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
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.