parity

The first personal computers were mainly used for recreation and the consequences of any undetected error was slight. However, if a machine is used for business or any serious  purpose it has to incorporate some method of error detection. The simplest and most commonly used method is party checking. This involves adding and extra  bit to every byte of data stored so as to make the number of 1 bits even. This a called even parity checking as opposed to making the total number of 1 bits odd i.e odd parity For example, if the data is 011101100 then the parity bit is 1 because there are five 1s in the data and making the parity bit 1 makes the total six which is even. If the data is 10100000 then the parity bit is 0 because the total number of 1s is already even. Each time data is read from memory is parity is checked. If a single bit has changed, either a 0 turning in to 1 or a 1 into a 0, since the data was stored  then the total number of 1  bits will not be even  and a parity error will be detected

The problem with parity checking is that while it is certain to detect  a change in a single bit ,if two bits change then this  leaves the total  number of 1 bits even and a parity error will nit occur. However, in modern computer  systems the probability of a single bit error is so small that the probability of two such  errors in the same  memory location a vanishingly small. Indeed, such errors are so rate that it is arguable that parity checking itself is redundant.

As is takes eight bits to store a single byte, adding an extra bit for parity checking makes the total number of bits need equal to nine. This is the reason that 386/486  systems need nine  1MByte chips or 9xMByte SIPs or SIMMs for every Mbytes of memory you also have an additional 1MByte dedicated to parity checking! In same machines you can disable parity checking and make use of the unused chips to increase the amount of memory available. As transient memory errors are very rare, and the memory is  tested for permanent faults every time you switch the machine on,  you might consider this a reasonable trade-off.


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