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:

Offset	Size	Name	Notes
0	BYTE	User number	0-15, or #E5 for deleted entries
1	8 BYTES	Filename	Unused characters are filled with space
9	3 BYTES	Extension	Unused characters are filled with space
C	DWORD	Zero	Contains all zeroes
10	BYTE	Block number	Tape only
11	BYTE	Last block	Tape only
12	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")
13	WORD	Data location	Address of the 2KB buffer
15	WORD	Load address
17	BYTE	First block	Set to #FF, only used for output files
18	WORD	Logical length	Actual file length is here
1A	WORD	Entry address
1C	36 BYTES	Unused
40	3 BYTES	Real length	24-bit number. Just a copy, not used!
43	WORD	Checksum	Unsigned sum of all bytes until this field
45	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.