1. When you turn on your computer, the power supply distributes current to the motherboard and the other devices located in the system unit case.
2. The surge of electricity causes the CPU chip to reset itself and look to the BIOS chip for instructions on how to proceed. BIOS stands for Basic Input/Output System. The BIOS is a set of instructions that provides the interface between the operating system and the hardware devices. The BIOS is stored in a read-only memory (ROM) chip.
3. The BIOS chip begins a set of tests to make sure the equipment is working correctly. The tests are completed, the BIOS instructions are copied into memory where they can be executed faster than in ROM.
4. After the POST tests are successfully completed, the BIOS begins looking for the operatingsystem. Usually, it first looks in diskette drive A. If an operating system disk is not loaded in drive A, the BIOS looks on drive C, the drive letter usually given to the first hard drive.