JIffyDOS Coding

Date: 2001-03-27 08:38:46

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


The contents of this email (including any attachments) may be
privileged and confidential. Any unauthorised use of the contents
is expressly prohibited. If you have received this email in error,
please advise us immediately (you can contact us by telephone
on +61 8 9441 2311 by reverse charge) and then permanently
delete this email together with any attachments. We appreciate
your co-operation.

Whilst Orbital endeavours to take reasonable care to ensure
that this email and any attachments are free from viruses or other
defects, Orbital does not represent or warrant that such is explicitly
the case

(C) 2000: Orbital Engine Company (Australia) PTY LTD and its

This message was sent through the cbm-hackers mailing list.
To unsubscribe: echo unsubscribe | mail cbm-hackers-request@dot.tml.hut.fi.

Archive generated by hypermail 2.1.1.