3. First steps

The first thing that happens when the 'on' switch is pressed is that the power supply fires up.

A power supply can take a second or more to become stable. Once it is producing a steady output, it sends a 'Hardware Reset' signal to the CPU. This signal travels along a wire directly to the CPU (it is part of the control bus).

The CPU Hardware Reset command is 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. This first instruction is loaded into the CPU program counter and booting is underway.

At this point, there is nothing in RAM main memory and the computer knows nothing about its own hardware.