ERROR DETECTION AND CORRECTION
The dynamic physical interaction of the electrical signals affecting the data pam of a memory unit may cause occasional errors in storing and retrieving the binary information. The reliability of a memory unit may be improved by employing error-detecting and error-correcting codes. The most common error detection scheme is the parity bit' A parity bit is generated and stored along with the data word in memory. The parity of the word is checked after reading it from memory. The data word is accepted if the parity of the bits read out is correct. If the parity checked results in an inversion. an error is detected, but it cannot be corrected. An error-correcung code generates multiple parity check bits thai are stored \\ ith the data word in memory. Each cbcck bit is a parity avera group of bits in the data word.When the word is read back from memory. The associated parity bits are also read from mernot) and compared with a new Hamming Code
One of the most common error-correcting codes used in RAMs was devised by R. W. Hamming.
In the Hamming code. k. parity bits are added 10an n-bit data word. forming a new word of n + k bits. The hit positions are numbered in sequence from I to n + k, These positions numbered as a power of2 arc reserved for the parity bits. The remaining bits are the data bits.
The code can beused with words of any length. Before giving the general characteristics of the code. we will illustrate its operation with a data word of eight bib .
Consider. for example. the 8-bil data word I10001no.We include ~ parity bits with the 8-bit word and arrange the 12 bits as follows: Bil position: I PI 2 3 4 5 6 7 8 9 10 II 12 Pl IP4 1 0 0 f\ 0 1 0 0