Z80 SPECIAL INSTRUCTIONS

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 l808_H.

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 1811_H. This is a backward jump; therefore, the offset value must be in 2’s complement. The offset value for the jump location NXTBYT (1808_H) is obtained as follows:

Program Counter:	1 8	11

Jump Location:	I 8	08
		09_H F7_H	(0 0 0 0 1 0 0 1)
2’s Complement of 09_H			(1111 0 111)
for backward jump:

Data Copy Between Accumulator and I/Os

Relative Jump Instructions

Flags and Decision Making

Z80 SPECIAL INSTRUCTIONS