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cc65/libsrc/runtime/add.s
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ArmAsm

;
; Ullrich von Bassewitz, 05.08.1998
;
; CC65 runtime: add ints
;
; Make this as fast as possible, even if it needs more space since it's
; called a lot!
.export tosadda0, tosaddax
.importzp sp, tmp1
tosadda0:
ldx #0
tosaddax:
ldy #0
clc
adc (sp),y ; lo byte
sta tmp1 ; save it
txa
iny
adc (sp),y ; hi byte
tax
clc
lda sp
adc #2
sta sp
bcc L1
inc sp+1
L1: txa ; Test high byte
bmi L2
bne L3
lda tmp1 ; Get low byte
rts
; Value is negative
L2: lda tmp1 ; Get low byte
ldy #$FF ; Force negative
rts
; Value is positive != 0
L3: lda tmp1 ; Get low byte
ldy #1
rts