From: Mark (mark_k_at_iname.com)
Date: 2000-03-31 03:05:19


A while ago I wrote:
> D2D can be used to read data from C64 disks in an unmodified Amiga 5.25"
> floppy drive. According to the documentation in the demo archive, reading
> from tracks 1-17 is really slow because the program must do some tricks to
> accountfor real 1541 drives having variable speed. (There is a program on
> Aminet, misc/emu/1541.lha, which can read 1541 disks but this requires that
> the Amiga drive's speed be reduced.)
>Does anyone have an original copy of Disk-2-Disk that they would be willing
>to sell? If you do, please contact me.

I managed to acquire an original copy of Disk-2-Disk V2.1 a couple of weeks
ago. I have been disassembling it with the eventual aim of figuring out how
it works.

So far I have determined that it does some funky stuff with the drive motor,
presumably "modulating" it to allow all tracks to be read on a 300rpm Amiga
drive. Pretty clever, and in theory using a similar technique it may even be
possible to read Macintosh 800K floppies in an Amiga drive.

Also there are at least two severe bugs in it, which can be easily fixed. One
will trash memory (a branch destination is two bytes too far), the other will
only show up in an error condition (branch destination is a text string
rather than the routine which displays that text string).

If anyone is interested I can post about how to fix these bugs.

Now, I need to find information on several aspects of the 1541 disk format. If
any of the following info is available online, please let me know!
 - explanation of how the GCR coding works
 - 1541 sector layout on disk (i.e. where are sectors in relation to index,
   what interleave is used)
 - what other bytes are written apart from the sector data itself (i.e. sector
 - 1541 filesystem (i.e. format of the BAM etc.)

-- Mark

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