cc65/libsrc/atari/break.s

;
; Christian Groessler, 27-Feb-2000
;
; void set_brk (unsigned Addr);
; void reset_brk (void);
;

       	.export	   	_set_brk, _reset_brk
	.destructor	_reset_brk
       	.export	       	_brk_a, _brk_x, _brk_y, _brk_sr, _brk_pc

	.include   	"atari.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	oldvec
	ora	oldvec+1	; Did we save the vector already?
        	bne	L1		; Jump if we installed the handler already

	lda	VBREAK
 	sta    	oldvec
 	lda 	VBREAK+1
 	sta	oldvec+1	; Save the old vector

L1:	lda	#<brk_handler	; Set the break vector to our routine
	sta	VBREAK
	lda	#>brk_handler
	sta	VBREAK+1
	rts

.endproc


; Reset the break vector
.proc	_reset_brk

	lda  	oldvec
	bne  	@L1
	ldx  	oldvec
	beq  	@L9		; Jump if vector not installed
@L1:	sta    	VBREAK
	stx  	VBREAK+1
@L9:	rts

.endproc



; Break handler, called if a break occurs

.proc	brk_handler

	sty	_brk_y
	stx	_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