In AMSDOS it is possible to store files in two ways: headerless and with a header. Headerless files are often files which were created with OPENOUT and SAVE"filename",a. Programs normally have a file header, which consist of 128 bytes and contain the following data:
| Field | Bytes | Name | Notes |
|---|---|---|---|
| System | 0 | User number | 0-15, or #E5 for deleted entries |
| 1..8 | Filename | Unused characters are filled with space | |
| 9..11 | Extension | Unused characters are filled with space | |
| 12..15 | Zero | Contains all zeroes | |
| 16 | Block number | Tape only, set to 0 | |
| 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 | As per cassette: the number of data bytes in the data record | |
| 21..22 | Data location | As per cassette: where the data was written from originally | |
| 23 | First block | Set to #FF, only used for output files | |
| User | 24..25 | Logical length | As per cassette: actual file length is here |
| 26..27 | Entry address | As per cassette: the execution address for machine code programs | |
| 28..63 | Unused | As per cassette | |
| 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 checksum, just add all bytes from 00 up to and including byte 66 together.
This is only a summary. All details are in Firmware Guide chapter 9, page 4.