Z80 SPECIAL INSTRUCTIONS

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Z80 SPECIAL INSTRUCTIONS

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The Z80 instruction set includes some instructions that perform more than one task. These instructions improve programming efficiency considerably. Some of these instructions are:

Mnemonics

Description

DJNZ d

Decrement B and Jump Relative on no zero (Z = 0)

The instruction decrements register B, and if B # 0, it jumps to the memory address specified by the offset value d.

LDI

Load and Increment

The instruction copies a data byte from the memory location shown by HL into the memory location indicated by DE_. Reg­isters HL and DE are incremented. and BC is decrement

LDIR

Load, Increment, and Repeat

This is similar to the instruction LDI, except that it is repeated until BC= 0.

LDD

Load and Decrement

The instruction copies a data byte from the memory location shown by HL into the memory location pointed to by DE. Registers HL, DE, and BC are decremented.

LDDR

Load, Decrement; and Repeat

This instruction is similar to LDD, except that it is repeated until BC= 0

Example

Modify the illustrative program Addition with Carry (Section 8. 7) using the in­struction DJNZ and the offset value.

Solution

The following mnemonics are repeated from a segment of the program in Figure; we assume that the segment is stored in memory locations starting from l808H.

Location

Label

Mnemonics

Comments

1808

NXTBYT:

LD D, (HL)

;Get data byte from input buffer

1809

ADDA,D

;Add data byte

180A

JP NC, SKIPCY

;If no carry, do not save CY

180D

INCC

;Save carry bit

180E

SKIPCY:

INC HL

;Point to next memory location

180F

DJNZ F7H

;Decrement counter B, and if B # 0, jump to location 1008 to get the next byte

Program Description and Calculation of the Offset Value In this program, the instruction DJNZ replaces two instructions-DEC B and JP NZ, NXTBYT-from the program in Figure 8.10. The instruction DJNZ assumes that register B is used as a counter. When the Z80 executes the 2-byte instruction DJNZ, the program counter holds the address 1811H. This is a backward jump; therefore, the offset value must be in 2's complement. The offset value for the jump location NXTBYT (1808H) is obtained as follows:

Program Counter:

1 8

11

Jump Location:

I 8

08

09H

F7H

(0 0 0 0 1 0 0 1)

2's Complement of 09H

(1111 0 111)

for backward jump:

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