AMSDOS Header
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:
Bytes | Name | Notes |
---|---|---|
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 | 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 number of data bytes in the data record. As per cassette |
21..22 | Data location | Where the data was written from originally. As per cassette |
23 | First block | Set to #FF, only used for output files |
24..25 | Logical length | Actual file length is here. As per cassette |
26..27 | Entry address | The execution address for machine code programs. As per cassette |
28..63 | Unused | As per cassette |
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.