Editing Modified frequency modulation (section)

==Overall format==
{{Main|Disk formatting}}
FM and MFM are used to indicate the position of individual bits in the original data, but the data does not in itself have any higher level of organization like "files". This is the purpose of the format of the disk. Disks are typically formatted into fixed-sized sectors which contain additional header information to link them back to files.

In the IBM formats, the start of a sector's header information and the start of the data itself are indicated with special "sync mark", a pattern of 0s and 1s that cannot appear in the data itself. This is accomplished by not encoding this data using the FM or MFM encoding, making it easy for the driver to spot it. The sync mark that is commonly used in MFM encoding is known as the "A1 sync" since the data bits form the start of the [[hexadecimal]] value A1 (10100001), but the fifth clock bit is different from the normal encoding of the A1 byte.

 Data:      '''1 0 1 0 0 0 0 1'''
 Clock:      0 0 0 1 1 1 0
 Encoded:   '''1'''0'''0'''0'''1'''0'''0'''1'''0'''1'''0'''1'''0'''0'''1'''
 Sync clock: 0 0 0 1 '''0''' 1 0
 Sync Mark: 100010010'''0'''01001
                     ^ Missing clock bit