is 75 percent
faster than CISC. It incorporates over
a million transistors and processes
data 64 bits at a time.
RLL
See Run-Length Limited.
ROM (Read-Only Memory)
Permanently stored data in the
computer memory. Also refers to
storage media that may only be read
(not erased or written to.)
Rotational Position Locking (RPL)
the spinning of multiple disks can be
coordinated. This is critical for faster
transfer rates in disk arrays.
RPM
Rotations per minute. In drives, the
measurement of the rotational speed
of the platter.
Run-Length Limited
A data compression encoding scheme
borrowed from the world of
mainframe computers to increase a
drive’s storage capacity beyond MFM.
SAF-TE
SCSI Accessed Fault-Tolerant
Enclosures. A specification, started
by Conner, for a standard method of
communicating with fault-tolerant
enclosures. It defined a set of SCSI
commands used to check the status of
fans, power supplies and temperature.
A small computer with its own SCSI
ID essentially monitors the
enclosure and tells the computer how
it’s doing.
SASI
Shugart Associates System Interface.
The forerunner of SCSI. SASI was
intended as a low-cost, peer-to-peer
interface.
S-Bus
Sun Microsystem’s high-performance
expansion bus for sparc-based
workstations. The S-Bus runs data
transfers at 70 MB/s or faster and
offers I/O configurations that can
sustain mainframe-like I/O rates. The
S-Bus supports Fibre Channel Loop
Architecture.
A drive feature that causes
synchronization of drive spindles, so
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SCA
Single Connector Attachment. A
high-density connector that carries
every type of signal passed along a
SCSI bus, including SCSI ID, LED and
spindle synchronization information.
SCAM
SCSI Configured Auto-Magically. A
protocol that allows a host adapter to
set the SCSI ID number of devices on
the SCSI bus, preventing ID conflicts,
and to set termination automatically.
Scatter/Gather
Refers to reading logically
contiguous blocks of data from a disk
and storing them at discontiguous
host computer memory addresses
(scatter) and writing data from
discontiguous host computer
memory addresses to consecutive
logical block addresses on a disk
(gather).
SCSI
Small Computer Systems Interface. A
standard interface via which
computers and their peripherals
communicate with each other.
SCSI BIOS Boot ROM
Used to initialize the SCSI chip on
startup and to support the transparent
operation of the adapter under DOS
through the standard INT13 BIOS
interface.
SCSI Bus
A facility, developed according to
SCSI specifications, for transferring
data between several devices located
between two end points, with only
one initiator and one target being able
to communicate at a given time.
Glossary
The SCSI bus provides high-speed,
parallel data transmission and is
typically used to connect hard disks,
tape backup drives, scanners and
printers to the host computer.
SCSI Disk Mode
Allows an internal hard disk to be
mounted and used as an external
drive by another Macintosh.
SCSI Host Adapter
Connects a SCSI controller to the I/O
bus of the host. A host adapter may be
integrated on the motherboard of the
system, or it may be implemented as
a separate board.
SCSI ID
A device’s unique address on the SCSI
bus is referred to as its ID, or
identification.
SCSI Manager
The SCSI Manager is part of the
Macintosh Operating System that
provides the interface between a
program such as a driver or formatter
and the actual hardware SCSI port.
SCSI Messages
Communication between the
initiator and target for the purpose of
interface management. A message
can be sent in both directions:
message-out from initiator to target,
or message-in from target to initiator.
There are three types of message
formats:
• single-byte
• two-byte
• extended (three-byte or longer)
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SCSI System Cable
Attaches the first SCSI device on the
bus to the computer.
Macintosh versions have a male 25pin connector to connect to the
computer and a 50-pin connector to
plug into the SCSI device. This cable
is often referred to as a 25-50 cable.
PC versions have a male 50-pin highdensity connector to connect to the
SCSI host adapter, and a 50-pin
connector to plug into the SCSI
device.
SCSI-3
SCSI-3’s main target is extending
SCSI-2’s functionality so that it is
suitable for much higher performance
rates, while maintaining backward
compatibility with present SCSI.
The SCSI-3 specification has
numerous goals, including better
cabling schemes for Fast and Wide
SCSI. Some of these cabling schemes
may be standardized before the SCSI3 specification is released.
Sector Skew
SCSI-SCSI Array Controller
A hardware-based disk array. The
drives are connected to a RAID
controller board installed inside the
disk array chassis. The host computer
recognizes the RAID controller board
as the SCSI storage device, rather
than the drives connected to it. Read
and write commands sent from the
computer are processed by the RAID
controller board, which in turn sends
the appropriate read and write
commands to the drives.
SCSI-2
SCSI-2 is an upgrade to the SCSI-1
standard that builds on the SCSI
specification in the following ways:
• defines extensions to the SCSI-1
specification
• provides more complete
standardization of SCSI-1 command
sets
• describes the necessary mechanical,
electrical, and functional qualities
that will allow for interoperability
of devices meeting the new
standard
To meet these demands, SCSI-2
devices require more intelligence and
firmware than their SCSI-1
counterparts.
370
The sector offset from one platter to
another in a hard drive.
Sectors, Alternate
Sectors set aside to replace bad
blocks. Also called “spares.”
Sectors
Sectors are the smallest subdivisions
of tracks, and usually contain exactly
512 bytes of data.
Sector Header
The first group of information fields
in a sector format. The header is
comprised of the following:
• a field for synchronizing the data
separator
• the address field, which contains
the cylinder, head, and sector
numbers
• the cyclic redundancy code
checksum, which is used to check
whether the address was read
properly
Selection Phase
The bus phase in which an initiator
selects a target device.
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Seek Time
The time it takes the read/write head
to move back and forth in search of
the appropriate track. Seek time does
not include latency or command
overhead. (See Access Time.)
Semiconductor
An element or compound whose
normal conductivity is between an
insulator and a conductor, but whose
resistance to an electrical current can
vary depending upon its level of
impurities.
Sense Data
Sense data is data returned by the
Request Sense command. This data is
used to diagnose the error that
occurred on the proceeding
command.
Sense Key
Error codes contained in sense data
that indicate a particular
classification of error.
Sense Request
This command retrieves sense data.
Sequential Access Device
Any device that must pass through
unwanted blocks of data to access the
targeted block, and can only access
one block at a time. For example, a
tape drive is a sequential access
device.
Sequential Test
A test that is useful for finding media
errors.
Serial Architecture
In a serial architecture, data is
transmitted one bit at a time.
Serial Data Transfer
Data bits travel single-file along one
wire in the cable.
Serial Port
A port that passes along data in
sequence, one bit at a time, as
opposed to a parallel port, which
passes data along in groups of eight,
sixteen, or thirty-two.
Serial Storage Architecture
A powerful high-speed serial interface
designed to connect high-capacity
data storage devices, subsystems,
servers and workstations. Only four
signal wires are required, compared to
68 for the closest SCSI equivalent.
SSA interfaces require no address
switches and no discrete terminators.
Server
See Network Server.
Servo
Information on a disk used for
guiding the head’s positioning over
data.
See also Voice Coil Actuator.
Servo Positioning Platter
A disk dedicated exclusively to
storing and maintaining servo
positioning information that guides
drive heads to accurate positions over
requested data.
Session
A group of data containing a Lead In
area, a Data area and a Lead Out area.
Glossary
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A session can be recorded in one
sitting or incrementally, depending
on the formatting software’s
capability. Each session carries about
15 MB of overhead for the Lead In and
Lead Out areas.
On a disc, a session is one contiguous
spiraling string of data. There may be
more than one session on a disc.
Shock Rating
A rating used to judge the ruggedness
of a drive.
Short Stroking
Using only half of a higher-capacity
drive to achieve faster seek times by
requiring the drive to seek across only
half the platter.
Signal Noise
Unwanted electrical signals that
interfere with a communications
channel. These can come from many
sources, including fluorescent lights,
electric motors, and electric or
magnetic fields.
• manage data transfer hardware,
such as DMA circuitry
• queue multiple operations for
single or multiple LUNs, “freezing”
the queuing of requests as
necessary to perform queue
recovery
• post results back to the initiating
device driver
• manage the selection,
disconnection, reconnection, and
data pointers of the SCSI HBA
protocol
Single Connector Attachment
See SCA.
Single-Ended
An electrical signal configuration
where information is sent through
one wire in a cable. The information
is interpreted by the change in the
voltage of the signal. Single-ended
interfaces allow cable lengths of up to
6 meters.
Compare with differential.
Single-Session Recording
Signal Resonance
Interference with a communications
signal by another signal oscillating at
the same frequency. This disturbs
signal integrity, making it difficult for
the CPU to interpret commands and
messages.
SIM
SCSI Interface Module. Part of the
CAM specification. The SIM function
processes SCSI requests and manages
the hardware-independent interface
to the HBA.
SIM services include:
In CD recording, where the data is
written to disc in one session.
Slave
When two devices or systems are in
such a relationship that one of them
has control over the other:
• The controlling device is the
master.
• The controlled device is the slave.
SLED
Single Large Expensive Drives. An
alternative to RAID.
• monitor I/O behavior and perform
error recovery if needed
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Sleep Mode
A safety feature available on some
computer models that parks the
drive head after a certain period of
inactivity.
Slew-Rate Control
Used to control rise/fall times of
signals.
Slider
A rigid block of material upon which
a disk head is mounted. It provides
lift, stability and mechanical strength
to the head assembly.
SMART
Self-Monitoring, Analysis and
Reporting Technology. SMART helps
protect user data from predictable
degradation in drive performance.
SMD-X
Storage Module Device (SMD-E and
SMD-H). Until SCSI’s rise to
prominence, SMD was the preferred
interface for drives used with highend server applications.
Soft Reset
The Reset condition kicks every
device off the bus. It is caused when a
SCSI device asserts the Reset control
signal. The Soft Reset option:
• tries to complete commands that
were in process
• holds on to all SCSI device
reservations
• maintains the SCSI device
operating mode
The Soft Reset option has the
advantage of allowing the initiator to
reset the bus without affecting other
initiators in a multiple-host system.
Glossary
Compare with Hard Reset.
Software-Based Disk Array
A RAID configuration that is created
and maintained through software.
Software Disk Cache
A cache that resides in the CPU’s
RAM. It is used for storing the most
frequently accessed disk
information. A software disk cache
speeds data retrieval but limits the
amount of RAM available for the
program itself. It is usually
configurable. Typically it can be
enabled/disabled and adjusted in size.
It also can be increased through the
installation of additional memory.
Spacial Re-Use
A function provided in Serial Storage
Architecture that allows data to be
transferred concurrently at high
speeds between many pairs of serially
connected peripherals, without the
need to route the data through the
processor.
Spindle
The shaft that platters are mounted
on.
Spindle Synchronization
In an array of disk drives, a feature
that ensures all spindles are spinning
in unison and at the same rate of
speed. Each platter of each drive will
be in precisely the right position
relative to the other drives. This
nearly eliminates latency, because the
head on drive B will be over the
appropriate sector of its platter just as
the head on drive A finishes reading
from or writing to the appropriate
sector on its platter.
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Sputtering
One of the techniques used to apply
the reflective metal layer to CD
media or the magnetic material to a
hard disk or floppy diskette. The
substrate disc is suspended in a
vacuum along with a mass of the
material to be deposited. This mass is
bombarded with an ion stream, which
dislodges atoms. The atoms are
attracted and cling to the substrate,
resulting in a thin, consistent
metallic coating.
SSA
See Serial Storage Architecture.
SSA IA
Serial Storage Architecture Industry
Association. It was formed in 1995 to
promote the acceptance of the SSA
standard within the industry. Its
membership includes more than 25
manufacturers of systems and
peripheral products.
Stepper Motor
An older model of head actuator that
moves the head in discrete steps. It is
a type of rotational pivot used in a
floppy drive. It operates somewhat
like a car’s transmission, converting
an incremental rotary movement into
linear travel.
ST-506
In 1983, Al Shugart’s Seagate
Technologies introduced the ST-506
as a 5.25-inch, 5 MB hard drive
implemented on the IBM XT. It
became the most popular of the lowcapacity MFM drives.
ST-412
Seagate’s successor drive to the ST506. It provided 12 MB of capacity.
Stamping
In CD production, manufacturing
data into a disc (as opposed to writing
data to a writable disc).
Status Phase
A Bus Phase in which the target sends
a byte of information to the initiator
that indicates the success or failure of
a command (or a series of linked
commands).
The Status Phase is critical for error
detection and defect management.
Step Pulse
A drive feature that moves the head
one track over, and speeds access
time by quickly positioning the head
over the correct track.
Stiction
Short for static friction. It happens
when the heads of the drive are stuck
to the platters of the drive. When you
try to power up the drive, the heads
cannot break free. Stiction is usually
caused by improper lubricants on the
drive platter.
Storage Peripherals
Devices attached to the host
computer with cables that store user
data.
Striping
See Data Striping.
Stub
A small length of a cable with
connectors. It is used for connecting
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an internal device, such as a hard
drive, to the internal SCSI port.
Subsystem
TAO
See Track-at-Once.
Tape Drive
A subset of the entire computer
system. A subsystem can include
drive arrays and controllers on add-in
boards that relieve the CPU of some
of its tasks.
Surge Suppressor
An intermediating strip of electrical
outlets that regulates the flow of
power to a system, filtering out power
surges and preventing costly damage
to system components.
Sustained Data Transfer Rate
The average data transfer rate over
time. It includes burst from the cache
(optimal) and cross-cylinder accesses
(suboptimal).
Synchronization
Causing to agree exactly in time or
rate.
Synchronization Bytes
Magnetic data that marks location
and time on a disk platter. These
bytes are added during the original
low-level formatting process of an
MFM drive.
Tagged Command Queuing
Part of the SCSI-2 specification,
Tagged Command Queuing allows
each LUN to queue up to 256 I/O
processes per initiator. This frees the
initiator from having to ask for a new
command each time it finishes an
operation.
Glossary
A type of storage, similar to an audio
tape recorder, that stores data on a
magnetic tape. Tape drives are usually
used for data back-up.
Target
A SCSI device, usually the peripheral,
that carries out the initiator’s request.
TB
See Terabyte.
T-Cal
See Thermal Recalibration.
Terabyte
A value equal to 1012 bytes. Written
out 1,000,000,000,000 bytes.
Termination
Electrical connection at each end of
the SCSI bus composed of a set of
resistors or other components. It is
used to provide closure to the bus and
reduce reflections or ringing.
Terminators for single-ended and
differential interfaces are different
and are not interchangeable.
Termination Power
The power supplied to a bus
terminator.
Thermal Recalibration
The process of recalculating the
positions of data on a hard disk
platter as those positions shift due to
the platter’s expansion under the heat
of operation.
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TIB
See Transfer Information Block.
Time-Out
SCSI devices operate under a
definable time period, beyond which
they will “time-out” and release the
bus.
Timing Mechanism
A mechanism that correlates a
platter’s constant speed with the
distance traveled, yielding a precise
calculation of the head’s position over
the platter.
TPI
Tracks per inch.
TPWR
See Termination Power.
Track
Invisible “grooves,” in the form of
concentric circles that store data on a
platter. On a hard disk, each track is a
single line of magnetic domains. On a
CD-ROM disc, each track is a line of
pits and lands.
Track Buffering
When data is requested from a
particular sector, the controller will
read the entire track and store it in its
RAM.
Track-at-Once
A method of writing to a CD where
the user can copy one track of data at
a time. Up to—but no more than—99
tracks of data can be stored on a disc,
space permitting. TAO recording can
be used with either single-session or
multisession recording. The finalized
376
disc is 100 percent backward
compatible with CD-ROM drives.
Track-to-Track Seek Time
The time it takes a drive head to
move from one track to another. This
usually falls in the 2 ms range.
Transfer Information Block
A block of information about a
pending data transfer, used to prevent
data transfer hiccups when the size of
the transfer outruns the size of the
track buffer.
Transformer
A device used to transfer electric
energy from one circuit to another.
Transistor
A digital, solid-state miniaturized
circuit used to modulate or control
the flow of an electric current.
Transparent
Activity that takes place in the
background, making all but the result
invisible to a user.
Trap
A command trigger that causes
automatic transfer of control to a
trap-handler program.
Travan
A subset of the mini-cartridge format
QIC (quarter-inch cartridge). At .315
inches, Travan is slightly wider than
standard QIC. It allows a 750 foot
tape length, which translates to larger
capacity—up to 10 GB for proposed
TR-5—and data transfer rates up to
1.2 MB/s.
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UDF
USB
See Universal Disk Format.
Ultra SCSI
See Universal Serial Bus.
Vapor Deposition
This is the same as Fast-20 SCSI. It
delivers twice the performance of
regular SCSI.
A technique for applying a chromelike covering onto high-speed, highcapacity hard drives.
See also Fast SCSI.
Variable Zone Recording
Uninterruptible Power Supply
An intermediary device connected to
a power source and a computer. When
there is a power failure, the UPS uses
a battery and power circuits to
produce the voltage required to keep
the computer running. Depending
upon the size of the battery and the
power requirements of the computer,
a UPS can power a computer
anywhere from a few minutes to
several hours.
A method of encoding that allows the
outer tracks of the platter to hold
more sectors than the inner tracks.
Similar to notch.
Vendor-Unique Commands
Commands left undefined within a
standard, that act as place-holders for
custom commands that will be
developed by individual vendors.
VESA
Video Electronic Standards
Association. See VESA Local Bus.
Universal Disk Format
An ISO 13346 standard for formatting
data on disks. This standard has been
spearheaded by the Optical Storage
Technology Association (OSTA). It
would allow disks to be read/written
on any platform, ensuring full data
interchange.
VESA Local Bus (VLB)
Video Electronic Standards
Association Local Bus. It was
designed to interface the Intel 80486
high speed processor bus to a local
high-speed expansion bus. It ran at
the processor speed and provided for
high-speed, 32-bit data transfers.
Universal Serial Bus
A 12 Mb/s interface supporting up to
63 peripheral devices. USB is
intended to support low-speed
peripherals such as keyboards and
pointing devices, and is not suitable
for mass storage or video applications.
USB supports automatic
configuration (“plug-and-play”) and
hot-plugging.
UPS
See Uninterruptible Power Supply.
Glossary
Virtual Memory
A technique whereby a computer
offloads some of the data/program
from its primary memory onto disk,
giving the impression that primary
memory can continue to accept/store
virtually unlimited amounts of data.
VLB
See VESA Local Bus.
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VLSI
Very-Large-Scale Integration. A chip
with 20- to 900-thousand logic gates
per chip. Transistors form logic gates
that control the flow of electrons
through chips.
Watermark
A reference point. In HSM ToolKit, a
watermark is a user-defined
percentage of disk capacity. When the
watermark is reached, HSM ToolKit
is triggered to migrate files to free up
some disk space.
Voice Coil Actuator
A type of actuator that moves the
read/write head in increments of
infinitely-controllable size. Uses
servo information for positioning.
Volume
Also known as a partition.
Represented by an icon on the
Desktop and used to store files and
folders of information.
Wedged Servo
See Embedded Servo.
White Book
The standard specification for Video
CD. JVC, Matsushita, Sony and
Philips coauthored this specification.
It is also known as the “Video CD
Standard.”
Wide SCSI
Volume Bit Map
A directory structure that keeps track
of allocation blocks that are in use
and that are free. It is used to
determine where to put data that
needs space and where to free up
space when a file is deleted.
Volume Information Block
A master directory block that
contains important information on
the volume, including:
• its name and type
• the number of files and folders on
the volume
• the allocation block size
• pointers to the location of files
There is a backup copy of this near
the end of the disk.
Warm Spare
A replacement drive for a RAID
system that requires the system to go
off-line but not power-down in order
to replace components.
378
A SCSI-2 option that makes possible
16- or 32-bit wide data transfers and
consequently much faster data
transfer rates. In order to handle the
extra width, an additional 29 signals
are necessary. It requires a 68-pin
cable connector.
Winchester 30/30
Considered to be the first true hard
drive technology. It was developed in
1973 by IBM. It was named after the
Winchester 30/30 rifle because the
hard drive used two 30 MB disks, one
fixed and one removable.
Winchester Drive
A fixed disk drive in which a sealed
data module houses the access arm
and the magnetic data disk. Most
modern hard drives use Winchester
technology.
Wired-or Signal
Or “Or-tied.” A control signal that
requires simultaneous control signals
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from more than one device to drive a
bus phase true or positive, preventing
electrical conflict. No devices drive
the signal to a false state. Instead, the
terminators pull the signal false
whenever it is not driven.
Word
Used to denote the number of bits
that constitute a common unit of
information in a particular computer
system.
WORM Drive
A CD-Recordable drive that writes to
and reads from compact discs.
Write-Once, Read Many (WORM)
Generic term for a type of drive
(WORM drive) that uses media that
can be written to only once, but that
can be read many times. Different
devices use different techniques for
preventing data from being erased.
Some drives use series of software
flags and special data bits that tell the
firmware of the drive not to write to
certain data blocks. Others physically
change the media so it cannot be
rewritten. Examples of WORM
drives include multifunction
magneto-optical drives, phase change
optical drives, and CD-Recordable
drives.
See also Write-Once, Read-Many.
Write-Through Cache
Write-Back Caching
Write-back caching keeps track of
what information in the cache has
been modified by the microprocessor.
This is done by marking modified
information with a “dirty bit.” When
displaced from the cache, all the
information with dirty bits is written
to primary memory. It’s likely that
the term “dirty bit” was coined to
denote a tag that marked information
that needed to be “cleaned out”
(written) to the disk or to memory.
Write Caching
Cache method by which the
controller transfers a write request to
its own cache, tells the CPU that the
task is complete, and then completes
the write at a later time. This speeds
operations because the controller
would not ordinarily tell the CPU it
had finished the command until it
actually had, and the CPU would
have to wait for notification before
going on to the next command.
Glossary
A variation of write caching, in which
data destined for the drive is written
to the disk but also stored in the
cache. If this data is needed again, it
can be read from the cache instead of
going to disk. This is often combined
with read caching to accelerate disk
performance.
XOR
“XOR” stands for “exclusive or,” a
function of Boolean logic used in
creating parity data. The XOR
function sets a parity bit to a 1 if and
only if either (but not both) of the
compared data bits is 1.
XOR Engine
A custom hardware engine that
speeds up generation of parity in
RAID 4 and 5 systems, which allows
a RAID drive to be recovered in the
event of a failure. XOR engines are
usually used on higher end RAID
subsystems. Other RAID subsystems
rely on an on-board processor to do
the XOR calculation.
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XPT
SCSI-2 CAM (Common Access
Method) Transport. It is a layer of
software that peripheral drivers and
application programs use to initiate
the execution of CAM functions.
Yellow Book
Standard specification for CD-ROM.
It allows for the presence of data
tracks on a CD. It builds on the Red
Book Standard. The Yellow Book
standard specifies that CD-ROM
must encode the first track as data. In
addition to the two layers of error
correction outlined in the Red Book,
data is further protected by a third
layer of error detection and
correction. When a disc conforms to
Yellow Book standard, it will usually
say “data storage” beneath the disc
logo.
Zip Drive
A drive with a 3.5” cartridge
containing a flexible disk that spins
within a cushion of filtered air.
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Index
Numerics
1:1 interleaving
1394 (FireWire)
2:1 interleaving
3:1 interleaving
3M 105
48, 222, 223
209, 211–212
48
49
A
access time 22, 33
command overhead 33
cylinder switch time 33
head switch time 33
latency 22, 33
seek time 33
active backplane 82, 95
actuator 26, 75, 132
double actuators 34
stepper motor 35
voice coil 36, 73
Adaptec 129, 135, 189, 200
agency standards 128
allocation block 187
American National Standards
Institute, see ANSI
AMI 229
ANSI 125, 128, 129, 193, 196, 202,
226, 235
Apple 125, 129, 137, 138, 153, 181,
212, 226, 244, 254, 255, 259,
263, 265
exceptions to SCSI 176
IDE implementation 234
SCSI implementation 173
architecture
isochronous 212
layered 180, 181, 189
loop 213
parallel 225
Index
serial 225
archive 269
arrays, see RAID
asynchronous event notification 199
ATA 126
ATAPI 238–239
autolocking 40
autoparking 40
Award 229
B
backplane
active backplane 82, 95
passive backplane 261
backup 62, 66, 270
differential backup 63
incremental backup 62
benchmark 272
Bernoulli Effect 101
Bigfoot 42
BIOS 285
bit 46, 132
bit-based striping 87
BLER 106
blind data transfer 282
block multiplexer 128
Blue Book 114
boot blocks 186
boot process
Macintosh 277
PC 285
boot sector 285
bootstrap 174
bus conditions 158–159
attention 158
hard reset 159, 327
reset 158
soft reset 159, 327
bus mastering 190
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bus phases 136, 179
and control signals 146
bus free 158
data 141
message out 158
selection 141
buses 243–262
Apple Desktop Bus 212
arbitration 169
block multiplexer 128
bus clock 244
bus mastering 244
bus reset 140, 142
CardBus 262
EISA 248
FIFO 246
hard reset 142
ISA 227, 246–247
MicroChannel 225, 247
Move16 246
NuBus 243–246, 254, 257
OEM channel 128, 132
PCI 217, 236, 237, 249–261
PCI Mezzanine 261
PCI on Macintosh 253
S-Bus 217
SCSI bus 132, 133, 141, 143, 150,
169
termination 153
VESA 237
VESA Local Bus (VLB) 248–249
byte 132
byte-based striping 87
C
cables 147–158, 216, 224
ACK 147
cable extender 148, 150
chip requirements 152
daisy chain 147, 155
differential 152
Enhanced IDE 237
fibre optic 218
GND 147
HDI-30 177
382
IDE 226
impedance 148, 155, 157
improper termination 155
length limitations 152
parity 147
peripheral interface cable 148, 149
recommendations 147
REQ 147
ribbon cables 147
SCSI disk adapter 177
SCSI system cable 148
SSA 219
stubs 150
cache 52–58
cache algorithm 56
CPU cache 53, 54
data transfer rates 53
hard drive cache 53, 55
hardware caching controller 58
level 2 cache 54
prefetch 56
RAM 54
RAM drives 58
read caching 56
read-ahead buffer 56
software disk cache 53, 54
track buffer 56
write caching 57
write-back cache 248
write-through cache 57
CAM 180, 189, 200–201, 226
capacitor 176
catalog tree 187
CCS 134
CD formats
CD Extra 114
CD-Bridge 114
CD-DA 113
CD-G 114
CD-i 113
CD-R 114
CD-ROM 113
CD-ROM XA 113
CD mastering 110
CD recording
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disc-at-once 114
incremental 115
Kodak multisession 117
multisession 116
multivolume/multisession 117
packet 116
session 115, 117
track 115, 117
track-at-once 115
CD standards
Blue Book 114
Green Book 113
Orange Book, Part II 110, 114
Orange Book, Part III 119
Red Book 113
White Book 114
Yellow Book 113
CDB, see command descriptor block
CDRFS format 108
CD-UDF format 108
central processing unit, see CPU
Centronics 153
chips 24
ASIC 250
BANDIT 254
CMOS 136, 220, 223, 285
Curio 138
DMA 137
ISP 138
MESH 138
mode 138
mode, DMA 138, 139
mode, normal 138, 139
mode, pseudo-DMA 138, 139
MUNI chip 245
nonvolatile flash memory 58
Pentium 252
RISC 137, 138
SCSI 138
SCSI chip 132, 135, 173, 176, 181,
184, 199, 200
Symbios Logic 136, 137
VLSI 226
CMOS 136
command descriptor block 159, 199,
Index
222, 295
Group 0 160, 295
Group 1 160, 296
Group 2 160
Group 5 297
pointers 160
command overhead 33
commands
ATA-3 commands 328
ATAPI commands 327
CD-ROM 312
command linking 198
communications device 313
Enhanced IDE commands 327
IDE commands 322
PCI commands 329
SCSI-2 commands 195–311
ST-506 commands 317
tagged command queuing 191, 198
tape device 313
vendor-unique 193, 311
common access method, see CAM
common command set 134, 196
Compaq 105, 191, 225
compression software 266
connectors 75, 130, 148, 152–153, 156,
198, 199, 219, 259, 260
Centronics 153
HDI-30 177
SCA 214
SCSI 152
Conner 95, 175, 225, 227
contacts
ANSI 290
PCI 291
SCSI Trade Assn. 290
contingent allegiance 199
controller 24, 29, 48, 74, 184, 223
command overhead 33
controller damage 61
DMA controller 137
hard drive cache 53, 55
hardware caching controller 58
see also chips
Copland 181
383
GtoS 2nd Ed. Book Page 384 Friday, March 27, 1998 12:05 PM
CPU 21, 48, 133, 184
and transfer rates 51
cache 53, 54
clock speed 137
MIPS 138
overhead 136, 137
CRC 241
cyclic redundancy check 241
cylinder switch time 33
D
data blocks 43
data density 45, 50
and operational speed 50
tracks per inch 50
data detection 47
PRML 47
data mapping 185
driver descriptor map 185
partition map 185
data reading, see reading
data recovery 67
data security
see also RAID
backup 62
disk duplexing 63
disk mirroring 64
recovery 67
data striping 84
data transfer 139
asynchronous 140, 140, 142, 309
block mode data transfer 244
bus reset 140
DMA 228, 236
Enhanced IDE 237
handshake 139
IDE 227
IDE limitations 229
NuBus 244
PIO 236
synchronous 140, 141–142
data transfer rates 50–51, 53, 141, 188
ATA-3 240
bandwidth 217, 220, 253
cables and 148
384
cache 53
data density 50
ESDI 224
Fibre Channel 216
HPPI 242
Intel chipsets 252
IPI 242
Macintosh 245
PC 192
PCI and NuBus 258
PowerPC to PCI 255
read channel 50
ST-506 222, 223
sustained 72
data writing, see writing
de facto standards 127, 134, 196
defragmentation 68
device drivers 59, 131, 181–183, 184,
224
ADD 189
HAM 189
input 183
LADDR 189
output 183
requests 182
device manager 184
differential 152, 216, 222
DIP switches 29
disk arrays, see RAID
disk capacity 40
disk duplexing 63
disk mirroring 64, 86
DMA 137, 138, 139, 179
pseudo-DMA 139
virtual DMA services 190
Dolby 120
DOS 190, 231
drive controller 29
drive head 23, 33
double actuators 34
flight height 31
giant magnetoresistance 32
head crashes 61
head disk assembly 31
inductive head 30, 32
FWB’s Guide to Storage
GtoS 2nd Ed. Book Page 385 Friday, March 27, 1998 12:05 PM
magneto-resistive head 32
slider 31
drives
ablative WORM 118
all digital drives 76
Bernoulli drives 72, 97, 101
CD-recordable drives 97, 109
CD-rewritable drives 97, 118
CD-ROM drives 97, 106
DVD drives 97, 120
floptical drives 97, 105
green drives 76
hard drives, see hard drives
hot swappable drives 81
Jaz drives 101
jukeboxes 97, 119, 131
magneto-optical drives 85, 97, 102
phase change optical drives 97, 104
RAM drives 58
removable cartridge drives 97, 99
removable drives 94, 97, 98
sequential access drives 266
tape drives 97, 122, 266
Winchester cartridge drives 85
WORM drives 102, 118
Zip drives 97, 101
drum storage 25
DTC 129
dust control 66
E
ECC 47, 106
edge connector 260
emergency boot disk 66
encoding 26, 44
ECC 47
frequency modulation 45
modified frequency modulation 46,
223
process 45–46
run length limited 46
variable zone recording 45
enhanced drive parameter table 236
EPRML 47
error correction code, see ECC
Index
Eurocard connector 259
expansion cards 179
disk controller 58
PCMCIA 42, 58, 100
see also host adapters
extents tree 187
F
fault tolerance 89
FCA 221
FCEL 221
fiber distributed data interface 126
Fibre Channel 209, 213–218, 220, 253
arbitrated loop 213
bandwidth 217
cable 216, 218
data transfer rates 216
direct disk attach 217
features 214
optical transmission 218
remote 217
Fibre Channel Association 213, 221
Fibre Channel Enhanced Loop 221
Fibre Channel Specification
Initiative 213
FIFO 246
file manager 184
file servers 217
file system formats
CDRFS 108
CD-UDF 108
Generic 107
HFS 107
Hybrid 107
ISO 9660 107
Rock-Ridge 107
UDF 119, 121
FireWire, see USB
floppy disks 25
FM encoding 45
form factor 40–43
full-height 40
half-height 40
one-third-height 40
formatter software 263
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GtoS 2nd Ed. Book Page 386 Friday, March 27, 1998 12:05 PM
formatting 43
1:1 interleaving 48
2:1 interleaving 48
3:1 interleaving 49
bad sectors 227
clusters 233
data blocks 43
grown defects list 44
high level formatting 190
logical blocks 43
low-level formatting 48
partitions 232
sectors 43, 223
spares 43
tracks 43
fragmentation 68
frequency modulation 45
Fujitsu 103
full-height drive 40
Future Domain 189
FWB 265
G
Generic format 107
giant magnetoresistance 32
GMR 32
Green Book 113
grown defects list 44
H
half-height drive 40
handshake 139
IDE 227
request/acknowledge 141
hard disk platters 38–43
hard drives 21–78
access times 22
advantages 21
all digital drives 76
armature 25
AV drives 58
bad sectors 227
capacity 72
controller 24
drive head 23, 33
386
drum storage 25
external components 30
form factor 40–43
formatting 43
full-height 40
green drives 76
half-height 40
hard drive cache 53, 55
hardware errors 60
head disk assembly 31
history 25–26
housing unit 40
IDE 177
indicators of reliability 69–73
internal components 28
limiting factors 37
mechanics 27–37
on/off policy 66
one-third-height 40
operational overview 23
platters 23, 33, 38–43, 73
positioning 67
power management 229
problems 280
prolonging the life of 66
sectors 43
software errors 60
temperature control 67
tracks 24, 43
trends 73–78
hard reset 159, 327
hard seek 47
hardware errors 60
controller damage 61
head crashes 61
media errors 61
spindle failure 62
stiction 62
temperature problems 62
hardware-based disk arrays 94, 95
head disk assembly 31
head parking 39
autolocking 40
autoparking 40
landing zone 40
FWB’s Guide to Storage
GtoS 2nd Ed. Book Page 387 Friday, March 27, 1998 12:05 PM
head switch time 33
Hewlett-Packard 213
HFS format 107
HFS partition 186
boot blocks 186
catalog tree 187
extents tree 187
volume bit map 187
volume information block 187
Hierarchical Storage Management, see
HSM
holographic memory 77
host adapters 157, 180, 189, 191, 192,
199, 200, 217, 225, 226
hot plugging 191, 214, 218
housing unit 40
Hybrid format 107
I
IBM 26, 32, 128, 134, 153, 189, 201, 212,
213, 218, 221, 222, 223, 225, 241,
246
IDE 129
IEEE 128
Imprimis 225
inductive head 30, 32
information transfer phases, see bus
phases
integrated circuits, see chips, see also
controllers
Intel 132, 137, 191, 237, 249, 258
interfaces
1394 (FireWire) 209, 211–212
ASPI 189, 191, 200
AT 225
ATA 201, 225, 227, 238, 262
ATA-2 235
ATA-3 240
ATAPI 238–239
BIOS INT13 285
CAM 189, 200, 226, 227
dumb 222
EATA 201
Enhanced IDE 235–237
ESCON 213
Index
ESDI 224, 225, 285
Fast ATA 235
Fast ATA-2 238
Fibre Channel 209, 213–218
FIPS-60 241
high-level interface 132
HPPI 242
I/O 193
IDE 129, 177, 189, 190, 225–235,
238, 262, 285
INT13 BIOS 190
IPI 129, 241
ISA 191
low-level interface 132, 212
parallel 225
peer-to-peer 129, 131
SASI 129
SCSI, see SCSI
serial 209, 210, 212, 218, 220, 222
SMD-E 241
SMD-H 241
SSA 209, 218–221
ST-412 222
ST-506 222, 225, 235, 285
USB 209, 211
interleaving 48–49, 86
1:1 interleaving 48, 222, 223
2:1 interleaving 48
3:1 interleaving 49
low-level formatting 48
sector skew 50
interrupts 175, 231
ISO 9660 format 107
isochronous 212
J
JTS Corporation 41
jumper pins 29
jumpers 189, 191, 227
JVC 212
K
Kerr Effect 103
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L
landing zone 40
latency 33, 57
Lempel-Ziv 124
LIMM-DOW 103
logical block 43
logical block address 236
logical unit number, see LUN
low-level formatting 48
LUN 131
M
magnetic domain 26, 30, 46
magnetic flux 46, 73, 223
magnetic recording 38
magneto-resistive head 32
magnets 38
master 227
Maxtor 224
mean time between failures, see MTBF
mean time to repair 71
media
Bernoulli 101
CD-R 111
CD-ROM 106
data density 26
dual layer 120
DVD 120
floptical 105
magneto-optical 103
phase change optical 104
removable cartridges 100
tape 122
WORM 118
memory
cache 52–58
CMOS 223, 285
declaration ROM 245
holographic memory 77
nonvolatile memory 21
parameter RAM 277
PCMCIA 42, 237
RAM 277, 285
ROM 174, 181, 185, 223, 234, 244
virtual memory 55
388
volatile RAM 21
messages
bus reset 159
MFM encoding 46
Microsoft 55, 189, 190, 191, 212, 233,
273
miniport driver 191
mode pages 48, 164
CD-ROM 239, 316
ECC 47
SCSI-2 314–316
tape device 239, 316
modified frequency modulation 46, 223
motherboard 200, 244, 277, 285
Motorola 137, 235
MPEG 121
MTBF 69–71
multiple-host system 127, 169
N
NCR 129
Nikon 103
Novell 189, 237
O
OEM channel 128, 132
one-third-height drive 40
open boot 255
open firmware 255
Orange Book, Part II 114
Orange Book, Part III 119
P
Panasonic 212
parallel disk array 87
parallel I/O 132
parity 80, 89, 143, 147, 175
parity disks 86, 87
partition map 185, 186, 277
partitions 232
passive backplane 261
PCI Mezzanine card 261
PCI SIG 249
PCI, see buses
PCMCIA 42, 58, 100, 237, 261–262
FWB’s Guide to Storage
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Philips 119
Phoenix 229, 236
PIO 228
platters 33
cobalt-nickel alloys 38
ferric oxide 38
glass 38
servo positioning platter 34
spindle 39
sputtering 38
vapor deposition 38
plug-and-play 191
PMC 261
pointers 160
active 160
current 160
saved 160
port bypass circuit 214
POST 159
Power Computing 256
power management 229
power on self test, see POST
power supply 81, 157
predictive failure analysis 65, 206
prefetch 56
PRML 47, 123
programmed in/out 228
punch cards 25
Q
Qlogic 135, 138
Quantum 42, 235, 238, 241
R
RAID 64, 79–96, 191
active backplane 82
applications 85, 86, 87, 88, 89
AutoRAID 94
bit-based striping 87
byte-based striping 87
caching 80
check disk 64
configurable rebuild 82
controller board 94
data striping 84
Index
disk failure 93
drive channels 82
example, level 5 89–92
fault tolerance 80, 89, 93
features 80–82
hardware based 94, 95
host adapter 95
hot spare 80
hot swappable backplane 95
hot swappable drives 81
hot swappable power supplies 81
key aspects 80
layered RAID 94
level 0 84
level 0 + 1 94
level 1 86
level 10 94
level 2 86
level 3 87
level 4 88
level 5 88–94
level 53 94
level 6 94
monitoring software 81
multiple hosts 80
overview of levels 82
parity 80, 89
parity disks 86
RAID on RAID 94
RAID software 264
ranks 82
redundant controllers 81
redundant fans 81
removable arrays 94
SCSI 79
serial SCSI 79
setup software 81
software based 95
spindle synchronization 82
tape arrays 94
warm spare 80
XOR engine 82
RAID Advisory Board 96
RAM 21
read caching 56
389
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read channel 50
read operation 31
read/write channel 37
reading 37, 47
floptical 105
read channel 50
scatter/gather 236
sector skew 49
Red Book 113
redundant array of independent disks, see
RAID
registers
ATAPI 328
IDE 321
RLL encoding 46, 223, 224
0,4,4 RLL 46, 47
2,7 RLL 46
3,9 RLL 46
ARLL 46
Rock-Ridge format 107
run length limited, see RLL encoding
S
SAF-TE 95
SASI, see interfaces
SCAM 191
scatter/gather 236
SCSI 125–221, 226
active high signals 152
active low signals 152
address, see ID
advanced technology
attachment 126
and PowerBooks 177
ANSI 125
Apple 125
Apple exceptions 176
Apple’s ANSI irregularities 175
Apple-style 173
asynchronous data transfer 140, 140,
142
backward compatibility 179, 193,
202
bus 127, 133
bus phases 136, 141, 179
390
bus reset 140, 142
bus termination 155
cable extender 150
cable limitations 133, 216
cables 148
CAM 200–201
commands 132
connectors 148, 152
contingent allegiance 199
control signals 143
daisy chain 130, 147
data signals 143
data transfer 139
device independence 131, 194
differential 133, 152, 216
disconnect capability 133
disk mode 177
dual-channel 153
Fast and Wide 253
Fast SCSI 141, 198, 237
Fast/Wide 150, 192
Fast-20 192
FDDI 126
fiber channel 126
handshake 139, 141, 175
high-level interface 132
HIPPI 126
host 127
host adapter 135, 189
initiator 127, 131, 141, 146, 158,
164, 170, 183, 199
IPI 126
low-level interface 132
LUN 131
mode pages 164
narrow 156
negotiation 142
original implementations 188
parameter blocks 180–181
parity 143
PC implementation 189–191
peer-to-peer 131
peripheral interface cable 149
protocol 131
ROM 181
FWB’s Guide to Storage
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SCAM 191
SCSI cables 147–158
SCSI chip 135, 173, 176, 181, 184,
200
SCSI filter 176
SCSI ID 130, 146–147, 191
SCSI Manager 131, 135, 173, 178,
180, 181, 184, 200, 259
SCSI-1 127, 133
SCSI-2 134, 137, 141, 144, 147, 150,
157, 160, 165, 169, 179, 192,
193–201
SCSI-2 commands 195–311
SCSI-2 mode pages 314–316
SCSI-3 147, 199, 202–208
serial SCSI 209–221
serial storage architecture 126
signal lines 143–146, 294–295
single-ended 133, 152, 196, 216
synchronous data transfer 140, 141–
142
system cable 148
target 127, 131, 141, 146, 158, 160,
164, 170, 183, 199, 212
termination 153–158, 176
Ultra SCSI 141, 262
variations 192
vendor-unique 196, 198
vendor-unique codes 193
Wide SCSI 156, 199
SCSI bus phases 169–173
and control signals 308
arbitration 170, 171
bus free 170, 171
command 170, 172
data-in/out 170, 172
message-in/out 170, 173
reselection 170, 171
selection 170, 171
status 170, 172
SCSI call chain 183–184
SCSI commands 159–303
command descriptor block 159, 295
mode select 164
mode sense 164
Index
SCSI Manager commands 179
sense data 161, 302
sense keys 161, 301
SCSI interface module, see SIM
SCSI interrupts 175
SCSI Messages
message reject 142
SCSI messages 164–168, 304–308
abort 166, 306
abort tag 167, 307
bus device reset 168, 308
clear queue 167, 307
command complete 165, 304
disconnect 166, 305
extended messages 165, 305
head of queue tag 167, 307
identify 168, 308
ignore wide residue 168, 307
initiator detected error 166, 305
linked command complete 167, 306
linked command complete (with
flag) 167, 306
message formats 164
message parity error 166, 306
message reject 166, 306
message-in 164
message-out 164
no operation 166, 306
ordered queue tag 167, 307
restore pointers 166, 305
save data pointer 166, 305
simple queue tag 167, 307
terminate I/O process 167, 307
SCSI problems, Apple
blind data transfers 175
fast reset 173
parity checking 175
unit attention condition 173
Seagate 34, 221, 222, 223, 235, 238
sector skew 49–50
sectors 43, 223, 227
seek time 33, 57, 72
self-monitoring, analysis and reporting
technology, see SMART
sequential access drives 266
391
GtoS 2nd Ed. Book Page 392 Friday, March 27, 1998 12:05 PM
serial port 130
Serial Storage Architecture, see SSA
servo 36–37, 73, 132
double embedded servo 37
embedded servo 37, 58
hybrid servo system 37
positioning information 37
servo positioning platter 34
voice coil actuator 36, 73
wedged servo 37
shock ratings 71
Shugart Associates 129
signals
ACK 157, 158
active high 152
active low 152
and bus phases 146
arbitration 144
assertion 144
attention 142
command complete 144
control signals 143, 170
data signals 143
IDE 319
negation 144
reflection 158
REQ 157, 158
SEL 158
signal lines 143–146, 294–295
ST-506 signals 318
SIM 200, 201
single connector attachment 75, 214
single-ended 152, 196, 216
slave 227, 238
SLED 79
slider 31
Small Computer System Interface, see
SCSI
SMART 65, 206, 240
soft reset 159, 327
software
AppleRAID 265
ATA Manager 234
backup 266
benchmark 272
392
card and socket 262
CD-R mastering 272
CD-ROM 265
CD-ROM ToolKit 265
compression 266
CoreTest 273
defragmentation 68
DiskPerf 273
Drive Rocket 273
Drive Setup 263
ECC 47
formatter 263
Hard Disk ToolKit 263
Heapfixer 283
HSM 267–272
RAID 81, 264
RAID ToolKit 265
SCANDISK 61
Slot Manager 244
SMARTdrive 55
software errors 60
SpinRite 49
utility software 66
VidTest 273
Ziff-Davis Benchmark 273
software disk cache 54
software-based disk arrays 95
Sony 119, 212
spares 43
spindle 39
spindle failure 62
synchronization 89
sputtering 38
SSA 126, 209, 218–221, 253
bandwidth 220
standards
agency 128, 224
ANSI 193, 196, 202, 224, 226, 235
ASNI 128
ASPI 200
de facto 127, 134, 196, 224
IEEE 128, 212, 223, 243
open 128
stepper motor 35
stiction 62
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storage
near-line 269
off-line 269
on-line 269
Sun Microsystems 138, 213, 255
SuperMac 256
surge suppressor 66
Symbios Logic 135, 136, 137
system heap 279, 283
T
tape drives 122
compression 122
DAT 122
digital audio tape 122
digital data storage 122
digital linear tape (DLT) 124
disadvantages 25
QIC 122
sequential access 25, 266
Travan 122
t-cal, see thermal recalibration
temperature problems 62
termination 153–158, 176, 198
active negation 158
active termination 157
examples 157
external 156
forced perfect termination 158
internal 156
termination power 156, 158
TPWR 176
Texas Instruments 212
thermal recalibration 58
Thinking Machines, Inc. 86
TPI 31, 50
TPWR 156
track buffer 49, 56
tracks 24, 43, 56
compact disc 115, 117
track buffer 49
tracks per inch 31, 50
transfer information blocks 175
transfer rates, see data transfer rates
troubleshooting
Index
drive not seen 275
drives 280
formatting 274
happy Mac 278
PC 286
removable media 284
sad Mac 279
spinup 276
system bomb 279
U
UDF format 119, 121
Ultra-ATA 241
uninterruptible power supply, see UPS
UNIX 217, 237
UPS 57, 66
USB 209, 211
V
vapor deposition 38
variable zone recording 45, 319
volume bit map 187
volume information block 187
volume limits
allocation block size 187
Macintosh 187
W
Western Digital 129, 223, 225, 235
White Book 114
Win32 309
Winchester 26
Windows 190, 217, 233, 237, 238, 240,
257, 266, 273
write caching 57
write-through cache 57
writing 32, 37
ablative WORM 118
CD-R 112
CD-ROM 106
data mapping 185
encoding 44
floptical 105
interleaving 48
magneto-optical 103
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phase change optical 104
scatter/gather 236
sector skew 49
variable zone recording 45
X
Xebec 129
XOR 82
XPT 180, 200
Xyratex 218
Y
Yellow Book 113
Z
Ziff-Davis 273
zones 45
394
FWB’s Guide to Storage
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