Booting has to happen in small steps because at the point of switch-on the machine is effectively dumb. It does not know about any hard disks it may contain, or any peripherals such as the screen, keyboard, mouse - absolutely nothing.

There is a small program called the BIOS (Basic Input Output System) stored in non-volatile memory (ROM) on the motherboard. It controls the start-up sequence of the computer and also configures hardware devices such as the printer, monitor etc.

The first thing that happens is the power supply fires up and once it is producing a steady output, it will send a 'Reset' command to the CPU. The CPU's reset command has been hard-coded by its designers to execute an instruction at a specific location within the BIOS chip. This location contains a 'Jump' command that points it to the starting address of the BIOS start-up program. Once the CPU has started executing the start-up program, booting is underway.

The BIOS will now do some basic system checks. Together, these checks are called POST short for "Power On Self Test". These checks include:

System memory is OK
System clock / timer is running
Processor is OK
Keyboard is present
Screen display memory is working
BIOS is not corrupted

This happens very quickly, but should there be a problem, the BIOS will emit beeps either from the computer's internal speaker or perhaps a small beeper on the motherboard itself. After all, at this point not even the screen is active. These 'beep codes' inform you of the problem. Each BIOS maker has a different set of beep codes.

Hopefully, the POST has passed all its checks, and so, lets move on to the next phase.

Challenge see if you can find out one extra fact on this topic that we haven't already told you

Click on this link: Booting Up