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:
| Size | Name | Notes |
|---|---|---|
| BYTE | User number | 0-15, or #E5 for deleted entries |
| 8 BYTES | Filename | Unused characters are filled with space |
| 3 BYTES | Extension | Unused characters are filled with space |
| DWORD | Zero | Contains all zeroes |
| BYTE | Block number | Tape only |
| BYTE | Last block | Tape only |
| 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") |
| WORD | Data location | Address of the 2KB buffer |
| WORD | Load address | |
| BYTE | First block | Set to #FF, only used for output files |
| WORD | Logical length | Actual file length is here |
| WORD | Entry address | |
| 36 BYTES | Unused | |
| 3 BYTES | Real length | 24-bit number. Just a copy, not used! |
| WORD | Checksum | Unsigned sum of all bytes until this field |
| 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.