Re: .DSK images with 40 tracks?

[pausch@saaf.se (Paul Schlyter)]

In article <20030311170738.27196.00002180@mb-ba.aol.com>,
Michael J. Mahon <mjmahon@aol.com> wrote:
 
> 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.
 
In one Apple II file system -- Apple CP/M -- there was no such
information; there the OS was supposed to KNOW the disk geometry.
An empty CP/M disk (no OS to boot with, no files ever written to it)
will simply have all its sectors filled with hex E5.
 
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