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:

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