Last modified on 15 October 2024, at 13:05

AMSDOS Header

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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.

When a file without a header is opened for input, a fake header is constructed in store as follows:

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
12..15 Zero Contains all zeroes
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 Firmware Guide chapter 9, page 4.