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There’s a lot that goes into booting up a computer system. When a computer is switched on, it has to run some tasks and processes before the desktop window can appear. The bootup steps involved can be broadly divided into five processes which will be discussed in this article.
For a system to boot successfully, it must pass through these five processes. It typically involves some quick checks of the hardware and pre-emptive launch of the operating system, and then, transfer of control to the user through the operating system. Let’s get more detailed information on these processes.
1. Hardware Start-Up/Power Up
This is the first step in any computer boot-up process. It is also the only step that is manually initiated by the operator/user. The operator supplies power to the system from an external power source, and on pressing the power button, power is supplied to the motherboard where it reaches the CPU and ROM. The CPU executes some start-up codes in ROM which then triggers the BIOS to run POST and subsequent booting processes.
2. Power-On Self-Test
This process can also be called POST. It is when the system BIOs does test checks on all hardware connected to the computer. This process is designed by computer manufactures; on booting a computer, it doesn’t recognize the operating system right away, instead, control is given to the BIOS to test run the hardware and launch the operating system, be it Windows or Linux.
The code that handles this process is located in the BIOS chip, some kind of hardware too in the computer. If POST is successful, the BIOS automatically starts executing the next boot process.
3. Find a Boot Device or I/O System
Some experts would simply describe this process as when the BIOS loads up the input/output system in a computer. The I/O system defines the rules by which communications between the CPU and other parts of the computer occur.
It is essentially the system that links up the user, the operating system, and the hardware (CPU) together. The I/O system is located in a file named “io.sys” found in the boot device. It contains information needed to boot the operating system.
4. Loading the Operating System
On completing the other three processes in the defined order, the OS is loaded into RAM by the information present in “io.sys”. Then other codes or presets present in the operating system will be executed.
At this stage control of the computer hardware is being handed from the BIOS to the operating system. It doesn’t matter the operating system being run in the computer, as all operating systems will follow the same boot process.
5. Transferring of Control to the User
This is where complete control is transferred to the OS and the user can interact with it through the UI. However, before the actual handover, the boot sector loads a code into 0000:7C00. When all these are completed, the operating system screen is finally displayed and the process is complete.
Depending on the computer, the whole boot-up process wouldn’t take more than 20 seconds to be completed. In most systems, it’s done in less than 15 seconds. That being said, this process can also be further expanded showing the intricate processes that are involved, but this is not necessary for the purpose of this article.