Re: [cc65] Uniform mass storage device discovery and file locations

Hello,

* On Mon, Aug 20, 2012 at 02:08:25PM +0200 Oliver Schmidt wrote:
> Hi,
> 
> Just for the records - here's again the code snipet and my questions -
> which the answer below refers to...
> 
> lda <device>
> jsr $ffb1 ; listen
> lda #$00
> sta $90   ; necessary ?
> lda #$ff  ; or rather #$6f ?

Yep, $6F.

$FF is the secondary address that is used for OPEN.

Having said this, I am not sure if this really makes a difference, as
the floppy (154x/157x) masks out unused bits at $E8B8:

E8B8 AND #$60
E8BA CMP #$60
E8BC BNE $E8FD

Only $E0 (secondary address for CLOSE) seems to be handled differently
($E8C7-). However, I fail to see how this code can even execute, as the
sequence above should already handle this. Or I am just too tired to
read the code correctly. ;)


I have not followed the rest of the code in the floppy, though. So there
might be some difference because of the high-nibble of the secondary
address that I am not aware of. It has been some years since I last
looked at this code in detail.

> jsr $ff93 ; lstnsa
> ldy $90
> jsr $ffae ; unlstn
> tya
> bpl <present>
> <not present>

> However I have several questions:
> - Is it really necessary to initialize $90

Yep.

In most cases, the status ($90) is cleared at $F30F in the C64 ROM: That
is, it is cleared whenever the KERNAL searches for a logical file number.
(OPEN 1,x,y or PRINT#1 or INPUT#1 or so). This only happens in
"high-level" accesses to the IEC, though.

It is also cleared at F3DF (opening a file on IEC), $F4A9 (LOAD), $F841
(tape routine), FE1E (general "set status" routine).

Thus, as you are using direct ("low-level") IEC routines, nothing will
reset $90 for you, so you have to do it yourself.

> - What is the right high nibble for "opening" the cmd channel here $F or $6 ?

$6F.

> - Is it correct to assume that unlstn doesn't trash Y ?

Yep, neither X nor Y is affected, only A (by contract):

http://www.devili.iki.fi/Computers/Commodore/C64/Programmers_Reference/Chapter_5/page_271.html
http://www.devili.iki.fi/Computers/Commodore/C64/Programmers_Reference/Chapter_5/page_304.html

> - Is it correct that $90 must be saved before calling unlstn ?

No, not really

Obviously, this routine only wants to make sure that an error that might
occur is not because of the UNLISTEN, but because of the LISTEN.

That's right if one thinks that the LISTEN might succeed ("I am here"
from the device), but the UNLISTEN might fail. I have problems to see
how this could occur, though, as long as other devices do not interfere.
Furthermore, if UNLISTEN fails, we have a communication problem, so all
bets are off anyway.

Regards,
Spiro.

-- 
Spiro R. Trikaliotis
http://www.trikaliotis.net/
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