Modified Frequency Modulation, commonly MFM, is a line coding scheme used to encode information on most floppy disk formats, which include the floppy disk formats used in most CP/M machines as well as PCs running DOS. In telecommunication, a line code is a code chosen for use within a communications system for transmission purposes. ... A floppy disk is a data storage device that is composed of a disk of thin, flexible (floppy) magnetic storage medium encased in a square or rectangular plastic shell. ... CP/M (Command Processor for Microcomputers) was an operating system for Intel 8080/85 and Zilog Z80 based microcomputers. ... This article or section does not cite its references or sources. ... Instructions on how to use the directory command. ...

MFM is a modification to the original FM (frequency modulation) scheme for encoding data on single-density floppy disks. Because the minimum spacing between flux transitions is a property of the disk and head design, MFM, which guarantees at most one flux transition per data bit, can be written at higher density than FM, which can require two transitions per data bit. It is used with a data rate of 250-500 kbit/s (500-1000 kbit/s encoded) on industry standard 5¼" and 3½" ordinary and high density diskettes. MFM was also used in early hard disk designs, before the advent of more efficient types of Run Length Limited (RLL) coding. Except for the steadily disappearing 1.44 MB floppy disk drives, MFM encoding is obsolete. Frequency modulation (FM) is a form of modulation which represents information as variations in the instantaneous frequency of a carrier wave. ... In telecommunications and computing, bit rate (sometimes written bitrate) is the frequency at which bits are passing a given (physical or metaphorical) point. It is quantified using the bit per second (bit/s) unit. ... Typical hard drives of the mid-1990s. ... Run Length Limited codes, or RLL codes are widely used in hard disk drives (and notably digital optical discs, such as CD, DVD and BluRay disc) to prevent long stretches of no transitions, and therefore decoding uncertainty, from creeping in. ...

Coding

MFM encoding can be thought of has having data bits separated by clock bits. The basic encoding rule is that (x, y) encodes to (x, x NOR y, y). On average this means that each data bit is encoded as two bits on disk, but some delimiters are required at the beginning and end of a sequence, so this limit is never quite reached in practice.

Note that the surrounding clock bits are sometimes known, but sometimes require knowledge of the adjacent data bits. A longer example:

 Data: 0 0 0 1 1 0 1 1 Encoded: ?0101001010001010 

(The bold bits are the data bits, the others are the clock bits.)

Notice that there is a minimum of 1 zero bit between adjacent ones (there are never two adjacent one bits), and the maximum number of zeros in a row is 3. Thus, MFM is a (1,3) RLL code.

This bit stream is then NRZI encoded to be written to disk, a 1 bit representing a magnetic transition, and a 0 bit no transition. Categories: Stub ...

A special "sync mark" is used to allow the disk controller to figure out where the data starts. This sync mark has two important properties: it has no runs of zeros shorter than 1 or longer than 3 (i.e. it follows the (1,3) RLL rules), and it will never occur in any bit position in any encoded data stream. The sync mark used is called an 'A1 sync' since it is similar to the encoding of the hexadecimal value A1 (10100001). In mathematics and computer science, hexadecimal, base-16, or simply hex, is a numeral system with a radix, or base, of 16, usually written using the symbols 0–9 and A–F, or a–f. ...

 Data: 1 0 1 0 0 0 0 1 Encoded: 100010010101001 Sync Mark: 100010010001001 ^ Missing clock bit 

See also

• Group Code Recording

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL. Group Code Recording (GCR) is a floppy disk data encoding format used by the Apple II and Commodore Business Machines in the 5¼ disk drives for their 8-bit computers (the best-known drives being the Disk II for the Apple II family and the Commodore 1541, used with the... The Free On-line Dictionary of Computing (FOLDOC) is an online, searchable encyclopedic dictionary of computing subjects. ... GNU logo (similar in appearance to a gnu) The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free content, designed by the Free Software Foundation (FSF) for the GNU project. ...