Hello All,
It looks like the following snippet is the key part of the JD routine, which
reads in data as bit-pairs every 10us. The trick of course is if this is to
be replicated for a VIC20 or C16/Plus4 then the data needs to be processed
in this relatively short time as there is no hand-shaking to hold off the
1541 once the Ok for the byte send has been initiated. Also, data is sent as
254byte packets... between pauses.
Not knowing enough about either the VIC20 or the Plus4:
1) are the port locations of DATA and CLOCK signals as convenient as they
are on the C64/C128?
2) the Plus4 runs at two speeds (?), is this controllable (since you
wouldn't want to be switching speeds during such critical open-loop timing)
beq $fb44
.fb4f lda $b2 ; lower three bits of $dd00
stx $dd00 ; store %00100xxx in $dd00
bit $dd00 ; bit test
bvc $faf0 ; branch is input clk=0
nop ; timing
sta $dd00 ; store %00000xxx in $dd00
ora $dd00 ; read two first bits
lsr a ; move right two times
lsr a
nop ; timing
ora $dd00 ; read next two bits
lsr a ; move right two times
lsr a
eor $b2 ; "trixx" to handle the lower three bits of $dd00
eor $dd00
lsr a ; move right two times
lsr a
eor $b2 ; "trixx" to handle the lower three bits of $dd00
eor $dd00
cpy $93 ; load/verify flag
bne $fb83 ; branch if verify
sta ($ae),y ; store loaded byte in memory
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