Every time the computer is powered on, the BIOS immediately performs a number of tasks from initializing the microprocessor to initializing and testing hardware to starting the operating system. Here is a sequence of steps of how the BIOS boots:
1.- When power is applied to the system, the power supply will generate a power good signal which is received by the motherboard timer. When the timer receives this signal, it stops forcing a reset signal to the CPU and the CPU begins processing instructions.
2.- The very first instruction performed by a CPU is to read the contents of a specific memory address that is preprogrammed into the CPU. Usually this address is FFFF:0000h. The code that the processor reads is actually a jump command (JMP) telling the processor where to go in memory to read the BIOS ROM.
3.- The POST (Power-On Diagnostic Test) is initialized and verify that all the critical components are functioning. The BIOS starts with a series of tests of the motherboard hardware. The order in which these tests are performed varies from motherboard to motherboard.
4.- When the POST sequence are completed, you are given the opportunity to access the system configuration utility. This utility allows you to configure and modify a range of features. These BIOS features control every aspect of the computer, from the speed at which the processor runs to the transfer mode of the hard disk.
5.- After the short delay, the BIOS will begin searching for an operating system. The BIOS will look for a DOS Volume Boot Sector on a valid master boot sector on all available drives. This can be anything from a hard disk to a CD-ROM drive.
6.- The master boot sector turns over the booting process to the operating system by loading the operating system’s boot sector. The operating system then starts loading up its core files.
7.- The core BIOS drivers are standardized and every piece of hardware is compatible with it.
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