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:
Byte 00: User number (value from 0 to 15 or #E5 for deleted entries) Byte 01 to 08: filename (fill unused char with spaces) Byte 09 to 11: Extension (fill unused char with spaces) Byte 16: first block (tape only) Byte 17: first block (tape only) Byte 18: file type (0:basic 1:protected 2:binary) Byte 21 and 22: loading address LSB first Byte 23: first block (tape only?) Byte 24 and 25: file length LSB first Byte 26 and 27: execution address for machine code program LSB first Byte 64 and 66: 24 bits file length LSB first. Just a copy, not used! Byte 67 and 68: checksum for bytes 00-66 stored LSB first Byte 69 to 127: undefined content, 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.