RE: Question about writing to 1541

From: Baltissen, GJPAA (Ruud) (
Date: 2005-05-03 13:35:01

Hallo allemaal,

Andre wrote:
> Second: Indeed in the write block routine ($f575) the last 
> "BVC *" at $F5CA waits for the last data byte (from ($30),y)
> to be _loaded_ into the shift register.

(After studying the schematics) Yep, you're absolutely right. So it is not
the end of the writing proces, but the beginning.

> Immediately after that the PCR is set to input again, so there
> is no time to shift the last data byte out to the RW-head
> completely.

Illogical, but that seems to be the case.

Regarding the two BVC's were the discussion started with: the first one
starts the proces of writing the last $55 and the second one makes sure it
is written and not dumped like the databyte during the proces mentioned by
Andre. I'm just wondering why the programmer cared so much for this $55 byte
and neglected this databyte. Only one thing comes to my mind: two different
programmers. One that likes to see things neatly closed and a sloppy one who
knew that the 1541 didn't do anything with the last byte and so didn't care
about it. It even can be a programming error but as it had no visible
result, nobody noticed it.

For Patryk:

>	LDA #$55
>	LDX $45
> L784:
>	BVC L784
>	STA $1C01
>	BNE L784

Where do you initialise $45? Anyway, your routine also proofs the statement
I made above. I also question the use of 'STA $1C01' here; it doesn't add
anything. IMHO it could have been placed immediatly after 'LDA #$55'. BTW,
the 1541 does the same. Why?

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