The CRC is initialised to FFFF. It is updated byte by byte and uses the CCITT-CRC16 algorithm. It is written after the ID and data fields of each sector in big-endian format (high byte first and then low byte).
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== HD floppy disks ==
It is theoretically possible to use HD floppy disks (1.44MB) on CPC by using either a Gotek drive or Amiga HD floppy drives spinning at 150rpm. However, nobody has ever written a CPC DOS to make use of it.
On a real floppy drive, if you want to fit more sectors on the same track, you have two options: you can make the FDC run faster, or you can make the disk spin slower.
Normally, HD disks use a faster FDC (with an automatic switch between the slow and fast modes). But, for example on Amiga systems with HD drives, they used floppy drives that spin slower for HD disks (150 rotations per minutes instead of 300) and this allow to use the same controller as before. By the time the floppy has done one complete rotation, you can write 18 sectors instead of just 9.
With a real floppy drive, this requires some mechanical modifications, and you may get into trouble if the spinning speed is not fast enough. But with the Gotek, this is not a problem. You can have as many sectors as you want, and the Gotek will generate the index pulse (simulating the floppy completing a turn) after it has sent them all. So it is essentially emulating a floppy that turns very slowly. The FDC has no problems handling that, but you may need to be a bit more relaxed than usual with the timeouts, as it will be some time before the sector you need will pass in front of the drive head, and the index pulse will also be slower than usual.
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