Acorn CP/M disc format

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Coeus
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Acorn CP/M disc format

Post by Coeus » Tue Jul 10, 2018 6:26 pm

I notice Acorn CP/M format discs have a fake DFS catalogue at the start on side 0. As far as I can work out this contains a single file that occupies the whole disk (side) and thus ensures the user cannot save files to the disc from DFS and overwrite CP/M files.

Shouldn't there be a similar fake catalogue for side 2, though? If so, this would need to go where DFS would look for it, i.e. track 0, sector 0. In CP/M terms that would place it at the end of the disc. Is this, perhaps, why the tracks go backwards on the second side of an Acorn CP/M disc? That would mean this fake catalogue could be hidden from CP/M by having the BPB for that disc be one track short. It would not be so easy if the tracks on the second side went forward from a CP/M perspective.

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jgharston
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Re: Acorn CP/M disc format

Post by jgharston » Tue Jul 10, 2018 10:25 pm

Coeus wrote:
Tue Jul 10, 2018 6:26 pm
I notice Acorn CP/M format discs have a fake DFS catalogue at the start on side 0.
...
Shouldn't there be a similar fake catalogue for side 2, though?
Nope, 'cos that would end up being in the middle of a CPM file.

Code: Select all

$ bbcbasic
PDP11 BBC BASIC IV Version 0.25
(C) Copyright J.G.Harston 1989,2005-2015
>_

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