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What Happens Inside a Computer After Power Is Turned On?

Computer technician with 19 years in the industry. I hold an A+ Certification, as well as a Bachelors of Science in Information Technology.

Boot Screen


What Happens Inside the Machine?

You push the power button and you may hear one short beep or a continuous beep. What does this beep indicate? Why does the computer beep when it boots up and what does it mean?

This Is What Happens

From the moment you push your power button, it perceives a set of automatic instructions that tells it what to do. Or better put, it goes through a series of system tests that the computer must pass in order for it to boot to the OS. Computers have four basic functions, and they are:

  1. Input (data)
  2. Process
  3. Storage
  4. Output

There are different devices or components that are assigned to carry out different functions. With this being said, a functional PC requires that the processes of software run on top of the hardware in this order:

  1. Hardware
  2. Bios
  3. Operating System
  4. Windows Management (Desktop)
  5. Applications

Your Bios (Basic Input/Output System) provides your PC with the necessary information to start.

Bios Screen


Accessing Bios

I could really go technical here and say that your Bios is a set of firmware instructions that control input and output parameters... but I'll keep it simple. Typically, to access the Bios, you can read the first bootup screen and it will give you the Bios access key. The Bios access key could vary based on manufacture and the type of computer you have. For example, the first screen above tells you to press Del to enter setup. This is where all of your setup configurations are, user configurable data such as, time, date, boot sequence, and installed components.

Bios is utilized to:

  • Check installed components
  • Enable or disable devices
  • Change boot sequence
  • Enable/Disable virtualization

So when you turn on your computer, the Bios performs what is known as hardware initialization and additionally supplies run-time services for the OS and applications.

Wait There's More

Okay! Now that we've discussed what happens upon startup, let's discuss what happens during startup or boot.

POST (Power On Self Test) is a basic or standard test that a computer runs during the boot process. POST will check if:

  • CPU is functional
  • RAM and graphics are accessible
  • If the keyboard is working
  • That Bios isn't corrupt

Post Bios


POST Workflow

When the devices and parameters are checked and returns no issue, then the PC will boot to the Operating System. However, if an error or issue is detected, the PC will display an error or give a beep code.

Tip: No beep could mean an issue with power supply or no power.

  • One short beep: System Passed Post
  • Continuous beep: Issue with power supply, motherboard, or keyboard.

What Is Post Doing


Putting It Together

As soon as your computer comes on, the Bios is then automatically awakened. The Bios sits on a (ROM) chip called CMOS (Complementary Metal Oxide Semiconductor). It holds the user configurable data like we discussed before: time, date, and system setup parameters.



It Is Sometimes Called RTC

CMOS is sometimes referred to as Real Time Clock (RTC). CMOS is onboard, which means motherboard. It's a battery powered semiconductor chip inside of a PC which stores information.

It is also referred to as CMOS RAM due to the fact that it utilizes a volatile but low-powered complementary metal-oxide-semiconductor SRAM. It doesn't need a refresh due to static ram usage and is often referred to as non-volatile. It's powered with a lithium battery, which additionally powers the real-time clock (RTC). However, if the CMOS utilizes SRAM and has non volatile memory, why would it need a source of power? Because it's non-volatile, it should keep or remember its info, even with no power source.

Moving Forward

Moving forward, all of these things work together during power on and boot up. However, the Bios and Post processes are mainly responsible.

  1. Power On
  2. CMOS initializes
  3. Bios Awakes (time, date, system setup parameter, components)
  4. Post Runs (Checks if PC is functional, keyboard working, Bios not corrupt, ram and graphics.)
  5. System test passed boots to OS, System tests fail (beep code or error message).


Now that I've explained a few actions that your computer takes upon turning on the power, you should have a general understanding of how the bootup process works. There is much more I could tell you because the whole process is wonderfully amazing. But I decided to keep it simple rather than discussing each concept of the operation. I hope this information is very informative and helpful.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


Pseudo. on November 14, 2018:


Your steps are out of order.

The power button activates the power supply in the PC, sending power to the motherboard and other components.

The PC performs a power-on self-test (POST). The POST is a small computer program within the BIOS that checks for hardware failures. A single beep after the POST signals that everything's okay. Other beep sequences signal a hardware failure, and PC repair specialists compare these sequences with a chart to determine which component has failed.

The PC displays information on the attached monitor showing details about the boot process. These include the BIOS manufacturer and revision, processor specs, the amount of RAM installed, and the drives detected. Many PCs have replaced displaying this information with a splash screen showing the manufacturer's logo. You can turn off the splash screen in the BIOS settings if you'd rather see the text.

The BIOS attempts to access the first sector of the drive designated as the boot disk. The first sector is the first kilobytes of the disk in sequence, if the drive is read sequentially starting with the first available storage address.

The boot disk is typically the same hard disk or solid-state drive that contains your operating system. You can change the boot disk by configuring the BIOS or interrupting the boot process with a key sequence (often indicated on the boot screens).

The BIOS confirms there's a bootstrap loader, or boot loader, in that first sector of the boot disk, and it loads that boot loader into memory (RAM). The boot loader is a small program designed to find and launch the PC's operating system.

Once the boot loader is in memory, the BIOS hands over its work to the boot loader, which in turn begins loading the operating system into memory.

When the boot loader finishes its task, it turns control of the PC (handoff)over to the operating system.

Chad Crouch from South Africa on September 19, 2017:

Very much interesting. I am a tech fanatic and must say good job with this article.