How does a computer work?

Powering on the computer
Computer power supply. When you first press the power button, the computer sends a signal to the computer power supply, which converts the alternating current (AC) to a direct current (DC). This supplies the computer and its components with the proper amount of voltage and electricity.
Once the computer and its components have received ample power and the power supply reports no errors it sends a signal (using transistors) to the motherboard and the computer processor (CPU). While this is happening, the processor will clear any leftover data in the memory registers and give the CPU program counter a F000 hexadecimal number. This number is the location of the first instruction and tells the CPU that it's ready to process the instructions contained in the basic input/output system (BIOS).

BIOS and the POST
When the computer first looks at the BIOS, it begins the power-on self-test (POST) sequence to make sure the components in the computer are present and functioning properly. If the computer does not pass any of these tests, it will encounter an irregular POST. An irregular POST is a beep code that is different from the standard one or two beeps. For example, an irregular POST could generate no beeps at all or a combination of different beeps to indicate the cause of the failure.
If the computer passes POST, it looks at the first 64-bytes of memory located in the CMOS chip, which is kept alive by the CMOS battery even when the computer is turned off. This chip contains information such as the system time and date and information about all the hardware installed in your computer.