{| class="wikitable"
|-
! Field !! Bytes !! Name !! Notes
|-
| rowspan="10" | System || 0 || User number || 0-15, or #E5 for deleted entries
|-
| 1..8 || Filename || Unused characters are filled with space
| 17 || Last block || Tape only, set to 0
|-
| 18 || File type || As per cassette: Bit0 = Protection (set if protected)
Bits1..3 = File contents (0:BASIC, 1:Binary, 2:Screen image, 3:ASCII, 4..7 are unallocated)
Bits4..7 = Version(ASCII files should be version 1, all other files should be version 0)
|-
| 19..20 || Data length || The As per cassette: the number of data bytes in the data record. As per cassette
|-
| 21..22 || Data location || Where As per cassette: where the data was written from originally. As per cassette
|-
| 23 || First block || Set to #FF, only used for output files
|-
| rowspan="3" | User || 24..25 || Logical length || Actual As per cassette: actual file length is here. As per cassette
|-
| 26..27 || Entry address || The As per cassette: the execution address for machine code programs. As per cassette
|-
| 28..63 || Unused || As per cassette
|-
| rowspan="3" | AMSDOS || 64..66 || Real length || Length of the file in bytes, excluding the header record. 24 bit number, least significant byte first. Just a copy, not used!
|-
| 67..68 || Checksum || Unsigned sum of all bytes until this field
| 69..127 || Unused ||
|}
These headers are detected by checksumming the first 67 bytes of the record. If the checksum is as expected then a header is present, if not then there is no header.
To calculate the checksumWhen a file without a header is opened for input, just add all bytes from a fake header is constructed in store as follows: {| class="wikitable"|-! Bytes !! Name !! Notes|-| 0 || User number || User number, #00 up ..#FF|-| 1..8 || Filename || Unused characters are filled with space|-| 9..11 || Extension || Unused characters are filled with space|-| 18 || File type || #16, unprotected ASCII version 1|-| 19..20 || Data location || Address of 2K buffer|-| 23 || First block || Set to #FF|} All other fields are set to and including byte 66 togetherzero.
This is only a summary. All details are in [[Firmware Guide]] chapter 9, page 4.
=== Expansion of the AMSDOS Header ===
Some OS and DOS expanded the classic AMSDOS header:
1. [[FutureOS Header]]:
* Contains and icon or information text
* File length is 24 Bit
* Defined Loading and Autostart (expansion) RAM configuration
* Additional file types
[[Category:Disc Operating System]]
[[Category:Programming]]