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cc65/libsrc/runtime/subeqsp.s

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;
; Ullrich von Bassewitz, 08.10.1998
;
; CC65 runtime: -= operator for ints on the stack
;
.export subeq0sp, subeqysp
.importzp sp
subeq0sp:
ldy #0
subeqysp:
sec
eor #$FF
adc (sp),y
sta (sp),y
pha ; Save low byte
iny
txa
eor #$FF
adc (sp),y
sta (sp),y
tax
pla ; Restore low byte
rts