What happens when you turn on a computer

In one line

When you press the power button, built-in firmware wakes up, checks the hardware, starts a small bootloader, and the bootloader loads the operating system into RAM.

Real-life analogy

Diagram

Step by step

Here is a puzzle. In lesson 1.3 we learned that RAM forgets everything when the power is off. And in lesson 1.4 we learned that the CPU runs instructions from RAM.

So when you switch on a computer, RAM is empty. Where does the CPU get its very first instruction? Let’s find out.

Step 1: power on

You press the power button. Electricity flows to the main board, the CPU, the RAM, the storage and the fans.

Step 2: firmware starts

The answer to our puzzle: every computer has a small program called firmware, stored on a chip on the main board. Unlike RAM, that chip keeps its contents without power.

The CPU is built to start by running this firmware. On most modern PCs it is called UEFI. Older PCs used BIOS, and many people still say “BIOS” for both.

Step 3: hardware check

The firmware runs a quick check of the hardware, often called POST (power-on self-test). It makes sure the essential parts are present and working, and gets them ready to use.

If something important is missing or broken, the computer may stop here, sometimes with beeps or an error message.

Step 4: find and start the bootloader

Next, the firmware looks in storage for a small program called the bootloader. Its only job is to start the operating system.

On Windows, this is the Windows Boot Manager. On Linux, a common one is GRUB.

Step 5: the operating system loads into RAM

The bootloader finds the core of the operating system, called the kernel, in storage and copies it into RAM. Then it hands over control.

The kernel sets up the rest of the system: it starts the drivers (small programs that know how to talk to each piece of hardware), and then starts the background services.

Step 6: ready to use

Finally you see the login screen or home screen. The manager is in, the counter is set up, and the restaurant is open. From here, everything works the way we saw in lesson 1.4.

Key takeaways

• At power-on, RAM is empty, so the first instructions come from firmware (UEFI or BIOS) on a chip that remembers.
• The firmware checks the hardware (POST), then starts the bootloader.
• The bootloader loads the operating system’s kernel into RAM and hands over control.
• The operating system starts drivers and services, and then the computer is ready to use.

Recap: unlock, lights on, check the kitchen, follow the checklist, set up the counter, open. Power, firmware, hardware check, bootloader, operating system, ready.

Now you understand what happens between pressing the power button and seeing your screen. That completes Module 1: you now know how a computer works inside. Next, we will connect computers together and look at how the Internet works.