Apple II Disk Image File Format

Introduction

An Apple II disk is a 5.25-inch floppy (the Disk II) of 35 tracks, each of 16 sectors of 256 bytes, 143,360 bytes in all; later machines also read 3.5-inch disks of 800 KB and hard disks. The disk image files that emulators and archives use for them come in four kinds, which hold the same disk with different amounts of detail. Inside the disk is either a DOS 3.3 or a ProDOS file system, and inside that are Applesoft, Integer BASIC and machine code programs, text and data. The 6502 (the 65C02 in the enhanced IIe and IIc) runs the code.

Each kind is a separate format here, with its own pages. The explorer opens all of them: drop a file on any page of this site and it works out which kind it is from its contents, and for a sector image which of the two sector orders it is in. The Sector order and File system options in the options bar override what it chose.

The extension clash. .dsk is also the Amstrad CPC disk image, a different format of a different machine (a Z80), described on the CPC .DSK page. The explorer tells them apart by content: a CPC disk starts with MV - CPC or EXTENDED CPC DSK, an Apple II sector image has no header and is exactly 143,360 bytes (163,840 for a 40-track disk). A CPC file is refused by the Apple II reading and shows as bytes with its own page. The Amstrad CPC viewer itself is not built yet. The extensions .po (also gettext translation files), .do and .nib (also Cocoa interface files) are claimed in the same careful way: only when a volume or nibbles actually parse.

Variations

In the explorer

All four are read into the same picture: the tree of tracks, sectors (or blocks, or nibble fields), a map of the disk coloured by what each sector is, the files with their types and addresses, and the files opening as nodes. See each variant's page for what its tree shows.

Not described

Known, not yet covered

• Viewer: 3.5-inch GCR disks in a .WOZ are listed track by track but their sectors are not decoded; the 5-and-3 encoding of 13-sector disks in a .NIB or .WOZ is recognised by its address field but its data field is not decoded.
• Viewer: WOZ FLUX and WRIT chunks are carried but not decoded, and quarter tracks other than the whole track are not shown.
• Viewer: other disk systems on Apple II disks (Apple Pascal, CP/M, Apple SOS and the many copy-protected and custom boot formats) are not read: they show as sectors, and a protected disk in a .NIB or .WOZ shows the sectors that decode and lists the others as problems.
• Viewer: the Applesauce .A2R flux images, .DC42/.DMG 3.5-inch images and Mac-format .HFS volumes on Apple II hard disks are not read.

Unknown

• Reference and viewer: only three real disk files were available (a DOS 3.3 System Master .dsk, a ProDOS utilities disk in a 2IMG file and an empty 800 KB ProDOS volume); there is no real .po, .nib, .woz or .d13 on this Mac, and no Apple II emulator source that reads them, so those readings are checked on built files and against the KEGS emulator's skew tables and 6-and-2 code. See each variant's page.

Also used for

• Amstrad CPC disk image – A CPC or Spectrum +3 floppy, track by track with each sector’s ID.

Links