Controlling the marquee
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You should let the system ROM be in charge of controlling the marquee. But if you want to try things, here's how it goes:
See the bit definitions of the REG_LEDDATA and REG_LEDLATCHES registers for more details.
The system ROM uses ~35us delays done with fall-through calls to NOPs.
move.b #$FF, REG_LEDLATCHES ; Set clock high
jsr wait
not.b d0 ; The panel # is sent inverted
move.b d0, REG_LEDDATA ; Set panel #
move.b #$F7, REG_LEDLATCHES ; Set clock low (latch panel #)
jsr wait
move.b #$FF, REG_LEDLATCHES ; Set clock back highwait:
movem.l d0-d7/a0-a5,-(a7) ; 120 cycles
movem.l (a7)+,d0-d7/a0-a5 ; 124 cycles
movem.l d0-d7/a0-a5,-(a7) ; 120 cycles
movem.l (a7)+,d0-d7/a0-a5 ; 124 cycles, total = 488 cycles = 40.7us
rts