Re: 1571 images as G64

From: Pete Rittwage <>
Date: Wed, 29 May 2013 18:51:38 -0400 (EDT)
Message-ID: <>
On Wed, May 29, 2013 3:23 pm, wrote:
> On 2013-05-29, at 18:44, Pete Rittwage wrote:
>>> If I understand correctly the specs of the G64 format, it should be
>>> flexible enough to accommodate any (?) GCR based disk image data, even
>>> if
>>> it is signed as "GCR-1541". If that is correct then the question is -
>>> has
>>> anyone seen this being put into some use? Like GCR images of 1571
>>> disks?
>>> Or Apple ][ (?)
>>> --
>> It's never been used for anything except single-sided Commodore disks,
>> so
>> anything else would be non-standard.  It would make much more sense to
>> extend the header and make it G71 for 168-track double-sided disks to
>> prevent confusion.  However, there are less 1571 disks that have copy
>> protection and need GCR images than you can count on one hand, that I
>> know
>> of.
> Pete, thank you for explanations. I haven't seen this format being used
> for 1571 (or other) either. But when reading the docs from
> "formats-20051127" [*] I understood that the file format could handle it,
> even without extending anything as number of tracks is a parameter in the
> header and then the data comes as GCR streams, plus it allows for
> speed-"zone" variations even on a per byte basis.
>> As for Apple, the specs aren't made for that. Technically, their GCR is
>> different than CBM- there is only one density, and the way the data is
>> constructed on the disk vs. what is read is odd (no sync marks per se,
>> every byte is a sync) AND they can have quarter-tracks.
> That's interesting ;-) So they used 80-track mechanism to access 20 tracks
> or what?
> But this should not break the format either, should it? One speed is fine
> and whatever is the structure of the GCR data should not be relevant
> either if only the GCR bits (read with proper speed) are stored. Or am I
> missing something?

Certainly you "could" do it.  I figured by the name of the extension and
the intro to the document, that it was always just designed as an
alternate/extension to the D64 format.  Calling it "G64" when used for
1571 or certainly Apple images makes less sense.  You could use the same
format, though, just with a different signature.  GCR-1571, GCR-APL2, or

In the Apple emulation world, they use "NIB" for raw files, which is
unrelated to NIB from mnib/nibtools.

On the Apple disk drive, they basically ripped out the Shugart board and
made a simple circuit controlled by software, so you have direct software
control over the actual field coils which move the heads.  While their
"official" DOS only uses 35 tracks (0-34) like CBM,  it is possible to
manipulate the heads to move not only 1/2 way in between tracks, but also
1/4.  It is still a wide head like the 1541, though, so it will destroy
data for 1.5 tracks when you write.

Pete Rittwage
C64 Preservation Project

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Received on 2013-05-29 23:00:03

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