RECORDS¶

A record can be either a header record or a data record. A header record serves as a "checkpoint" in the file, with a magic value that can be used to recognize archive files.

A header record has a fixed size of 28 bytes, as follows:

  28 bytes:    magic string

A data record has a variable size, as follows:

  2 bytes:     file number


  2 bytes:     attribute ID


  4 bytes:     data size (N)


  N bytes:     data

A header record is distinguished from a data record by the magic string. The file number 0x414d, corresponding to the characters "AM", is forbidden and must be skipped on writing.

Attribute ID 0 (AMAR_ATTR_FILENAME) gives the filename of a file. This attribute is mandatory for each file, must be nonempty, must fit in a single record, and must precede any other attributes for the same file in the archive. The filename should be a printable string (ASCII or UTF-8), to facilitate use of generic archive-display utilities, but the format permits any nonempty bytestring. The filename cannot span multiple records.

Attribute ID 1 (AMAR_ATTR_EOF) signals the end of a file. This attribute must contain no data, but should have the EOA bit set.

CONNECTION TO DATA MODEL¶

Each file in an archive is assigned a file number distinct from any other active file in the archive. The first record for a file must have attribute ID 0 (AMAR_ATTR_FILENAME), indicating a filename. A file ends with an empty record with ID 1 (AMAR_ATTR_EOF). For every file at which a reader might want to begin reading, the filename record should be preceded by a header record. How often to write header records is left to the discretion of the application.

All data records with the same file number and attribute ID are considered a part of the same attribute. The boundaries between such records are not significant to the contents of the attribute, and both readers and writers are free to alter such boundaries as necessary.

The final data record for each attribute has the high bit (the EOA bit) of its data size field set. A writer must not reuse an attribute ID within a file. An attribute may be terminated by a record containing both data and an EOA bit, or by a zero-length record with its EOA bit set.