Hardware Reference
In-Depth Information
borneportablecomputer,forexample,usedthesesingle-densityfloppydiskdrives,which
stored about 80KB of data on a single disk. Although it was popular until the late 1970s,
FM encoding is no longer used.
Modified FM Encoding
Modified Frequency Modulation encoding was devised to reduce the number of flux re-
versals used in the original FM encoding scheme and, thus, to pack more data onto the
disk. MFM encoding minimizes the use of clock transitions, leaving more room for the
data. It records clock transitions only when a stored 0 bit is preceded by another 0 bit;
in all other cases, a clock transition is not required. Because MFM minimizes the use of
clock transitions, it can double the clock frequency used by FM encoding, which enables
it to store twice as many data bits in the same number of flux transitions.
Because MFM encoding writes twice as many data bits by using the same number of flux
reversalsasFM,theclockspeedofthedataisdoubledandthedriveactuallyseesthesame
number oftotal fluxreversals aswith FM.This means adrive usingMFMencoding reads
and writes data at twice the speed of FM, even though the drive sees the flux reversals
arriving at the same frequency as in FM.
Because it is twice as efficient as FM encoding, MFM encoding also has been called
double-density recording
. MFM is used in virtually all PC floppy disk drives today and
was used in nearly all PC hard disks for a number of years. Today, virtually all hard disks
use variations of RLL encoding, which provides even greater efficiency than MFM.
Table 8.2
shows the data bit-to-flux reversal translation in MFM encoding.
Table 8.2 MFM Data-to-Flux Transition Encoding
Run Length Limited Encoding
Today's most popular encoding scheme for hard disks, called
Run Length Limited
, packs
up to twice the information on a given disk than MFM does and three times as much in-
formation as FM. In RLL encoding, the drive combines groups of bits into a unit to gen-
erate specific patterns of flux reversals. Because the clock and data signals are combined
