* In MFM encoding, IDAM and DAM 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.
* Gaps are necessary to accommodate variations in rotation speed between different drives and avoid overlapping.
The main competitor of the µPD765 FDC chip on the market was the WD179x FDC chip family. Its main differences are:
* the 765 has 3 scan commands, while the 179x has none.
* the 765 has 5 status registers, while the 179x only has one.
* the 765 has multi-byte "command phase" and "result phase", while commands are issued to the 179x by writing a single byte to the Command Register, after having set parameters in its dedicated registers. After the command completes, the Status Register contains all the status information in a single byte.
* the 765 has drive select pins to directly select one of four floppy drives, while the 179x has none. Instead, external logic must be used to handle multiple drives.
* 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.
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