[[Category:Cpctech.org]]<div style="border: 1px solid rgb(228, 222, 222); margin: 0px 0px 5px; padding: 0.5em 1em; background-color: rgb(249, 249, 249);"><center> '''''This artikel article originally came from Kevin Thackers' archive at [http://www.cpctech.org.uk http://www.cpctech.org.uk].''''' </center></div>
= Disk image file format =
== Disc Information block ==
The "Disc Information block" is always at offset 0 in the disk image file. If track data exists, then this will immediatly immediately follow the Disc Information Block and will start at offset &100 in the disc image file.
The "Disc Information block" has the following structure:
! bytes
|-
| 00 - 0c 0b
| "Track-Info\r\n"
| 1312
|-
| 0d 0c - 0f
| unused
| 34
|-
| 10
*"number of sectors" is used to identify the number of valid entries in the sector information list.
*"sector size" parameter is used to calculate the location of each sector's data. Therefore, The data allocated for each sector must be the same. 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. Note that sector size's unit is 256-bytes. So, a sector size of 2, translates to 256 << (2-1), or 512 bytes.
*Sector data always follows Track Information Block at offset &100 from the start of the track information block.
*Sector data is stored in the same order as the sectors in the sector info block.
**b0 MA (Missing Address Mark)
*The following bits are used from NEC765 status register 2:
**b5 b6 CM (Control Mark)
**b5 DD (Data Error in Data field)
**b0 MD (Missing address Mark in Data field)
! bytes
|-
| 00 - 0c 0b
| "Track-Info\r\n"
| 1312
|-
| 0d 0c - 0f
| unused
| 34
|-
| 10
{| border="1"
|-
! Date Data rate
! description
|-
{| border="1"
|-
! Date rate Mode
! description
|-
:3. How to determine the actual rate. 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.:Also, the actual rate is twice longer than indicated in the datasheet as the NEC765 clock runs at 4MHz on CPC instead of 8MHz for the datasheet.
:Examples:
! Actual rate
|-
| 4Mhz 4MHz
| FM
| 4us 8µs per bit
|-
| 4Mhz 4MHz
| MFM
| 2us 4µs per bit
|}