cc65/libsrc/runtime/mod.s

;
; Ullrich von Bassewitz, 07.08.1998
;
; CC65 runtime: modulo operation for signed ints
;

; When negating values, we will ignore the possibility here, that one of the
; values if $8000, in which case the negate will fail.

       	.export		tosmoda0, tosmodax
	.import		popsargs, udiv16, negax
	.importzp	ptr1, tmp1

tosmoda0:
      	ldx	#0
tosmodax:
	jsr	popsargs	; Get arguments from stack, adjust sign
      	jsr	udiv16		; Do the division
        lda     ptr1            ; Load low byte of result
      	ldx	ptr1+1          ; Load high byte of result

; Adjust the sign of the result. tmp1 contains the high byte of the left
; operand, tmp2 contains the high byte of the right operand. The sign of
; the result of the modulo operation is the same as that of the left
; operand

        bit     tmp1
        bpl     Pos             ; Jump if sign of result positive

; Result is negative

        jmp     negax           ; Adjust the sign

; Result is positive

Pos:    rts