cc65/libsrc/apple2/break.s

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

        .export         _set_brk, _reset_brk
        .destructor     _reset_brk

        ; Be sure to export the following variables absolute
        .export         _brk_a: abs, _brk_x: abs, _brk_y: abs
        .export         _brk_sr: abs, _brk_pc: abs

        .include        "apple2.inc"

_brk_a = $45
_brk_x = $46
_brk_y = $47
_brk_sr = $48
_brk_sp = $49
_brk_pc = $3A

.bss
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     BRKVec
        sta     oldvec
        lda     BRKVec+1
        sta     oldvec+1        ; Save the old vector

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

.endproc


; Reset the break vector
.proc   _reset_brk

        lda     oldvec
        ldx     oldvec+1
        beq     @L9             ; Jump if vector not installed
        sta     BRKVec
        stx     BRKVec+1
        lda     #$00
        sta     oldvec          ; Clear the old vector
        stx     oldvec+1
@L9:    rts

.endproc



; Break handler, called if a break occurs

.proc   brk_handler

        sec
        lda     _brk_pc
        sbc     #$02            ; Point to start of brk
        sta     _brk_pc
        lda     _brk_pc+1
        sbc     #$00
        sta     _brk_pc+1

        clc
        lda     _brk_sp
        adc     #$04            ; Adjust stack pointer
        sta     _brk_sp

        lda     _brk_sr         ; Clear brk
        and     #$EF
        sta     _brk_sr

        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