Changes
/* FDC Track Format */
In MFM 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 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.
The trick is that the FDC cannot unmagnetize portions of a sector or leave portions of a sector unmagnetized to recreate this effect on another floppy disk.
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=== Error Detection ===
The FDC765 happens to use the exact same algorithm for error detection as the one the Amstrad firmware uses to check each 256-byte tape segments.
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).
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