Disk Format Definition/Meaning:
The formal of information recorded on magnetic disk, allowing a
system to recognize, control, and verify the data. There are two levels at which
formats are defined.
(a) The way in which the data stream is divided into separately addressable
portions, called sectors, with address marks and data marks to differentiate
between the different types of information within the sector, and with a
cyclic
redundancy check or error-correcting code also provided.
(b) The way in which the binary information is encoded as a pattern of magnetic
flux reversals.
Since recordings on disks are made as a bit serial stream on a single track at a
time, special provision has to be made to allow the reading system to maintain
synchronization. This is achieved by the encoding format, which either includes
clock pulses or encodes the data in such a way that there cannot be a case
where there is a succession of eight or more bit cells without a magnetic flux
transition occurring. The read electronics can maintain synchronization for
short periods without transitions. The common methods of encoding are as
follows.
Frequency modulation (FM; F2F) is a form of self-clocking recording.
The beginning of each bit cell is marked by a clock pulse recorded as a change
in the direction of the magnetic flux. If the cell is to represent a binary 1 a
second pulse or transition is written at the center of the cell, otherwise there
is no further change until the start of the next cell. If the frequency of the
clock is F then a stream of Is will result in a frequency of 2F (hence F2F
recording). In this form of recording the minimum separation between transitions
is half of one cell and the maximum is one cell. In modified frequency
modulation (MFM) a binary 1 is always represented by a transition at the center of a bit cell but there is not always a transition at the
boundary of the cell. A transition is written at the start of a bit cell only if
it is to represent a binary 0 and does not follow a binary 1. Thus the minimum
separation between transitions is one cell and the maximum is two cells. For the
same spacing of flux transitions the MFM method allows twice as many bits to be
encoded in a unit distance; it is thus sometimes referred to as a double-density
recording.
Modified modified frequency modulation (M2FM) is a modified form
of MFM that deletes flux transitions between two 0s if they are followed by a 1.
Run-length limited encoding is a form of NRZ (nonreturn to zero) recording in
which groups of data are mapped onto larger groups before recording. A
frequently used method known as OCR (group code recording) breaks the data
stream into groups of four bits and maps these onto five bit groups. Because of
the resulting redundancy, the five bit groups can be selected to limit the
number of consecutive 0s and thus control the maximum spacing between flux
transitions. Other similar codes are EIR (error indicating recording), a form of
four to six mapping that uses only the groups with odd parity, i.e. three or
five 1s, and 3PM (three-phase modulation), which has a minimum sequence of two
0s and a maximum of eleven. See also formatter.
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