Overview
The VAPI disk format is used to accurately preserve Atari disk images. This page attempts to capture what (incomplete) information has been released about the format thus far. If any inaccuracies or additional information is known, please let me know so this page can be updated.
Structure
The general structure of an ATX file is a file header followed by one or more track records.
File Header (FH)
The file header can vary but is usually 48 bytes:
| 'A' 1 byte | 'T' 1 byte | '8' 1 byte | 'X' 1 byte | version_number 2 bytes (little-endian) | reserved 22 bytes | size 4 bytes (little-endian) | reserved 16 bytes |
size: offset from start of file to first track record.
Track Record (TR)
A track record is comprised of a track header (TH), a sector list (SL), sector data (SD) and optional extended sector data (XSD).
Track Header (TH)
The track header can vary but is normally 32 bytes:
| record_size 4 bytes (little-endian) | record_type 2 bytes (little-endian) | reserved 2 bytes | track_number 1 byte | reserved 1 byte | sector_count 2 bytes (little-endian) | reserved 8 bytes | header_size 4 bytes (little-endian) | reserved 8 bytes |
record_type: can be one of the following values:
| Value | Type |
|---|---|
| $00 | data track |
track_number: zero-indexed (nominally 0-39).
sector_count: number of physical sectors in track.
header_size: offset from start of track header to start of track data in bytes.
Sector List (SL)
A sector list is comprised of a sector list header (SLH) and one or more sectors headers (SH).
Sector List Header (SLH)
The sector list header can vary but is normally 8 bytes:
| size 4 bytes (little-endian) | record_type 1 byte | reserved 3 bytes |
record_type: can be one of the following values:
| Value | Type |
|---|---|
| $01 | sector list |
Sector Header (SH)
The sector header is 8 bytes:
| sector_number 1 byte | sector_status 1 byte | sector_position 2 bytes (little-endian) | start_data 4 bytes (little-endian) |
sector_status: 1 byte broken down as follows:
| Bit | Description |
|---|---|
| 7 | Reserved (must be zero) |
| 6 | 1 = extended data present for sector |
| 5 | 1 = deleted sector (DAM $F8) |
| 4 | 1 = missing sector data |
| 3 | 1 = FDC CRC error |
| 2 | 1 = FDC lost data error |
| 1 | Reserved (must be zero) |
| 0 | Reserved (must be zero) |
sector_position: zero-based starting angular position of sector in 8us intervals (1/26042th of a rotation or ~0.0138238 degrees). Nominally 0-26042.
start_data: offset in bytes from the start of the track header to the sector data (SD). Note: no data is present if sector data is missing (sector status bit 4)
Extended Sector Data (XSD)
The extended sector data is 8 bytes and repeats once for every sector in the track that had bit 6 set in the sector header:
| size 4 bytes | type 1 byte | sector_index 1 byte | weak_offset 2 bytes (little-endian) |
| Value | Description |
|---|---|
| $10 | weak sectors present |
sector_index: zero-based index of physical sector within track.
weak_offset: offset of the first byte within the sector to be replaced with random data during read. This data should be different every time the sector is read.