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 is consists consist of 128 bytes and contains contain the following data:
<pre>{| class="wikitable"Byte 00: |-! Field !! Bytes !! Name !! Notes|-| rowspan="10" | System || 0 || User number|| 0-15, or #E5 for deleted entriesByte 01 to 08: filename|-| 1..8 || Filename || Unused characters are filled with space|-Byte 09 bis | 9..11: || Extension|| Unused characters are filled with spaceByte 18: type|-byteByte 21 and 22: loading address| 12..15 || Zero || Contains all zeroesByte 24 and 25: file length|-Byte 26 and 27: execution address for machine code programs| 16 || Block number || Tape only, set to 0Byte 64 and 65: checksum for byte 00 |-| 17 || Last block || Tape only, set to byte 660Byte 67 and 68|-| 18 || File type || As per cassette: Bit0 = Protection (file lengthset if protected)</pre>
To calculate the checksumBits1..3 = File contents (0:BASIC, just add byte 00 to byte 66 to each other1: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|-| rowspan="3" | 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|-| rowspan="3" | 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. When a file without a header is opened for input, a fake header is constructed in store as follows: {| class="wikitable"|-! Bytes !! Name !! Notes|-| 0 || User number || User number, #00..#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 zero. This is only a summary. All details are in [[CategoryFirmware Guide]] chapter 9, page 4. === Expansion of the AMSDOS Header ===Some OS and DOS expanded the classic AMSDOS header:Programming 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]]