File: revers.s

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;
; Ullrich von Bassewitz, 07.08.1998
;
; unsigned char revers (unsigned char onoff);
;
        .include "atari.inc"
        
        .export         _revers
        .export         _revflag

_revers:
        ldx     #$00            ; Assume revers off
        tay                     ; Test onoff
        beq     L1              ; Jump if off
        ldx     #$80            ; Load on value
L1:     ldy     #$00            ; Assume old value is zero
        lda     _revflag        ; Load old value
        stx     _revflag        ; Set new value
        beq     L2              ; Jump if old value zero
        iny                     ; Make old value = 1
L2:     ldx     #$00            ; Load high byte of result
        tya                     ; Load low byte, set CC
        rts

        .bss

_revflag:
        .res    1