1 Basic Computing Concepts (5) Storage Devices

1 Basic Computing Concepts (5) Storage Devices
1 Basic Computing Concepts (5) Storage Devices
Secondary Storage
Secondary Storage refers to all media that retains digital data or
programs. It is therefore non-volatile. This media’s storage has to be
loaded in the RAM to be executed and worked-upon by the processor. In
contemporary usage ‘memory’ usually refers to RAM and ‘storage’ to
secondary storage.
5.1 Storage Access Methods
How data files are stored in secondary
storage varies with the types of media and
devices you are using. Data files may be
stored on:
sequential-access storage,
direct-access (random-access) storage
5.1.1 Sequential-Access Storage
Tape is an example of sequential-access storage media. When
operating in a sequential environment, a particular record can be read only
by first reading all the records that come before it in the file. When you
store a file on tape, the 125th record cannot be read until the 124 records
in front of it are read. The records are read in sequence. You cannot read
just any record at random. This is also true when reading punched cards or
paper tape.
5.1.2 Direct-Access Storage
Direct-access (random-access) storage allows you to access the 125th
record without first having to read the 124 records in front of it. Magnetic
disks are examples of direct-access storage media. Data can be obtained
quickly from anywhere on the media.
5.2 Fundamental Storage Technologies
The most commonly used data storage technologies are
semiconductor, magnetic, and optical, while paper still sees some limited
usage. Some other fundamental storage technologies have also been used in
the past or are proposed for development.
Semiconductor (a semiconductor is somewhere between a conductor
and an insulator of electricity)
o Flash memory
o Magnetic disk
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 Floppy disk
 Hard disk
o Magnetic tape
o CD, CD-ROM, DVD, BD-ROM (Read only storage, off-line storage)
o CD-R, DVD-R, DVD+R, BD-R (Write once storage, off-line
o CD-RW, DVD-RW, DVD+RW, DVD-RAM, BD-RE (Slow write, fast
read storage, off-line storage)
o Ultra Density Optical or UDO (similar in capacity to BD-R or BDRE, slow write, fast read, off-line storage)
Magneto-optical disc storage
3D optical data storage
o Paper tape
o Punched cards
o Barcode
Others (uncommon)
5.3 Flash Memory
Flash memory is a very popular nonvolatile, rewritable memory chip used for
storage. Extremely durable, flash memory is
used in myriad portable devices, including
digital cameras, digital music players,
smartphones and tablet computers. USB drives
(pen drives) are flash memory chips, and the
solid state drives (SSDs), which are
increasingly replacing hard disks in laptops,
are also flash memory chips.
Years ago, flash
replaced the permanent read-only-memory (ROM) chip
on a PC motherboard that held the BIOS. Instead of
having to open the case and physically replace the
ROM BIOS chip, updated BIOS code could be
downloaded into the flash chip
Evolving from the EEPROM, flash was invented
by Toshiba and named after its ability to erase a block
of a data "in a flash."
5.4 Floppy Disk
The floppy disk was introduced by IBM in 1971. It was officially called
a "diskette," but nicknamed "floppy". Until the early 1990s, the floppy was
the primary method for distributing applications, for backup and for
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transferring data between machines. By the mid1990s, the floppy gave way to the CD-ROM for
software distribution, while local networks and the
Internet became popular for backup and data
exchange. The floppies in the drive used to rotate
at about 300 RPM. The following table summarises
the floppy types and capacities.
Floppy Types (from recent to oldest)
Storage Capacity
Highest Lowest
3.5” rigid
1.44 MB 400 KB Sony
5.25” flexible 1.2 MB
100 KB Shugart
8” flexible
1.2 MB
100 KB IBM
5.5 Hard Disk
A hard-disk is a magnetic disk (or disks) as shown in the diagram. The
term ‘hard’ is used to distinguish it from a soft, or ‘floppy, disk’. A hard
disk can hold gigabytes of data. A single hard disk usually consists of
several platters. Each platter requires two read/write heads, one for each
side. All the read/write heads are attached to a single access arm so that
they cannot move independently. Each platter has the same number of
tracks, and a track location that cuts across all platters is called a cylinder.
For example, a typical 84 megabyte hard disk for a PC might have two
platters (four sides) and 1,053 cylinders (sector, see diagram).
The primary computer storage medium, which is made of one or more
aluminum or glass platters, coated with a ferromagnetic material. Most hard
disks are "fixed disks," which have platters that reside permanently in the
drive. Almost all computers have an internal hard disk, and external units
can be plugged in for additional storage or backup.
The other type of hard disk is a "removable disk" encased in a
cartridge, allowing data to be ejected from the drive for external storage or
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transfer to another party. Before high-speed Internet connections,
removable SyQuest, Jaz and Zip cartridges were often used.
Hard disks provide fast retrieval, because they rotate constantly at
high speed from 4,000 to 15,000 RPM. Either to preserve battery life in
laptops or to promote longevity, hard disks can be configured to turn off
after a defined period of inactivity.
Hard disk access
times range from 3 ms
to about 15 ms, whereas
CDs and DVDs range
from 80 ms to 120 ms.
The platters in a
drive are separated by
disk spacers and are
clamped to a rotating
spindle that turns all
the platters in unison.
The spindle motor is
built right into the
spindle or mounted
directly below it and
spins the platters at a
constant set rate.
The space between the platter and the head is so minute that even one
dust particle or a fingerprint could disable the spin.
All the heads are attached to a single head actuator, or actuator arm,
that moves the heads around the platters.
The time taken to access a given byte of information stored on a hard
disk is typically a few thousandths of a second, or milliseconds. By
contrast, the time taken to access a given byte of information stored in
random access memory is measured in billionths of a second, or
nanoseconds. Rotating optical storage devices, such as CD and DVD drives,
have even longer access times.
5.6 Magnetic Tape
The magnetic tape is a sequential storage
medium used for data collection, backup and
archiving. Like videotape, computer tape is made of
flexible plastic with one side coated with a
ferromagnetic material. Tapes were originally open
reels, but were superseded by cartridges and
cassettes of many sizes and shapes.
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Tape has been more economical than disks
for archival data, but that is changing as disk
capacities have increased enormously. If tapes
are stored for the duration, they must be
periodically recopied or the tightly coiled
magnetic surfaces may contaminate each other.
Although most tapes are used for archiving
rather than routine updating, some drives allow
rewriting in place if the byte count does not
change. Otherwise, updating requires copying
files from the original tape to a blank tape
(scratch tape) and adding the new data in
Legacy open reel tapes used nine
linear tracks (8 bits plus parity), while
modern cartridges use 128 or more tracks.
Data are recorded in blocks of contiguous
bytes, separated by a space called an
"interrecord gap" or "interblock gap." Tape
drive speed is measured in inches per
second (ips). Over the years, storage
density has increased from 200 to 38,000
5.7 The CD as a Medium of Data Storage
CD ROM (Compact Disc Read Only Memory) is similar to an audio
CD and works in much the same way, but stores data instead of music. A
typical data CD can store up to 700Mb, 486 times more information than a
standard 1.44Mb 3.5" floppy disk.
A CD Writer (CD burner) can save data
or audio to a special type of recordable CD
(CD-R or CD-RW).
You will often see CD Writers advertised
as 48X-12X-50X, this means it will burn
(write) at 48X, ReWrite (write again over an
existing rewritable CD) at 12X, and read at
50X the normal CD speed. For instance, a 48X
CD Writer will write a CD 48 times faster than
a 1X CD. 1X CD-ROM takes about 70 minutes
to be read completely.
CD-R (Compact Disc-Recordable) refers to a recordable CD ideal for
backing up data.
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CD-RW (Compact Disc Re-Writable) refers to a recordable CD which
allows the data to be overwritten numerous times (handy for daily backups).
The CD Writer (also Reader) has been superseded in technology by the
DVD/CD Writer/Reader.
5.8 The DVD as a Medium of Data Storage
The different variations on the term DVD (e.g. +R, -R, -ROM, and so
on) describe the way data is stored on or written to the disc itself. These are
called physical formats.
DVD+R and DVD+RW formats are
supported by Philips, Sony, Hewlett-Packard,
Dell, Ricoh, Yamaha and others. DVD+R is a
recordable DVD format similar to CD-R. A
DVD+R can record data only once and then
the data becomes permanent on the disc. The
disc cannot be recorded onto a second time.
DVD+RW is a re-recordable format similar to
CD-RW. The data on a DVD+RW disc can be
erased and recorded over numerous times
without damaging the medium.
DVDs that have been made using a +R/+RW device can be read by
most commercial DVD-ROM players.
The DVD-R, DVD-RW and DVD-RAM formats are supported by
Panasonic, Toshiba, Apple Computer, Hitachi, NEC, Pioneer, Samsung and
Sharp. These formats are also supported by the DVD Forum (The DVD
Forum is an international organization composed of hardware, software,
media and content companies that use and
develop the DVD and formerly HD DVD
formats. It was initially known as the DVD
Consortium when it was founded in 1995).
DVD-R is a recordable DVD format
similar to CD-R and DVD+R. A DVD-R can
record data only once and then the data
becomes permanent on the disc. The disc
cannot be recorded onto a second time.
DVD-RW is a re-recordable format
similar to CD-RW or DVD+RW. The data on a DVD-RW disc can be erased
and recorded over numerous times without damaging the medium. DVDs
created by a -R/-RW device can be read by most commercial DVD-ROM
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DVD-RAM discs can be recorded and erased repeatedly but are
compatible only with devices manufactured by the companies that support
the DVD-RAM format. DVD-RAM discs are typically housed in cartridges.
DVD-ROM was the first DVD standard to hit the market and is a read only format. The video or game content is burned onto the DVD once and
the DVD will run on any DVD-ROM-equipped device. DVD-ROMs are similar
to CDs.
The DL in DVD+R DL and DVD-R DL stands for Dual Layer. This
technology is supported by a range of manufacturers.
As the name suggests, dual layer technology provides
two individual recordable layers on a single-sided
DVD disc. Dual Layer is more commonly called
Double Layer in the consumer market, and can be
seen written as DVD+R DL or DVD-R DL (also called
DVD+R9 and DVD-R9).
The dual layered discs (DVD+R9 and DVD-R9)
can hold 7.95GB and double sided dual layer (called DVD-18) can hold
A Note on DVD Burners: Until 2003
consumers would have to choose a
preferred DVD format and purchase the
DVD media that was compatible with the
specific DVD burner. In 2003 Sony
introduced a multi-format DVD burner
(also called a combo drive or DVD-Multi)
and today many manufacturers offer
multi-format DVD burners that are
compatible with multiple DVD formats.
There are other DVD formats such as DVD-VCD, DVD-SVCD and DVDMP3.
5.9 Magneto-Optic Disk
This is a rewritable optical disk that is
used in combination with magnetic technology.
Employed in a variety of storage and archival
applications, including large disk libraries,
magneto-optic (MO) disks are housed in
removable cartridges that have a 30-year shelf
life. Capable of up to a million rewrites, MO
access times are in the sub-25ms range and are
faster than pure optical CD-RWs and DVDRAMs.
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MO discs come in 3.5" and 5.25" cartridges. The latter are double
sided, but must be removed and flipped over. Capacities of 3.5" cartridges
vary but can hold up to 1.3GB. 5.25" disks can hold up to 9.1GB.
Data are written on an MO disk by a laser and a magnet. Reading is
accomplished by means of reflection of light on bits.
5.10 UDO
An UDO (Ultra Density Optical) disk is a 5.25" rewritable optical disc
technology from Plasmon, Broomfield, CO. It also supports write-once
(WORM) media. UDO disks can support up to
Although conceptually similar to the
rewritable CD/DVD drives used by consumers,
UDO was designed with more hardy engineering
for commercial applications and long-term
storage. For example, rewritable UDO uses
eight layers in the disk compared to four, which
is typical for rewritable consumer media.
Designed to supersede magneto-optic (MO)
disks in capacity, UDO's first blue laser 30GB
drives offered a big storage jump over 9.1GB, the final format for 5.25" MO
drives. UDO's phase change technology provides a stable recording layer
that is expected to last 50+ years compared to 25 for MO.
5.11 Blu-ray
A Blue-ray disk (BD) is a highcapacity optical disc for high-definition
(HD) movies. Developed primarily by Sony,
Blu-ray and DVD discs have the same
diameter and look similar, but Blu-ray's
blue-violet laser reads pits a third the size
of a DVD on tracks packed much tighter
together. Blu-ray players support DVDs
and CDs.
A BD comes in ROM (read-only
memory), R (recordable) and RE (rewritable) formats. Blu-ray disks can hold
up to 50GB.
5.12 Comparison Tables
Below are two tables that compare the main secondary storage media.
The tables may not show the latest technologies but are still valid in
comparing technologies.
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Drive Cost
Media Cost
Very low
Very low
Very High
Very High
Very Fast
Very High
USB Flash
Drive Cost
Media Cost
1.44 MB
0.04 Mbps
10p per disk
100 Mbps
10 Mbps
30 Mbps
30 Mbps
1 Mbps
Hard Disk
Hard Disk
USB Flash
Approx 6,000 FD's
Approx 2,000 FD's
700 MB Approx
500 FD's
9 GB Approx 6,300
256 MB Approx
200 FD's
CD -R Very Low
CD -RW Low
CD -R 13p
CD -RW 40p
DVD R £1.50
DVD RW £2.50
(Acronyms: MB = Megabytes, GB = Gigabytes, Mbps= Megabits per second, R
= Recordable, RW = Rewriteable, FD = Floppy disks, FDD = Floppy disk
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