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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:

Size Name Notes
BYTE User number 0-15, or #E5 for deleted entries
8 BYTES Filename Unused characters are filled with space
3 BYTES Extension Unused characters are filled with space
BYTE Block number Tape only
BYTE Last block Tape only
BYTE File type 0:BASIC 1:Protected 2:Binary (ASCII files don't have headers)
WORD Data length  ???
WORD Load address
BYTE First block Tape only?
WORD Logical length Actual file length is here
WORD Entry address
36 BYTES Unused
3 BYTES Real length 24-bit number. Just a copy, not used!
WORD Checksum Unsigned sum of all bytes until this field
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.