cc65/libsrc/plus4/break.s

;
; Ullrich von Bassewitz, 27.09.1998
;
; void set_brk (unsigned Addr);
; void reset_brk (void);
;

       	.export	   	_set_brk, _reset_brk
	.export	       	_brk_a, _brk_x, _brk_y, _brk_sr, _brk_pc
        .import         brk_jmp

	.include      	"plus4.inc"


.bss
_brk_a:		.res	1
_brk_x:		.res   	1
_brk_y:		.res   	1
_brk_sr:	.res	1
_brk_pc:	.res	2

oldvec:       	.res	2      	   	; Old vector


.data
uservec:    	jmp	$FFFF  	   	; Patched at runtime


.code

; Set the break vector
.proc	_set_brk

	sta	uservec+1
  	stx	uservec+2      	; Set the user vector

        lda	#<brk_handler  	; Set the break vector to our routine
	sta	brk_jmp+1
       	lda	#>brk_handler
	sta	brk_jmp+2
	rts

.endproc


; Reset the break vector
.proc	_reset_brk

        lda     #$00
        sta     brk_jmp+1
        sta     brk_jmp+2       ; Reset the vector
        rts

.endproc



; Break handler, called if a break occurs. Note: Y is not on the stack!

.proc	brk_handler

       	sty	_brk_y
	pla
	sta	_brk_x
	pla
	sta	_brk_a
	pla
	and	#$EF		; Clear break bit
	sta	_brk_sr
	pla	    		; PC low
	sec
	sbc	#2 		; Point to start of brk
	sta	_brk_pc
	pla	    		; PC high
	sbc	#0
	sta	_brk_pc+1

	jsr	uservec		; Call the user's routine

	lda	_brk_pc+1
	pha
	lda	_brk_pc
	pha
	lda	_brk_sr
	pha
	ldx    	_brk_x
	ldy	_brk_y
	lda	_brk_a
	rti	   		; Jump back...

.endproc