Repeat: Difference between revisions

=={{header|6502 Assembly}}==

This routine is a bit messy, and assumes the called routine doesn't clobber the zero-page memory used to maintain it. This can be modified to push/pop those values before/after the routine is executed.

<lang 6502asm>macro RepeatProc,addr,count ;VASM macro syntax

lda #<\addr

sta z_L ;a label for a zero-page memory address

lda #>\addr

sta z_H ;a label for the zero-page memory address immediately after z_L

lda \count

jsr doRepeatProc

endm

doRepeatProc:

sta z_C ;another zero-page memory location

loop_RepeatProc:

jsr Trampoline_RepeatProc

dec z_C

lda z_C

bne loop_RepeatProc

rts

Trampoline_RepeatProc:

db $6c,z_L,$00

;when executed, becomes an indirect JMP to the address stored at z_L and z_H. Some assemblers will let you type

;JMP (z_L) and it will automatically replace it with the above during the assembly process.

;This causes an indirect JMP to the routine. Its RTS will return execution to just after the "JSR Trampoline_RepeatProc"

;and flow into the loop overhead.</lang>