Introduction
A .dsk, .do or .po file is the contents of an Apple II floppy, sector after sector, with no header and no signature. The three names differ in the
order of the sixteen sectors of a track: .dsk and .do keep DOS 3.3's order, .po keeps ProDOS's. Most emulators
(Apple II JS, AppleWin and others) boot them. A file that starts MV - CPC or EXTENDED CPC DSK is not this format: it is an Amstrad CPC disk (the
CPC .DSK page).
Structure
0Track 0 16 sectors of 256 bytes (the boot sector is the first in the file in either order)4,096 bytes4,096Track 1 and on up to track 34; the DOS 3.3 VTOC is track 17 sector 0 at 69,6324,096 bytes each
| Size | What it is |
|---|---|
| 143,360 | 35 tracks of 16 sectors: the usual Apple II floppy (DOS 3.3 or ProDOS) |
| 163,840 | 40 tracks of 16 sectors: some disks and emulator images |
| 116,480 | 35 tracks of 13 sectors (.d13): a DOS 3.2 disk. The sectors are in the order they are on the track, so there is no sector order to choose |
| any multiple of 512 | A ProDOS volume of blocks (a 3.5-inch disk of 819,200 bytes, a hard disk image .hdv or .po): block n at n × 512, no tracks |
In the explorer
The explorer finds the order by trying the sectors under both and keeping the one in which a DOS 3.3 VTOC and catalogue read without problems (and the sectors the files claim add up to the sectors the VTOC says are used), or in which a ProDOS volume directory parses; when neither does, the extension decides. The options bar has the order and the file system to override it. The tree shows the tracks and, in them, each sector labelled by what it is, with the VTOC and catalogue fields; for a ProDOS volume in ProDOS order it shows 512-byte blocks, with the directory entries.
Checked on a real DOS 3.3 System Master disk, which opens with 19 files and no problems in DOS order, and in a re-skewed copy in ProDOS order (the same 19 files and the same bytes in each); on a real ProDOS volume; and on built images in both orders.
Not described
Known, not yet covered
- Viewer: disks whose sectors are in neither order (some copy-protected or custom-format disks, and the Apple Pascal and CP/M disks) show as sectors only; there is no third order to choose.
- Viewer: a 13-sector disk is read only as a
.d13sector image, from its DOS 3.2 VTOC; the sector order of DOS 3.2 images made by other tools is not described.
Unknown
- Reference: there is no real
.po,.door.d13file on this Mac; those names are read from the extension and built files, and the 13-sector reading is checked on one image built by hand.
Variants
- Apple II sector image (.DSK, .DO, .PO) – the sectors with no header, in DOS or ProDOS order
- 2IMG disk image – a 64-byte header in front of the sectors
- NIB nibble image – the nibbles of each track as the drive reads them
- WOZ disk image – the bit stream of every track, in chunks
Links
Software
- Apple II JS – Apple II emulator that runs in the browser and loads .dsk and .po files (any browser)
- AppleWin – Apple II emulator (Windows)
- CiderPress II – tool to view, extract and make Apple II disk images (Windows, macOS, Linux)