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Merge pull request #606 from pfusik/ldec

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Optimize deceaxy
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Oliver Schmidt 2018-03-10 11:28:46 +00:00 committed by GitHub
commit be30c0bd70
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1 changed files with 23 additions and 13 deletions
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36
libsrc/runtime/ldec.s
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@ -1,5 +1,6 @@
;
; Ullrich von Bassewitz, 29.12.1999
; Piotr Fusik, 09.03.2018
; originally by Ullrich von Bassewitz
;
; CC65 runtime: Decrement eax by value in Y
;
@ -11,16 +12,25 @@ deceaxy:
sty tmp1
sec
sbc tmp1
sta tmp1
txa
sbc #0
tax
lda sreg
sbc #0
sta sreg
lda sreg+1
sbc #0
sta sreg+1
lda tmp1
rts
bcs @L9
; Borrow from X.
dex
cpx #$FF
bne @L9
; X wrapped from zero to $FF, borrow from sreg.
dec sreg
cpx sreg
bne @L9
; sreg wrapped from zero to $FF, borrow from sreg+1.
dec sreg+1
; Done.
@L9: rts