In this text I will try to explain what is going on inside your computer when you turn it on.

When you have pushed the on button on your PC, it starts to work.

An electric signal follows a permanently programmed path to the CPU or the Central Processing Unit. In daily speech we call it the microprocessor. Companies such as Intel, AMD and Cyrix manufacture microprocessors. When the signal reaches the CPU, it clears leftover data from the chip's internal memory registers. The signal also resets a CPU register called the program to a specific hexadecimal number. If your computer is an AT or later, which most computers are today, the specific number is F000. This number in the program counter tells the CPU to address to the beginning of the boot program that is stored permanently in a set of ROM (read-only memory) chips in the PCs BIOS (basic input/output system).

The CPU uses the address F000 to find and start the ROM BIOS boot program.

Now the ROM BIOS boot program starts a series of systems checks called the Power-On Self-Test or POST. The CPU starts to check itself and the POST program by reading the code, which are placed at different locations and checking them against identical permanent records.

The CPU then sends signals over the system bus. The bus is the circuit on the motherboard that connect all components with each other. These signals are to make it sure that all components attached on the motherboard are all functioning.

The CPU checks the system's timer, which is important for making sure that all the PCs operations function are synchronised.

The POST checks the graphic memory on the display adapter and the video signals that control the display. The code in the display adapter is now integrated in the system's overall BIOS and memory configuration. It's first at this point that you

can see something on your screen.

The POST continues to check the RAM (Random Access Memory). The CPU writes data to each RAM chip, reads it and compares it with the data it sent to the chips

in the first place.

The CPU checks that your keyboard is attached and if any keys have been pressed.

The POST sends signals on the bus to any disk drives and listens for a response to determine what drives are available.

The POST compares its result with a record stored in a specific CMOS chip that are supplied by a battery. All earlier settings in the BIOS are stored in this CMOS chip.

Some systems have their own BIOS. The systems own BIOS will now recognise

and incorporate their BIOS as its own BIOS and memory use.

Your PC is now ready to load the Operating system.