Run Length Limited

(storage)Run Length Limited - (RLL) The most popular scheme for encoding data on magnetic disks. RLL packs up to 50% more data on a disk than MFM.

IBM invented RLL encoding and used it in mainframe disk drives. During the late 1980s, PC hard disks began using RLL. Today, virtually every drive on the market uses some form of RLL.

Groups of bits are mapped to specific patterns of flux. The density of flux transitions is limited by the spatial resolution of the disk and frequency response of the head and electronics. However, transitions must be close enough to allow reliable clock recovery. RLL implementations vary according to the minimum and maximum allowed numbers of transition cells between transitions. For example, the most common variant today, RLL 1,7, can have a transition in every other cell and must have at least one transition every seven cells. The exact mapping from bits to transitions is essentially arbitrary.

Other schemes include GCR, FM, Modified Frequency Modulation (MFM). See also: PRML.

run a risk
run across
run afoul
run aground
run along
run around
run away
run bases
run batted in
run by
run down
run dry
run for
run into
-- Run Length Limited --
run low
Run of stairs
run off
run on
run out
run over
run roughshod
run short
Run steel
run through
run time
run up
run-length encoding
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