Changes
/* FDC Track Format */
* There are 2 types of sectors based on their DATA AM value. Data sectors are marked by an FB byte, while deleted sectors are marked by an F8 byte.
* Gaps are necessary to accommodate variations in rotation speed between different drives and avoid overlapping.
In MFM (Modified Frequency Modulation) encoding, the IDAM and DATA AM are always preceded by three A1 bytes to help the FDC lock onto the data stream after a gap and accurately read the following datas. This is needed because MFM is more compact and harder to read than earlier encoding methods.
The CRC error-detecting code 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).
At ~300 RPM, an MFM track contains about 6250 bytes (200 ms per track / 32 µs per byte).
The main competitor of the µPD765 FDC chip on the market was the WD179x FDC chip family. The primary difference between 765 and 179x controllers is that the 765 only does standard track formats (preamble, marks and data fields), while the 179x will write anything you tell it in the write track (formatting a track) mode.
