{| class="wikitable"
|-
! Size Field !! Bytes !! Name !! Notes
|-
| BYTE rowspan="10" | System || 0 || User number || 0-15, or #E5 for deleted entries
|-
| 1..8 BYTES || Filename || Unused characters are filled with space
|-
| 3 BYTES 9..11 || Extension || Unused characters are filled with space
|-
| BYTE 12..15 || Block number Zero || Tape onlyContains all zeroes
|-
| BYTE 16 || Last block Block number || Tape only, set to 0
|-
| BYTE 17 || File type Last block || Tape only, set to 0:BASIC 1:Protected 2:Binary (ASCII files don't have headers)
|-
| WORD 18 || Length 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)
|-
| WORD 19..20 || Load address Data length || As per cassette: the number of data bytes in the data record
|-
| BYTE 21..22 || First block Data location || Tape only?As per cassette: where the data was written from originally
|-
| WORD 23 || Logical length First block || Actual file length is hereSet to #FF, only used for output files
|-
| WORD rowspan="3" |User | Entry address |24..25 | | Logical length || As per cassette: actual file length is here
|-
| 36 BYTES 26..27 || Unused Entry address || As per cassette: the execution address for machine code programs
|-
| 3 BYTES 28..63 || Real length Unused || 24-bit number. Just a copy, not used!As per cassette
|-
| WORD rowspan="3" |AMSDOS | Checksum |64..66 | Unsigned sum | Real length || Length of all the file in bytes until this field, excluding the header record. 24 bit number, least significant byte first. Just a copy, not used!
|-
| 59 BYTES 67..68 || Checksum || Unsigned sum of all bytes until this field|-| 69..127 || Unused || Free to use
|}
To calculate These headers are detected by checksumming the checksum just add all first 67 bytes from of the record. If the checksum is as expected then a header is present, if not then there is no header. When a file without a header is opened for input, 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]]