[CCTALK] Commodore 1541 Drive

From: Cameron Kaiser <spectre_at_stockholm.ptloma.edu>
Date: Fri May 17 03:31:45 2002

> > That being said, since the 1541 is an intelligent peripheral I suppose you
> > could reprogram it to read Apple floppies, but this would require a lot of
>
> I'm not convinced you can. The lowest levels of the GCR encoding/decoding
> are done in hardware both for the Apple Disk II and the 1541, and I don't
> think they're compatible. And on most 1541s the logic is inside a
> Commodore gate array chip (There is a version which uses TTL + ROM,
> though), so it would be non-trivial to modify.

If you mean a dedicated IC doing the GCR encoding/decoding in the 1541, the
6502 is actually doing the decoding when it's in FDC mode, and this can be
easily intercepted (see "Inside Commodore DOS") -- the idea is write your own
driver code into RAM (what little RAM there is), leave an EXECUTE job code
to point to your routine, and when the 6502 next drops into FDC mode, your
routine will take over the unit exclusively and you can do your work without
interference.

The ROM routines for GCR interconversion start at $f6d0 (starting at page
418 in the Inside C= DOS memory map).

That being said, you'd have to resort to driving the 6522s yourself to
manipulate the bitstream directly -- very low-level, and of course with the
small amount of RAM in the 1541 this would require impressively tight code.
But it is, theoretically, possible at least on some level, even if it's
merely decoding the bytes and handing them off to the C64 for processing
(putting the "DOS" in the 64).

-- 
----------------------------- personal page: http://www.armory.com/~spectre/ --
 Cameron Kaiser, Point Loma Nazarene University * ckaiser_at_stockholm.ptloma.edu
-- Had this been an actual emergency, we would have fled already. -------------
Received on Fri May 17 2002 - 03:31:45 BST

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