CompTIA A+® Certification All-In-One For Dummies®, 2nd Edition

3.4. Understanding Error-Checking Memory

Two primary types of error-checking memory have been used in systems over the years. The following sections introduce you to these two types of error-checking memory. Be sure to become familiar with them for the exam.

3.4.1. Parity versus nonparity

Parity memory is a type of error-checking memory, which is memory that verifies the information stored in memory is what is actually read from memory at a later time. Nonparity memory, comparatively, is memory that does not perform any kind of error checking to ensure that the data written to memory is what is actually read when it is retrieved.

Here's how parity memory works. The two types of parity memory are odd and even. Both parity methods function the same way but differ in the sense of whether they look for an odd number of bits or an even number of bits. This discussion uses odd parity as the example.

With parity memory, for every byte (8 bits) of data written to memory, there is an additional ninth bit — the parity bit. When storing information to memory, the number of the enabled data bits (bits set to 1) written to memory are added up.

With odd parity, if an even number of data bits are enabled, the parity bit is set to 1 (enabled) so that there is an odd number of enabled bits in total written to memory. If the result of all the enabled data bits is odd, the parity bit is set to 0 (disabled) so that the odd number of enabled bits is retained.

After the parity bit has been set, the ...