* The ID Address Mark (IDAM) marks the beginning of a sector's header.
* The Data Address Mark (DATA AM) marks the beginning of the actual data in a sector.
* Gaps are necessary to accommodate fluctuations in rotating speed due to mechanical factors.
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.