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:
| Offset | Size | Name | Notes |
|---|---|---|---|
| 0 | BYTE | User number | 0-15, or #E5 for deleted entries |
| 1 | 8 BYTES | Filename | Unused characters are filled with space |
| 9 | 3 BYTES | Extension | Unused characters are filled with space |
| C | DWORD | Zero | Contains all zeroes |
| 10 | BYTE | Block number | Tape only |
| 11 | BYTE | Last block | Tape only |
| 12 | BYTE | File type | 0:BASIC 1:Protected 2:Binary (ASCII files don't have headers but a fake header is built in memory with the file type #16, "Unprotected ASCII v1") |
| 13 | WORD | Data location | Address of the 2KB buffer |
| 15 | WORD | Load address | |
| 16 | BYTE | First block | Set to #FF, only used for output files |
| 17 | WORD | Logical length | Actual file length is here |
| 19 | WORD | Entry address | |
| 1B | 36 BYTES | Unused | |
| 3F | 3 BYTES | Real length | 24-bit number. Just a copy, not used! |
| 42 | WORD | Checksum | Unsigned sum of all bytes until this field |
| 44 | 59 BYTES | Unused | Free to use |
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.