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cc65/libsrc/apple2/revers.s

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;
; Ullrich von Bassewitz, 2005-03-28
;
; unsigned char __fastcall__ revers (unsigned char onoff)
;
.export _revers
.include "apple2.inc"
_revers:
tax ; Test onoff
beq normal ; If zero, "normal" must be set
ldx #$3F+1 ; Set "inverse"
normal: dex ; $00->$FF, $40->$3F
lda #$00 ; Preload return code for "normal"
ldy INVFLG ; Load current flag value
stx INVFLG ; Save new flag value
bmi :+ ; Jump if current value is $FF (normal)
lda #$01 ; Return "inverse"
: ldx #$00
rts