Boot Sequence and POST
It is worth spending time looking at the sequence of events when starting a system.
The BIOS (Basic Input/Output System) is a collection of software utilities and programs that can be invoked by the operating system or application software to perform many hardware-related tasks.
The instructions supplied by the BIOS are what the PC executes during its power-on or boot-up sequences until it is able to fetch and execute instructions on its own. When the system starts the boot sequence, the BIOS starts a Power-On Self-Test, or POST, which verifies the data in the CMOS to the physical devices it can detect on the system.