BOOTING – What happens when you press power button

By ioplus3 

When Computer first boots up you see Basic Input/Output System (BIOS) or Unified Extensible Firmware Interface (UEFI), depending on what Firmware your motherboard has. Firmware is part hardware part software. It is a "Software" that is "Hard-coded" on an electronic chip which lists steps to follow when power is switched on. This can be a Read Only Memory or Flash Memory (a kind of memory that retains data in the absence of power).

Both basically are the first electronic chip that powers up when you press the Power button on CPU cabinet.

BIOS /UEFI first checks for settings stored in CMOS Chip, which stores the BIOS/UEFI settings even when the power is not available using a Micro lithium cell called CMOS battery. CMOS Battery also powers up the REAL TIME CLOCK (RTC) chip which keeps track of Date and time.

After this BIOS/UEFI will run POWER ON SELF TEST (POST) that check all the hardware inside your computer is functioning correctly. If yes you may here one beep & If something is not working properly it will give error messages with several repeated beeps which you can decode using Motherboard troubleshoot Manual.

Once POST completes the BIOS looks for Master Boot Record (MBR) & UEFI looks for GUID Partition Table (GPT) on a Bootable device, which is most of the time a Hard Disk Drive or a Solid-State Drive. The MBR/GPT hold information on how logical partitions are organized on the Drive and contains a code to function as BOOTLOADER for the installed operating system.

BOOTLOADER then starts copying Operating System files into RAM from the Bootable device. As OS loads into the RAM from the Bootable device, the BIOS/UEFI gives complete control of Hardware to the OS. This is the time when you see the OS logo on Screen. When OS is completely loaded into the RAM, it is ready to take inputs from USER and you see Login Screen. Login Screen will appear if you have set OS User Password or have multiple users. If you are the only user & using the computer without a password, you may not see the login screen and get directly to the OS Desktop screen.

The above entire process is called BOOTING.

You can access BIOS/UEFI & change its settings by pressing a key just after you pressed the power button. most of the time it’s F2 or DELETE key. A message may appear when you power up the computer that shows what key to press to Access BIOS/UEFI. If Computer is not giving any indication for which key to use & pressing F2 or DEL is not working, then you must look for it in Manual.

BIOS is being replaced by UEFI nowadays. This is because BIOS can only support Hard drives up to 2.2TB while UEFI can support 7.02 ZB (1 Zettabyte = 1 Billion Terabyte). You can buy 4TB in 8K rupees & I personally have two 4TB Hard disks. UEFI also provides a Graphical user interface where you can use Mouse while BOIS is a Text-based user interface where you must work with just Keyboard.