.DSK File Format

Introduction

The extended DSK file is a copy of a 3-inch floppy disk of the Amstrad CPC or of the ZX Spectrum +3, in the format that was added to the original CPCEMU DSK so that copy-protected disks with tracks of different sizes could be stored. It starts with EXTENDED CPC DSK File. Instead of one track size for the whole disk it has a table with one byte for every track and side, and every sector entry says how many bytes of the sector are stored.

Because the same extension is used for the standard file, for Apple II sector images and others, the explorer tells them apart by the first bytes of the file (EXTENDED for this one, the 8 characters the emulators check; the standard file starts MV - CPC).

Structure

A 256-byte disk information block, then the track blocks in the order track 0 side 0, track 0 side 1, track 1 side 0 and so on (one side only for a single-sided disk). A track whose table entry is 0 is not stored and takes no space. See Disk, track and sector blocks.

In the explorer

Drop a .dsk file that starts with EXTENDED on the explorer to see:

• the tree: the disk information block with its fields and the track size table, every stored track block with its track information block (each sector entry with its ID, status bytes and stored length) and every sector’s data, with several stored copies of a sector marked as a weak sector;
• the disk (creator, tracks, sides, how many track blocks are not stored, sectors and bytes), a table of the tracks and a table of the sectors with the status bits MAME uses named;
• the problems found: a track block that is not Track-Info or is cut short, track and side numbers that disagree with the place, more than 29 sectors, a stored length that is not a multiple of the size the size code gives, a table longer than its 204 entries, a track block beyond the end of the file, extra bytes.

Not described

Known, not yet covered

• Reference: the individual bits of the controller status registers ST1 and ST2 beyond the three MAME uses; the data rate and recording mode bytes some later writers store at track offsets 0x12 and 0x13; the “Offset-Info” blocks some later extensions add (not in the pages we read).
• Viewer: the file system inside the disk (AMSDOS/CP/M directory, +3DOS) is not read, so no directory, files or BASIC programs are shown and nothing is disassembled; the weak-sector copies are not shown as separate nodes.

Unknown

• Reference: the sector ID ranges of non-+3 disks come from one wiki line ($40-49, $C0-C9), not confirmed by a second source; the viewer’s “first sector ID” line is a hint built on it.
• Viewer: one real file (an extended DSK written by Caprice32, 42 tracks of nine 512-byte sectors) is read and compared with an independent decoding; no copy-protected disk with weak sectors or unformatted tracks was available, so those branches are checked on built files only.

Also used for

• Amstrad CPC disk image (standard) – A CPC or Spectrum +3 floppy (MV - CPCEMU), track by track with each sector’s ID.

Links