Learn how a Computer Starts !

Welcome ! In this blog, I will explain how a computer starts or how basically boot process happen. Even though you will find it through many sources I will keep it simple. So Let's start.

It all starts with a Power button when you press the power button electricity is supplied to all the components. The steps that happen are :

Step - 1 BIOS STARTUP: BIOS stands for Basic INPUT OUTPUT SYSTEM Every computer has BIOS pre-programmed into its hardware. It’s different than an operating system. Operating systems can be installed, uninstalled, and updated long after you’ve bought the computer. Bios configurations are saved in Firmware which relies on ROM which is powered by CMOS Battery. Bios provide necessary instructions to start the computer and perform power-on self-test (POST). In this test it first checks the components like I/O devices, CPU, RAM, HDD etc are working properly or not. These components send an OK signal if they all are working properly.

STEP-2: After the BIOS startup has been initialized and all checks have been passed. The BIOS loads the operating system into ram and how it finds out is through IRQ numbering the hard drive on which the OS is there can be of number 14 or 15. after it finds out the path of the OS the handshake happens between firmware and OS in MBR( MASTER BOOT RECORD ) which has a space of 512kb on the same hard drive as the OS. Once the MBR is ready the bootloader code is executed. THEN the firmware handles the system configuration and components to OS and it loads it into RAM.

STEP -3 AFter the OS is loaded into RAM the OS loads the required Drivers for operating of components that are compatible with the required version of the Hardware Compatibility List. Finally, Login Screen appears and you can login with your Password.