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The native floppy disk format/layout or "filesystem" used by Amstrad microcomputers such as the CPC and PCW range and by the Sinclair Spectrum +3 are versions of one that came from CP/M.

In Amstrad parlance it might be referred to as "AMSDOS" and in Spectrum parlance "+3DOS" or "PLUS3DOS". But those names probably more accurately refer the "disk operating systems", ie the extra Basic commands and system APIs used to access the disks, than the format of the disks.

It could perhaps just be "The CP/M format/layout/filesystem". But from reading around, the Amstrad and Speccy each had extra features not from CP/M and there were quite a few versions of CP/M adding features, some of what either came after the Amstrad and Spectrum variants, or just added features those platforms eschewed.

If the Amstrad and Spectrum variants were branches of a specific stage of CP/M with a version number of name, that could well be what I'm looking for. There do seem to be a few suffixed versions of CP/M out there when I Google, such as CP/M-86 and CP/M 2.2. I've also noticed "FDOS" and "BDOS" in use.

When referring to the versions used by the Amstrad and Speccy, which term would not be a misnomer, not be anachronistic, etc?

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I don't remember them having a “name”. Disk interchangeability was less of a thing than it became between 16-bit computers. Amstrads were very much not interchangeable, since they had the Hitachi 3" drives, used by Tatung (UK) and very few others. Even the PCW9512+ with its 720 K 3½" drive used a format derived from the earlier CPC disks.

The format is CP/M*-ish*, circa CP/M-80 2.2. It supported user numbers, but didn't expressly display file dates. While CP/M+ (aka CP/M-80 3.1) on the CPC6128 and PCW machines could create file dates, the machines didn't have RTCs so the feature was seldom used.

Even amongst CP/M machines there was little disk interoperability. As long as the BDOS commands matched the FDC, you could have CP/M filesystems on any size of disk, hard or soft sectored, single or double sided, 40/80 tracks, 8/9/… sectors per track. Exchanging data between machines generally meant a null modem and xmodem/kermit.

So, if anything, I'd call these Amstrad format disks.

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  • OK that makes sense, thanks. I'm writing some software that works with DSK disk images of these filesystems but I'm not sure what to shoot for or what to call it and I didn't use any of those machines back in the day. It started Speccy +3 only but I've added some Amstrad features and so far at least I'm not interested in CP/M stuff. – hippietrail Jun 1 at 0:43
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The Amstrad CPC manual (chapter 7 part 12) listed three "disk formats" supported by AMSDOS, which it named "System", "Data only" and "IBM" (CP/M 2.2 only).

All three of these formats had:

  • 40 tracks
  • 64 directory entries
  • 512 bytes per sector, no matter the track.

And the format-specific characteristics:

  • System had 9 sectors per track, two reserved tracks for CP/M boot (the manual notes the format is called "Vendor" instead when the System tracks are empty)
  • Data Only had 9 sectors per track too, but no reserved tracks.
  • IBM had 8 sectors per track, and one reserved track. The manual says it's compatible with the "single side" format used by CP/M in IBM PC.
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  • Yes these three seem just to be subflavours of the AmsDos flavour, and are also supported on the Spectrum +3. But there are differences too. The Spectrum has an optional disk specification that I don't think the Amstrad has. The file headers are totally different though the same size. The Spectrum doesn't use timestamps. I'm not sure if the boot sectors work the same. I better look at the Amstrad one now... Various CP/M flavours seem to have extra features crammed in. I don't think Amstrad or Spectrum use Bc in the directory entry or 16-bit logical extents. – hippietrail May 31 at 13:34

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