The Free On-line Dictionary of Computing (30 December 2018):

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.

   (http://cma.zdnet.com/book/upgraderepair/ch14/ch14.htm).

   (2003-08-12)