What are the operation performed when the computer is turned on?

When a computer is switched on, the power supply is taken out of standby mode and runs up to full power mode. Voltage is applied to the motherboard and peripheral units. At the same time a reset pulse is applied to the motherboard. This will start the Central Processor Unit, which applies clock pulses to the onboard chips.

On the motherboard the BiOS chip starts its program.

The BiOS checks the available amount of memory and checks that the keyboard and mouse are connected. This is known as the POST (Power On Self Test).

It then checks for available long term memory storage, such as disk drives and removable drives.

Depending on how it is set up, it will choose a drive and check whether a valid operating system is available.

If an operating system is available and bootable, this program will now run.

Control is now taken over by the O/S on the drive.

What happens next will depend on the O/S installed.