USER`S MANUAL

ADDRESS SPACES S3F80N8_UM_REV1.10

USING THE REGISTER POINTS

Register pointers RP0 and RP1, mapped to addresses D6H and D7H, are used to select two movable 8-byte working register slices in the register file. After a reset, they point to the working register common area: RP0 points to addresses C0H–C7H, and RP1 points to addresses C8H–CFH.

To change a register pointer value, you load a new value to RP0 and/or RP1 using an SRP or LD instruction.

(see Figures 2-5 and 2-6).

With working register addressing, you can only access those two 8-bit slices of the register file that are currently pointed to by RP0 and RP1.

The selected 16-byte working register block usually consists of two contiguous 8-byte slices. As a general programming guideline, it is recommended that RP0 point to the "lower" slice and RP1 point to the "upper" slice

(see Figure 2-5). In some cases, it may be necessary to define working register areas in different (noncontiguous) areas of the register file. In Figure 2-6, RP0 points to the "upper" slice and RP1 to the "lower" slice.

Because a register pointer can point to either of the two 8-byte slices in the working register block, you can flexibly define the working register area to support program requirements.

)

PROGRAMMING TIP — Setting the Register Pointers

RP0

← no change, RP1 ← 48H

RP0

← 00H, RP1 ← no change

← no change, RP1 ← 0F8H

Register File

Contains 24

8-Byte Slices

0 0 0 0 1 X X X

RP1

0 0 0 0 0 X X X

RP0

8-Byte Slice

8-Byte Slice

FH (R15)

8H

7H

0H (R0)

16-Byte

Contiguous

Working

Register block

Figure 2-5. Contiguous 16-Byte Working Register Block

2-6

S3F80N8_UM_REV1.10 ADDRESS

8-Byte Slice

F7H (R15)

F0H (R8)

1 1 1 1 0 X X X

RP1

0 0 0 0 0 X X X

RP0

Register File

Contains 24

8-Byte Slices

8-Byte Slice

7H (R7)

0H (R0)

16-Byte non-

Contiguous

Working

Register block

Figure 2-6. Non-Contiguous 16-Byte Working Register Block

)

PROGRAMMING TIP — Using the RPs to Calculate the Sum of a Series of Registers

Calculate the sum of registers 80H–85H using the register pointer. The register addresses from 80H through 85H contain the values 10H, 11H, 12H, 13H, 14H, and 15 H, respectively:

RP0

← R0 + R1

R0

← R0 + R3 + C

R0

← R0 + R5 + C

The sum of these six registers, 6FH, is located in the register R0 (80H). The instruction string used in this example takes 12 bytes of instruction code and its execution time is 36 cycles. If the register pointer is not used to calculate the sum of these registers, the following instruction sequence would have to be used:

80H

← (80H) + (82H) + C

80H

← (80H) + (85H) + C

Now, the sum of the six registers is also located in register 80H. However, this instruction string takes 15 bytes of instruction code rather than 12 bytes, and its execution time is 50 cycles rather than 36 cycles.

2-7

ADDRESS SPACES S3F80N8_UM_REV1.10

REGISTER ADDRESSING

The S3F8-series register architecture provides an efficient method of working register addressing that takes full advantage of shorter instruction formats to reduce execution time.

With Register (R) addressing mode, in which the operand value is the content of a specific register or register pair, you can access any location in the register file. With working register addressing, you use a register pointer to specify an 8-byte working register space in the register file and an 8-bit register within that space.

Registers are addressed either as a single 8-bit register or as a paired 16-bit register space. In a 16-bit register pair, the address of the first 8-bit register is always an even number and the address of the next register is always an odd number. The most significant byte of the 16-bit data is always stored in the even-numbered register, and the least significant byte is always stored in the next (+1) odd-numbered register.

Working register addressing differs from Register addressing as it uses a register pointer to identify a specific

8-byte working register space in the internal register file and a specific 8-bit register within that space.

MSB

Rn

LSB

Rn+1 n = Even address

Figure 2-7. 16-Bit Register Pair

2-8

S3F80N8_UM_REV1.10 ADDRESS

Special-Purpose Registers

FFH

Control

Registers

E0H

D0H

C0H

BFH

System

Registers

CFH

RP1

Register

Pointers

RP0

Each register pointer (RP) can independently point to one of the 24 8-byte "slices" of the register file. After a reset, RP0 points to locations C0H-C7H and RP1 to locations C8H-CFH (that is, to the common working register area).

NOTE:

In the S3F80N8 microcontroller, only Page 0 is implemented.

General-Purpose Register

Prime

Registers

C0H

80H

00H

Can be Pointed by Register Pointer

Figure 2-8. Register File Addressing

2-9