lda     SWCHA
    lsr
    lsr
    lsr
    lsr
    and     #%11
    ldy     lastTrack
    sta     lastTrack           
    eor     NextTrackTbl,y  
    beq     .down           
    eor     #%11
    bne     .endTC         
.up:                        
    ...
.down:                   
    ...
.endTC
    ...
NextTrackTbl
    .byte   %01
    .byte   %11
    .byte   %00
    .byte   %10