#34: Looping in Z80 Assembly (Sinclair ZX Spectrum)
Looping in Z80 assembly language using 8 bit and 16 bit counters.
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=========================
8 Bit Loop Counter Program
org 30000
MAIN ;Main program loop.
LD HL, Counter ;Get address of loop count verifier.
LD BC, 0 ;Initialize BC to zero (B to be used as loop counter).
LD B, 10 ;Initialize loop counter to 10.
Loop ;Start of loop.
INC (HL) ;Increment loop count verifier.
DJNZ Loop ;Decrement loop counter.
CALL SUB1 ;[Print loop count verifier]
RET ;Return to BASIC.
;******** Subroutines ************
SUB1 ;Print loop count verifier.
LD A, 2 ;Set output channel value to select top of screen.
CALL ROM1 ;[Activate output channel]
LD BC, (Counter) ;Get value of loop counter verifier.
CALL ROM2 ;[Print value in BC (loop counter verifier)].
RET ;Return from SUB1
;***** ROM Routines ******
ROM1 EQU 5633 ;Activate output channel per A.
ROM2 EQU 6683 ;Print value in BC (9999 max).
;***** Data ********
Counter DW 0 ;Initialize loop count verifier to zero (8 bit).
;*** end ***
16 Bit Loop Counter program:
org 33000
MIAN ;Main program loop.
LD BC, 0 ;Initialize loop counter verifier.
LD HL,300 ;Initialize loop counter.
Loop ;Start of loop.
INC BC ;Increment loop counter verifier.
DEC HL ;Decrement loop counter (does not affect any flags).
LD A, H ;Copy H to A for comparison.
OR L ;Check if H and L are both zero.
JR NZ, Loop ; Jump if HL is not zero (loop not finished).
CALL SUB1 ;[Print loop count verifier]
RET ;Return to BASIC.
;****** Subroutines ********
SUB1 ;Print loop count verifier.
LD A, 2 ;Set output channel value to select top of screen.
PUSH BC ;Save BC value onto stack.
CALL ROM1 ;[Activate output channel]
;LD BC, ;Get value of loop counter verifier.
POP BC ;Retreive BC value from stack.
CALL ROM2 ;Print contents of BC (loop counter verifier).
RET ;Return from SUB1
;**** ROM Routines ******
ROM1 EQU 5633 ;Activate output channel per A.
ROM2 EQU 6683 ;Print value in BC (9999 max).
;*** end ***
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