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RLL Encoding

Today's most popular encoding scheme for hard disks, called Run Length Limited (RLL), packs up to 50% more information on a given disk than even MFM does and three times as much information as FM. In RLL encoding, groups of bits are taken as a unit and combined to generate specific patterns of flux reversals. By combining the clock and data in these patterns, the clock rate can be further increased while maintaining the same basic distance between the flux transitions on the disk.

During the late 1980s, the PC hard disk industry began using RLL encoding schemes to increase the storage capabilities of PC hard disks. Today, virtually every drive on the market uses some form of RLL encoding.

Instead of encoding a single bit, RLL normally encodes a group of data bits at a time. The term Run Length Limited is derived from the two primary specifications of these codes, which is the minimum number (the run length) and maximum number (the run limit) of transition cells allowed between two actual flux transitions. Several schemes can be achieved by changing the length and limit parameters, but only two have achieved any real popularity: RLL 2,7 and RLL 1,7.

Even FM and MFM encoding can be expressed as a form of RLL. FM can be called RLL 0,1 because there can be as few as zero and as many as one transition cell separating two flux transitions. MFM can be called RLL 1,3 because as few as one and as many as three transition cells can separate two flux transitions. Although these codes can be expressed in RLL form, it is not common to do so.

RLL 2,7 initially was the most popular RLL variation because it offers a high density ratio with a transition detection window the same relative size as that in MFM. This method allows for high storage density with fairly good reliability. In very high-capacity drives, however, RLL 2,7 did not prove to be reliable enough. Most of today's highest-capacity drives use RLL 1,7 encoding, which offers a density ratio 1.27 times that of MFM and a larger transition detection window relative to MFM. Because of the larger relative window size within which a transition can be detected, RLL 1,7 is a more forgiving and more reliable code, which is required when media and head technology are being pushed to their limits.

Another little-used RLL variation called RLL 3,9 -- sometimes called Advanced RLL (ARLL) -- allowed an even higher density ratio than RLL 2,7. Unfortunately, reliability suffered too greatly under the RLL 3,9 scheme. The method was used by only a few controller companies that have all but disappeared.


A disk track is too large to manage effectively as a single storage unit. Many disk tracks can store 50,000 or more bytes of data, which is very inefficient for storing small files. For that reason, a disk track is divided into several numbered divisions known as sectors. These sectors represent slices of the track.

Different types of disk drives and disks split tracks into different numbers of sectors, depending on the density of the tracks. Hard disks usually can use 17 to 100 or more sectors per track. Sectors created by standard formatting procedures on PC systems have a capacity of 512 bytes, but this capacity may change in the future. Sectors are numbered on a track starting with 1, unlike the heads or cylinders which are numbered starting with 0.

When a disk is formatted, additional ID areas are created on the disk for the disk controller to use for sector numbering and identifying the start and end of each sector. These areas precede and follow each sector's data area, which accounts for the difference between a disk's unformatted and formatted capacities. These sector headers, inter-sector gaps, and so on, are independent of the operating system, file system, or files stored on the drive. For example, an older 38MB hard disk has a capacity of only 32MB when it is formatted. Modern IDE (Integrated Drive Electronics) and SCSI (Small Computer System Interface) hard drives are preformatted, so the manufacturers now only advertise formatted capacity. Even so, nearly all drives use some reserved space for managing the data that can be stored on the drive.

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