Memory for disk arrays
If, for example, the amount is equal to four words (in binary record 100), you
can write that number in binary form, will require three-level, it means that
the size of contracts checklist is three words. Therefore, if you have four
discs to store data, the need for an additional three-disc for storing control
data. Similarly, if the seven disks for data (binary record 111) will need three
disk storage control words. If the figures given by the eight discs (binary
record 1000), the need to drive four-horse quiz information. Hamming code,
forming the word checklist, based on the use of porazryadnoy operations "excluding
" OR (XOR) (also used the name "disparity") - logs-agency unit gives XOR
operation with the noncoincidence operands (0 and 1) and zero when they
coincidence (0 and 0 or 1 and 1). The very word checklist, obtained by the
algorithm Hemminga - is a result of inversion porazryadnoy exclusive OR
operation rooms of the information bits words, the values of which are equal 1.
To illustrate, consider the original 1101 floor. The first (001), third (011)
and fourth (100) volume level is a unit of the word. It is therefore necessary
to hold porazryadnuyu exclusive OR operation for 100 = 110. 011 these rooms
bits: 001 The very same word checklist (Hamming code) is produced when
porazryadnom invertirovanii the result, that is equal to 001. When a newly
calculated data and code Hemminga compared with the source code. To compare the
two codes used porazryadnaya "exclusive OR operation." If the result of
comparison in all bits equal to zero, the reading is correct, otherwise its
value is room mistakenly taken basic-level code. Let us, for example, the
original floor well 1100000. Because the units are at the sixth (110) and
seventh (111) positions checklist layer of equals 110.
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