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37 lines
551 B
ArmAsm
37 lines
551 B
ArmAsm
;
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; Piotr Fusik, 09.03.2018
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; originally by Ullrich von Bassewitz
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;
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; CC65 runtime: Decrement eax by value in Y
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;
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.export deceaxy
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.importzp sreg, tmp1
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deceaxy:
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sty tmp1
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sec
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sbc tmp1
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bcs @L9
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; Borrow from X.
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dex
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cpx #$FF
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bne @L9
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; X wrapped from zero to $FF, borrow from sreg.
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dec sreg
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cpx sreg
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bne @L9
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; sreg wrapped from zero to $FF, borrow from sreg+1.
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dec sreg+1
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; Done.
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@L9: rts
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