Booting Process

What happens between the time that the user powers up the computer and when the icons appear on the desktop?

1. the CPU initializes itself, which is triggered by a series of clock ticks generated by the system clock
2.  Part of the CPU’s initialization is to look to the system’s ROM BIOS for its first instruction in the startup program
3. The ROM BIOS stores the first instruction, which is the instruction to run the power-on self test (POST), in a predetermined memory address
4. POST begins by checking the BIOS chip and then tests CMOS RAM
5. If the POST does not detect a battery failure, it then continues to initialize the CPU, checking the inventoried hardware devices (such as the video card), secondary storage devices, such ashard drives and floppy drives, ports and other hardware devices, such as thekeyboard and mouse, to ensure they are functioning properly
6. Once the POST has determined that all components are functioning properly and the CPU has successfully initialized, the BIOS looks for an OS to load.
7. The BIOS typically looks to the CMOS chip to tell it where to find the OS, and in most PCs, the OS loads from the C drive on the hard drive even though the BIOS has the capability to load the OS from a floppy disk, CD or ZIP drive. The order of drives that the CMOS looks to in order to locate the OS is called the boot sequence, which can be changed by altering the CMOS setup
8. A small routine available in ROM  called Boot Strap Loader  reads a special  load routine residing in the boot sector (track 0,Sector 0,)  of the hard disk  into  RAM..

This program loads the rest of the OS into memory.