begin 777 usa-map.gif

The three octal digits (777, 644, or whatever) specify the unix owner/group/other protection bits for read/write/execute. The file name follows. Lines before this are to be ignored.

The "begin line" is followed by a stream of data octets. The source bit stream is converted, 6 bits at a time, by placing the 6 bits into the low-order 6 bits of an octet and adding 0x20 (hexadecimal 20) with the result that each encoded octet is in the range 0x20-0x5f. This range is reasonably safe, since it corresonds to ASCII "printable" characters (special characters and capital letters). (However, this includes the blank/space and other potentially troublesome octets. See MIME's Base64.)

The data octets are arranged into lines that begin with a representation of the count of data octets on the line. The count in binary has 0x20 added and is one octet in length. For example if the line has 45 data characters (the maximum), then the first octet is 45+32 or 0x2D+0x20. (This displays as the ASCII letter 'M' (0x4D).)

The lines are separated by an octet whose value is octal 012, decimal 10, 0x0A, the ASCII nl ("newline") graphic.

The file ends with the line:

end

There may be other special keywords included.