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_. RegÂisters 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 inÂstruction 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) |
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for backward jump: |