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5.2 / ERROR CORRECTION 169

5.2 ERROR CORRECTION

A semiconductor memory system is subject to errors. These can be categorized as
hard failures and soft errors. A hard failure is a permanent physical defect so that
the memory cell or cells affected cannot reliably store data but become stuck at 0 or
1 or switch erratically between 0 and 1. Hard errors can be caused by harsh environ-
mental abuse, manufacturing defects, and wear.A soft error is a random, nondestruc-
tive event that alters the contents of one or more memory cells without damaging the
memory. Soft errors can be caused by power supply problems or alpha particles.
These particles result from radioactive decay and are distressingly common because
radioactive nuclei are found in small quantities in nearly all materials. Both hard
and soft errors are clearly undesirable, and most modern main memory systems in-
clude logic for both detecting and correcting errors.
Figure 5.7 illustrates in general terms how the process is carried out. When
data are to be read into memory, a calculation, depicted as a function f, is performed
on the data to produce a code. Both the code and the data are stored. Thus, if an
M-bit word of data is to be stored and the code is of length K bits, then the actual
size of the stored word is M + K bits.
When the previously stored word is read out,the code is used to detect and pos-
sibly correct errors. A new set of K code bits is generated from the M data bits and
compared with the fetched code bits.The comparison yields one of three results:

• No errors are detected.The fetched data bits are sent out.
• An error is detected, and it is possible to correct the error. The data bits plus
error correction bits are fed into a corrector, which produces a corrected set of
M bits to be sent out.
• An error is detected,but it is not possible to correct it.This condition is reported.

Codes that operate in this fashion are referred to as error-correcting codes.A code
is characterized by the number of bit errors in a word that it can correct and detect.

Error signal

Data out M
Corrector

Data in M M K
f
K Memory K Compare
f

Figure 5.7 Error-Correcting Code Function