The POST( Power On Self Test ) is a collection of test programs. The functions of these programs is testing the various functional units in the PC and verifying whether they are working properly or not. The POST programs are automatically executed by the PC whenever the PC is started or Reset. The idea behind this is gaining confidence with the hardware before using it.
The POST is stored in ROM BIOS on the motherboard. The microprocessor starts instruction processing from the memory address FFFF0 H from where the POST begins. The POST is a series of simple programs designed to test and catch faults in different hardware components and circuits.
The BIOS will begin its POST duties when the CPU is reset. The first memory location the CPU tries to execute is known as the reset vector. In the case of a hard reboot, the North bridge will direct this code fetch(request) to the BIOS located on the system flash memory. For a warm boot, the BIOS will be located in the proper place in RAM and the North bridge will direct the reset vector call to the RAM. If the tests are successful, the POST arranges for loading the operating system from a diskette. If any hardware error is noticed the fault to the user in four different ways.