cc65/libsrc/runtime/lsubeq.s

;				  
; Ullrich von Bassewitz, 07.04.2000
;
; CC65 runtime: -= operator
;
; On entry, the low byte of the address of the variable to decrement is
; in ptr1, the high byte is in Y, and the decrement is in eax.
;

       	.export	       	lsubeq1, lsubeqa, lsubeq
       	.importzp   	sreg, ptr1


lsubeq1:
       	lda	#$01

lsubeqa:
	ldx	#$00
	stx	sreg
	stx	sreg+1

lsubeq:	sty	ptr1+1		    	; Store high byte of address
	ldy	#$00		    	; Address low byte
	sec

	eor	#$FF
	adc	(ptr1),y	    	; Subtract byte 0
	sta	(ptr1),y
       	pha			    	; Save byte 0 of result for later

	iny			    	; Address byte 1
	txa
	eor	#$FF
	adc	(ptr1),y	  	; Subtract byte 1
	sta	(ptr1),y
	tax

	iny			    	; Address byte 2
	lda	(ptr1),y
	sbc	sreg
	sta	(ptr1),y
	sta	sreg

	iny			    	; Address byte 3
	lda	(ptr1),y
	sbc	sreg+1
	sta	(ptr1),y
	sta	sreg+1

	pla			  	; Retrieve byte 0 of result

	rts			  	; Done