This document describes the standard disk image format. It has the file +extension ".DSK". + +
The "Disc Information block" is always at offset 0 in the disk image file. +If track data exists, then this will immediatly follow the Disc Information Block and will start at +offset &100 in the disc image file. + +
The "Disc Information block" has the following structure: +
| offset | +description | +bytes + |
|---|---|---|
| 00-21 | +"MV - CPCEMU Disk-File\r\nDisk-Info\r\n" | +34 | +
| 22-2f | +name of creator | +14 | +
| 30 | +number of tracks | +1 | +
| 31 | +number of sides | +1 | +
| 32-33 | +size of a track + (little endian; low byte followed by high byte) | +2 | +
| 34-ff | +not used (0) | +204 | +
Notes: +
The tracks are always ordered in this way regardless of the disc-format described by the disc +image. +
Each Track Block comprises a Track Information Block and sector data. +The sector data is always at an offset of &100 bytes from the start of the +track block. The data for the next track in the disc image immediatly +follows the data for the current track. + +
The first Track Block is located at offset &100 in the disk image file. +The track block starts with the Track Information Block and has this form. + +
| 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: +
If the track contains different sized sectors, then the data allocated must be the size +of the biggest sector. The "sector size" parameter is used to calculate +the location of the sector data. +
| 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 | +notused (0) | +2 | +
Notes: +
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. + +
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. +
The "DISK INFORMATION BLOCK" is always located at offset 0 in the disk image file, and has +the following structure: + +
| offset | +description | +bytes + |
|---|---|---|
| 00 - 21 | +"EXTENDED CPC DSK File\r\nDisk-Info\r\n" | +34 | +
| 22 - 2f | +name of creator (utility/emulator) | +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: +
| offset | +description | +bytes | +
|---|---|---|
| 01 | +high byte of track 0 length (equivalent to track length/256) | +1 | +
| ... | +... | +... | +
NOTES: +
| 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: +
| 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: +
Extension to TRACK INFORMATION BLOCK: +
| offset | +description | +bytes | +
|---|---|---|
| 12 | +Data rate. (See note 1 and note 3) | +1 | +
| 13 | +Recording mode. (See note 2 and note 3) | +1 | +
Notes: +
| Date rate | +description | +
|---|---|
| 0 | +Unknown. | +
| 1 | +Single or double density | +
| 2 | +High Density | +
| 3 | +Extended density | +
Existing files should have zeroes in these bytes; hence the use of 0 for unknown. + +
| Date rate | +description | +
|---|---|
| 0 | +Unknown. | +
| 1 | +FM | +
| 2 | +MFM | +
Existing files should have zeroes in these bytes; hence the use of 0 for unknown. + +
The NEC765 floppy disc controller is supplied with a single clock. When reading from and writing to a disc using the NEC765 you can choose FM or MFM +recording modes. Use of these modes and the clock into the NEC765 define the final rate at which the data is written to the disc. +
When FM recording mode is used, data is read from or written to at a rate which is double that of when MFM is used. The time for each bit will be twice the time for MFM. +
Examples: +
| NEC765 Clock | +FM/MFM | +Actual rate | +
|---|---|---|
| 4Mhz | +FM | +4us per bit | +
| 4Mhz | +MFM | +2us per bit | +
For sector size N="7" the full 16K will be stored. It is assumed that sector sizes are defined +as 3 bits only, so that a sector size of N="8" is equivalent to N="0". + +
Some copy protections have what is described as 'weak/random' data. Each time the sector is read one or more bytes will change, the value may be random between consecutive reads of the same sector. +
To support these formats the following extension has been proposed. +
Where a sector has weak/random data, there are multiple copies stored. The actual sector size field in the SECTOR INFORMATION LIST describes the size of all the copies. +To determine if a sector has multiple copies then compare the actual sector size field to the size defined by the N parameter. For multiple copies the actual sector size field will have a value which is a multiple of the size defined by the N parameter. +The emulator should then choose which copy of the sector it should return on each read. + + + + +