FDCOPY file format

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.

Uncompressed file format

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".

Compressed file format

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.

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.

Track header word

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):

	DEFW	length		;Track header
	DEFS	length		;Track data, <length> bytes

Track data blocks

The header is followed by one or more data blocks. A block can either be compressed or uncompressed.

A compressed block is 3 bytes long, and is formed:

	DEFW	repeats | 08000h	; Number of repetitions of the byte 
					; Bit 15 set to indicate compressed
	DEFB	byte			; The byte to repeat

An uncompressed block is formed:

	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.

FDCOPY creates a compressed block for a run of 5 or more identical bytes.

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.