<html> <head> <title>FDCOPY file format</title> </head> <body> <H1>FDCOPY file format</H1> <p>FDCOPY.COM is a program supplied on Amstrad's website to convert DOS-formatted floppies to/from disc image files. When using image files, there are two formats it can use - compressed and uncompressed. <h2>Uncompressed file format</h2> <p>This is simply a dump of all the bytes on the disc, identical to that which would be produced by "dd if=/dev/fd0 of=filename". <h2>Compressed file format</h2> <p>FDCOPY detects the compressed form by checking if the filename ends in .CFI; it can also be forced to use compression with the undocumented /CI option. There is no 'magic number' to detect if an image file is compressed. <p>In the compressed format, the disc image is stored as a sequence of tracks. The image file doesn't contain a track count anywhere; FDCOPY depends on the first track containing a DOS boot spec to work out how many tracks there are. LIBDSK contents itself with just checking for EOF after processing each track. <h3>Track header word</h3> <p>Each track starts with a 2-byte little-endian word, which gives the number of disc bytes in this track (not including the length word itself): <pre> DEFW length ;Track header DEFS length ;Track data, <length> bytes </pre> <h3>Track data blocks</h3> <p>The header is followed by one or more data blocks. A block can either be compressed or uncompressed. <p>A compressed block is 3 bytes long, and is formed: <pre> DEFW repeats | 08000h ; Number of repetitions of the byte ; Bit 15 set to indicate compressed DEFB byte ; The byte to repeat </pre> <p>An uncompressed block is formed: <pre> DEFW length ; Number of bytes in this block ; at most 7FFFh, but FDCOPY doesn't ; create blocks with more than 2400h ; bytes in them. DEFS length ; The bytes. </pre> <p>FDCOPY creates a compressed block for a run of 5 or more identical bytes. <p>There is no end-of-track marker; the end of the track can only be detected by comparing the track length word with the number of bytes decoded.
