A program that tests computer hardware and peripheral devices for correct operation. Some faults, known as hard faults, are relatively easy to find, and the diagnostic program will diagnose them correctly every time. Other faults, called soft faults, can be difficult to find, because they occur under specific circumstances rather than every time the memory location is tested.
Most computers run a simple set of system checks when the computer is first turned on. The PC tests are stored in ROM and are known as power-on self tests (POSTs). If a POST detects an error condition, the computer stops and displays an error message on the screen.