Extended DSK image definition

The extended DSK image is a file designed to describe copy-protected floppy disk software. It's definition was defined by Marco Vieth, Ulrich Doewich and Kevin Thacker.

This format has been widely adopted and is one of the major file formats used (the other major format is the standard disk image with the "MV - CPC" identifier).

The extended disk image format should be used for copy-protected disc software, or in place of a standard disk image if the resulting image is smaller than in the standard disk image form.

Extended DiSK Format (Rev.5)

The disc image has the following format:

DISK INFORMATION BLOCK (256 bytes)

TRACK INFORMATION BLOCK * number of tracks * number of sides

The track blocks are stored in increasing order 0..number of tracks, with alternating sides interleaved if the disc image describes a double sided disk. e.g. if the disk image represents a double sided disk, the order of tracks is: track 0 side 0, track 0 side 1, track 1 side 0, track 1 side 1.... track (number of tracks-1) side 0, track (number of tracks-1) side 1

The tracks are always ordered in this way regardless of the disc-format described by the disc image.

The location of the track information block is found by using the track size table.

DISK INFORMATION BLOCK


offset	description	bytes
00 - 21	"EXTENDED CPC DSK File\r\nDisk-Info\r\n"	34
22 - 2f	name of utility/emulator that created the file	14
30	number of tracks	1
31	number of sides	1
32 - 33	unused	2
34 - xx	track size table	number of tracks*number of sides

NOTES:

An extended DSK image is identified by the "EXTENDED" tag. The track size at offset 32h and 33h is ignored for extended format DSK images.
Track data starts at offset 100h.
The "EXTENDED" tag is present to prevent existing emulators which support the standard DSK image from interpreting the data wrong and possibly crashing.
"\r" is the C programming language equivalent of ASCII character 13.
"\n" is the C programming language equivalent of ASCII character 10.

TRACK OFFSET TABLE


offset	description	bytes
01	high byte of track 0 length (equivalent to track length/256)	1
...	...	...

NOTES:

Depending on the information in the disk information block, the table contains

track lengths for a single sided floppy disc
track lengths for a double sided floppy disc

track lengths are stored in the same order as the tracks in the image e.g. In the case of a double sided disk: Track 0 side 0, Track 0 side 1, Track 1 side 0 etc...
A size of "0" indicates an unformatted track. In this case there is no data, and no track information block for this track in the image file!
Actual length of track data = (high byte of track length) * 256
Track length includes the size of the TRACK INFORMATION BLOCK (256 bytes)
The location of a Track Information Block for a chosen track is found by summing the sizes of all tracks up to the chosen track plus the size of the Disc Information Block (&100 bytes). The first track is at offset &100 in the disc image.

TRACK INFORMATION BLOCK


offset	description	bytes
00 - 0c	"Track-Info\r\n"	13
0d - 0f	unused	3
10	track number	1
11	side number	1
12 - 13	unused	2
14	sector size	1
15	number of sectors	1
16	GAP#3 length	1
17	filler byte	1
18 - xx	Sector Information List	xx

NOTES:

Identical to the original DSK format definition.

SECTOR INFORMATION LIST


offset	description	bytes
00	track (equivalent to C parameter in NEC765 commands)	1
01	side (equivalent to H parameter in NEC765 commands)	1
02	sector ID (equivalent to R parameter in NEC765 commands)	1
03	sector size (equivalent to N parameter in NEC765 commands)	1
04	FDC status register 1 (equivalent to NEC765 ST1 status register)	1
05	FDC status register 2 (equivalent to NEC765 ST2 status register)	1
06 - 07	actual data length in bytes	2

NOTES:

Identical to the original definition except for the addition of the sector data length. This value is in bytes and stored in little endian notation. (low byte followed by high byte)
The location of each sectors data is found by adding the size of the previous sectors, plus the size of the 256 byte header.
For 8k Sectors (N="6"), only 1800h bytes is stored.