Re: .DSK images with 40 tracks?

[mjmahon@aol.com (Michael J. Mahon)]

In article <Pine.A41.4.44.0303112110440.81632-100000@zivunix.uni-muenster.de>,
Holger Picker <pickerh@uni-muenster.de> writes:

>On 11 Mar 2003, Ron Kneusel wrote:
>
>> Date: 11 Mar 2003 09:23:34 -0800
>> From: Ron Kneusel <oneelkruns@hotmail.com>
>> Newsgroups: comp.sys.apple2
>> Subject: .DSK images with 40 tracks?
>>
>> Are there any emulators out there which would support 40 track disk images?
>> Someone has emailed me asking about extending DSK2FILE to make images of
>> 40-track disks.
>>
>> Curious...
>>
>> Ron
>> oneelkruns@hotmail.com
>>
>Hi,
>
>well, at least it shouldn't be too much of a problem for an emulator to
>support 40 tracks. Whether you've got 30, 35, or 40 tracks, from the
>algorithm's point of view that's all the same. What's important though is
>how you would like to load and store a 40 track disk image. The standard
>.dsk image format (as well as .do or .po) only stores 35 tracks. Either you
>would have to come up with a new format or extend a previously existing one.
>(I'm sorry, I have not taken a deeper look into formats like 2em yet.)

Extending by simply appending the extra track data is the obvious
way to go.  The issue would be how the emulator detects the kind
of disk image it's working with.  I suspect that many of them use the
file size as a primary determinant, and then a few are clever enough
to look at a few "signature" bytes in the image to determing the
sector order and OS type.

A minor kink in the works is that perhaps the most common error
in disk image files is the prepending of a "header" of 128 bytes,
sometimes _multiple copies_ of the header!  So a "smart" program
that processes disk images would have to scan through, eliminating
the detectable headers, then verify format/type by looking at signature
bytes, then set number of tracks based on remaining file size.

Of course, some consistency checks are possible, since the VTOC
or other root directory information has "number of tracks" information
once the file system type is determined.

> Could
>I ask you, please, which "real" disk drive supported a 40 track disk image?
>Were 40 tracks possible on a standard disk ][ drive? I know of a few AppleII
>emulators that are currently still under development, e.g. AppleWin,
>AppleOasis, Dapple, Kegs... If they do not support 40 tracks yet and 40
>tracks are considered standard, then 40 tracks should and could easily (IMO)
>be implemented.

Most half-height drives could do 40 tracks without problems, and many
full-height drives could, too.  Only the very early Apple Disk ][ drives using
the SA390 mechanism ran into trouble, and even they could usually
squeeze in an extra track or two.

-michael

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