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 checksumWhen a file without a header is opened for input, just add all bytes from a fake header is constructed in store as follows: {| class="wikitable"|-! Bytes !! Name !! Notes|-| 0 || User number || User number, #00 up ..#FF|-| 1..8 || Filename || Unused characters are filled with space|-| 9..11 || Extension || Unused characters are filled with space|-| 18 || File type || #16, unprotected ASCII version 1|-| 19..20 || Data location || Address of 2K buffer|-| 23 || First block || Set to #FF|} All other fields are set to and including byte 66 togetherzero.
This is only a summary. All details are in [[Firmware Guide]] chapter 9, page 4.
=== Expansion of the AMSDOS Header ===
Some OS and DOS expanded the classic AMSDOS header:
1. [[FutureOS Header]]:
* Contains and icon or information text
* File length is 24 Bit
* Defined Loading and Autostart (expansion) RAM configuration
* Additional file types
[[Category:Disc Operating System]]
[[Category:Programming]]