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VAPI / ATX Disk Image Format

The VAPI disk format is used to accurately preserve Atari disk images.


Overview

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

version_number: e.g. 1

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_size: size of the whole track record in bytes (including header).

record_type: can be one of the following values:
ValueType
$00data 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
size: size of sector list in bytes.

record_type: can be one of the following values:
ValueType
$01sector 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_number: 1-indexed (note: if the track has duplicate sectors, there will be multiple sectors with the same sector number).

sector_status: 1 byte broken down as follows:
BitDescription
7Reserved (must be zero)
61 = extended data present for sector
5Reserved (must be zero)
41 = missing sector data
31 = FDC CRC error
21 = FDC lost data error
1Reserved (must be zero)
0Reserved (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:

type
1 byte		reserved
4 bytes		sector_index
1 byte		weak_offset
2 bytes (little-endian)

type: can be on of the following values:
ValueDescription
$08weak 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.
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