MFM

MFM is an acronym for "Modified Frequency Modulation" and is a data encoding used on floppy discs. It is used by the NEC 765.

MFM is stored as flux transitions on the disc. The flux transitions are converted by the read/write head into a 1 or 0.

MFM stores data as a stream of cells which contain a data bit and a clock bit. The data separator synchronises to a continuous pattern of zeros, and when synced, will transmit data and clock bits to the FDC as separate signals.

The value of a clock bit is the NOR of the previous data bit and the current data bit. Data is transmitted starting with bit 7 down to bit 0.

e.g. <clock for bit 7> <bit 7> <clock for bit 6> <bit 6> <clock for bit 5> <bit 5>...

<clock for bit 5> = <bit 6> EOR <bit 5>

Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).