* The Data Address Mark (DATA AM) marks the beginning of the actual data in a sector.
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).