Difference between revisions of "AMSDOS Header"
From CPCWiki - THE Amstrad CPC encyclopedia!
PulkoMandy (Talk | contribs) m |
|||
| Line 3: | Line 3: | ||
{| class="wikitable" | {| class="wikitable" | ||
|- | |- | ||
| − | ! | + | ! Bytes !! Name !! Notes |
|- | |- | ||
| − | | 0 | + | | 0 || User number || 0-15, or #E5 for deleted entries |
|- | |- | ||
| − | | 1 | + | | 1..8 || Filename || Unused characters are filled with space |
|- | |- | ||
| − | | 9 | + | | 9..11 || Extension || Unused characters are filled with space |
|- | |- | ||
| − | | | + | | 12..15 || Zero || Contains all zeroes |
|- | |- | ||
| − | | | + | | 16 || Block number || Tape only |
|- | |- | ||
| − | | | + | | 17 || Last block || Tape only |
|- | |- | ||
| − | | | + | | 18 || 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") |
|- | |- | ||
| − | | | + | | 19..20 || Data location || Address of the 2KB buffer |
|- | |- | ||
| − | | | + | | 21..22 ||Load address || |
|- | |- | ||
| − | | | + | | 23 || First block || Set to #FF, only used for output files |
|- | |- | ||
| − | | | + | | 24..25 ||style="white-space: nowrap;"| Logical length || Actual file length is here |
|- | |- | ||
| − | | | + | | 26..27 || Entry address || |
|- | |- | ||
| − | | | + | | 28..63 || Unused || |
|- | |- | ||
| − | | | + | | 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 || Free to use |
|} | |} | ||
| + | |||
| + | 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. | To calculate the checksum just add all bytes from 00 up to and including byte 66 together. | ||
Revision as of 11:24, 11 July 2024
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 |
| 17 | Last block | Tape only |
| 18 | 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") |
| 19..20 | Data location | Address of the 2KB buffer |
| 21..22 | Load address | |
| 23 | First block | Set to #FF, only used for output files |
| 24..25 | Logical length | Actual file length is here |
| 26..27 | Entry address | |
| 28..63 | Unused | |
| 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 | Free to use |
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.
