* The ID Address Mark (IDAM) marks the beginning of a sector's header on a floppy disk.
* The Data Address Mark (DATA AM) marks the beginning of the actual data in a sector on the disk.
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 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).