DOS 3.3 File Format

Introduction

Apple DOS 3.3 (1980) is the disk operating system of the Apple II: it gives a 5.25-inch floppy of 35 tracks of 16 sectors of 256 bytes (143,360 bytes) a catalogue of files with names, types and a lock. It has no sub-directories. DOS 3.2, its predecessor, did the same on disks of 13 sectors a track (116,480 bytes), with an older sector encoding. The file system is not a file format of its own: it is what the sectors of a .DSK, .PO, .2MG, .NIB or .WOZ disk image hold, and every one of those can carry it.

Everything is addressed by track and sector, counted the DOS way (the DOS logical sector, not the position the sector has on the track). A .DSK or .DO file keeps its sectors in exactly that order, so track t sector s is at t × 4096 + s × 256.

Structure

Track 17 is in the middle of the disk to keep the head's travel short. Sectors 0 and 15 are at the same place in DOS order and in ProDOS order, which is why a VTOC alone cannot tell the two orders apart: the explorer also follows the catalogue chain and checks that the sectors the files claim account for the sectors the VTOC says are in use.

In the explorer

Drop a disk image. The tree has every track and, in it, every sector labelled (boot, VTOC, catalogue sector, track/sector list, sector n of a named file, free, or in use but not part of any file). The VTOC and the catalogue sectors have their fields, entry by entry. The Files table lists every file; its Open button shows an Applesoft or Integer BASIC program as a listing, a binary file disassembled at its load address, text as text. The picture of the disk colours each sector by what it is. Deleted files are noticed; their data is not read.

Not described

Known, not yet covered

• Viewer: deleted files are listed in the catalogue sector but not recovered, and DOS 3.3 disks of other sizes (800K DOS 3.3 volumes of third-party systems, 40-track disks beyond what the VTOC says) are read only as far as the VTOC's own track count.
• Viewer: the 5-and-3 data field of 13-sector disks (DOS 3.2) in a .NIB or .WOZ is not decoded; a 13-sector disk is read only from a sector image (.D13).
• Reference: the file types S, R, and the second Applesoft and binary types (a $20, b $40) are named, but their contents are shown as raw sectors (files).
• Viewer: the boot sector and the DOS image on tracks 0–2 are labelled but not disassembled.

Unknown

• Reference: only one real DOS 3.3 disk was available (the System Master): the catalogue chain over more than three sectors, two-sector track/sector lists, random-access text with holes, deleted entries and the 13-sector release are read on built images only.

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