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UM008007-0715 Specific Z80 Instruction Examples

Z80 CPU

User Manual

Specific Z80 Instruction Examples

Example 1

When a 737-byte data string in memory location DATA must be moved to location BUF-

FER, the operation is programmed as follows:

LD HL, DATA ;START ADDRESS OF DATA STRING

LD DE, BUFFER ;START ADDRESS OF TARGET BUFFER

LD BC, 737 ;LENGTH OF DATA STRING

LDIR ;MOVE STRING–TRANSFER MEMORY POINTED

;TO BY HL INTO MEMORY LOCATION POINTED

;TO BY DE INCREMENT HL AND DE,

;DECREMENT BC PROCESS UNTIL BC = 0

Eleven bytes are required for this operation and each byte of data is moved in 21 clock

cycles.

Example 2

A string in memory (limited to a maximum length of 132 characters) starting at location

DATA is to be moved to another memory location starting at location BUFFER until an

ASCII $ (used as a string delimiter) is found. This operation is performed as follows:

LD HL, DATA ;STARTING ADDRESS OF DATA STRING

LD DE, BUFFER ;STARTING ADDRESS OF TARGET BUFFER

LD BC, 132 ;MAXIMUM STRING LENGTH

LD A, '$' ;STRING DELIMITER CODE

LOOP: CP (HL) ;COMPARE MEMORY CONTENTS WITH

;DELIMITER

JR Z, END-$ ;GO TO END IF CHARACTERS EQUAL

LDI ;MOVE CHARACTER (HL) to (DE)

;INCREMENT HL AND DE, DECREMENT BC

JP PE, LOOP ;GO TO

LOOP IF MORE CHARACTERS

END: ;OTHERWISE, FALL THROUGH

;NOTE: P/V FLAG IS USED

;TO INDICATE THAT REGISTER BC WAS

;DECREMENTED TO ZERO

Nineteen bytes are required for this operation.

Example 3

A 16-digit decimal number is shifted as depicted in Figure 24. This shift is performed to

mechanize BCD multiplication or division. The 16-digit decimal number is represented in

packed BCD format (two BCD digits/byte) The operation is programmed as follows:

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 325 326

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