A .DSK file contains all the bytes of an Apple DOS 3 formatted floppy disk. The file does not have a header which can make it difficult to verify. The simplest form of verification is to check the file size.
A typical DOS 3 disk has 35 tracks. Each track has 16 sectors and each sector has 256 bytes. This gives each disk a total of 143,360 bytes.
Sector 0 of track 17 contains the Volume Table of Contents. The VTOC contains the information needed to start traversing the disk and accessing the individual files stored on it.
| Volume Table of Contents | ||
|---|---|---|
| Byte | Length | Description |
The 2nd and 3rd bytes of the VTOC indicate where the catalog starts. Each catalog sector links to the next catalog sector and holds information for 7 files.
| Catalog Sector | ||
|---|---|---|
| Byte | Length | Description |
Each catalog sector contains 7 file entries and each file entry uses 35 bytes.
| File Entry Bytes | ||
|---|---|---|
| Byte | Length | Description |
Each file has a sector list that is used to keep track of all the sectors used to store a file's contents. The first two bytes of a file entry point to the sector that starts the list.
| Sector List Sector | ||
|---|---|---|
| Byte | Length | Description |
The last 200 bytes of the VTOC holds the sector usage bit maps. A single bit is used to indicate if a sector is available (1) or used (0). There is a bit for each sector on the disk. The bit maps are laid out in big-endian 16-bit integers and separated by 2 unused bytes.
This means, for a typical disk with 16 sectors per track, you read a 16-bit integer. The first integer represents the sectors in track 0. The low bit of the integer represents sector 0 and the high bit represents sector 15. Then you skip 2 bytes and read another 16-bit integer. The second integer represents the sectors in track 1. You repeat this pattern for each track.
| Sector Usage Bit Maps | ||
|---|---|---|
| Byte of VTOC | Length | Track Represented |