Re: CP/M disk format question

[pausch@saaf.se (Paul Schlyter)]

In article <48779f9e$0$17148$742ec2ed@news.sonic.net>,
Andy McFadden  <fadden@fadden.com> wrote:
>All of this is in the context of Apple II CP/M on a 5.25 floppy.
>
>CP/M disks use 1K blocks, and have a directory starting at block 12 (track 3
>on a 5.25" disk).  The first 3 tracks are reserved for the boot image.  That
>leaves 140K - 12K = 128 blocks.

All of those quantities are actually configurable - their values are
stored in the DPB (Disk Parameter Block) in the BIOS, where each disk
has its own DPB.

I hooked up 80 track DSDD disks to my Apple II in the 80's and made
all of the disk into a 640K CP/M disk volume.  Then I had to make the
CP/M blocks 2K large, since a CP/M disk with 1K blocks cannot be as
large as 640K.

So you should really write: "All of this is in the context of an
Apple II CP/M on a 5,25" floppy in its standard 140K configuration".

>I've been sent an apparently valid disk image that uses block numbers above
>128.  It appears that such values wrap around, storing data in track 0-2.
>
>Is this the expected behavior?  This is of interest for the CiderPress
>disk read routines, which are currently unable to extract such a file
>in its entirety.

I would call that behaviour an undocumented bug.  The system tracks 0-2
should be accessible only through BIOS calls, not through BDOS calls.
The bug is probably in the BIOS.


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