Atari data cassette

Revision as of 17:35, 18 March 2013

Atari data cassettes store programs or data for the Atari 400 or 800 computers.

File structure

A file stored on cassette has this overall structure:

• 20-second leader (mark tone)
• A series of data record frames
• An end-of-file frame

Data record frame

The data record frame has this structure:

• Pre-record write tone (PRWT)
• Data record body
• Post-record gap (PRG)

Pre-record write tone

The pre-record write tone has two forms, depending on whether the write OPEN mode was "stop/start" or "continuous write".

For stop/start, the PRWT is 3 seconds of mark tone.

For continuous write, the PRWT is .25 second of mark tone.

Post-record gap

The post-record gap also has two forms for the two write OPEN modes.

For stop/start, the PRG is up to 1 second of unknown tones.

For continuous write, the PRG is from 0 to n seconds of unknown tones, depending on program timing.

An inter-record gap (IRG) consists of the PRG of the first record followed by the PRWT of the next record.

Data record body

The data record is a fixed-length record of 132 bytes, stored as as sequence of bits going from high bit to low bit. This is in the following order:

• Two speed measurement bytes: 0 1 0 1 0 1 0 1
• Control byte (FC indicates full data record, FA indicates partial data record [only found in last record of file], FE indicates an all-zero end-of-file record)
• 128 data bytes (last byte is set to number of data bytes in record for partial data records)
• Checksum

Links

• Atari reference manual (lots of tech details)