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57 lines
895 B
ArmAsm
57 lines
895 B
ArmAsm
;
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; Ullrich von Bassewitz, 07.04.2000
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; CC65 runtime: -= operator
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;
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; On entry, the low byte of the address of the variable to decrement is
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; in ptr1, the high byte is in Y, and the decrement is in eax.
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;
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.export lsubeq1, lsubeqa, lsubeq
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.importzp sreg, ptr1
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lsubeq1:
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lda #$01
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lsubeqa:
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ldx #$00
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stx sreg
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stx sreg+1
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lsubeq: sty ptr1+1 ; Store high byte of address
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ldy #$00 ; Address low byte
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sec
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eor #$FF
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adc (ptr1),y ; Subtract byte 0
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sta (ptr1),y
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pha ; Save byte 0 of result for later
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iny ; Address byte 1
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txa
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eor #$FF
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adc (ptr1),y ; Subtract byte 1
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sta (ptr1),y
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tax
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iny ; Address byte 2
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lda (ptr1),y
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sbc sreg
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sta (ptr1),y
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sta sreg
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iny ; Address byte 3
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lda (ptr1),y
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sbc sreg+1
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sta (ptr1),y
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sta sreg+1
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pla ; Retrieve byte 0 of result
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rts ; Done
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