Modified Frequency Modulation (MFM) is a method of recording digital data on magnetic media. In the olden days of sub-100 Megabyte hard drives, most used MFM to record the data onto the disks. MFM used a set of rules that specified the way data was represented as magnetic flux changes. These rules were used to pack as many bits of data in the smallest amount of media area. MFM was self-clocking, and the controller was able to sync automatically to the data stream. Other methods of recording data was RLL and, to a lesser extent, NRZ.