BIOS vs UEFI

UEFI and BIOS are used to boot computer system. Although UEFI is a newer software solution, it is not always necessary to use UEFI.

Some mother board provides led or bottom-right corner on monitor to display boot code status

	BIOS (Basic Input/Output System)	UEFI (Unified Extensible Firmware Interface)
Release Date	1975.	2002.
Operating Mode	16-bit.	32-bit/64-bit.
User Interface	Basic UI navigation using the keyboard.	Provides a user-friendly graphical UI with mouse support.
Partition Support	Up to four physical partitions.	Up to 128 physical partitions by using GPT partitioning scheme
Partition Size Limit	2 TB.	18 exabytes (~18.8 million terabytes).
Performance	Performs best on older 16-bit systems.	Faster boot time and better performance compared to BIOS.
Storage	System initialization information stored in a dedicated chip on the motherboard.	System initialization information stored in a .efi file on the hard drive. which is kept on a special disk partition called the EFI System Partition (ESP)
Security	Password protection	Secure Boot feature preventing unauthorized apps from booting
Pros and Cons	dual booting is much simpler	allows to utilize the UEFI shell environment
Boot process	power on CPU starts up from BIOS firmware (POST) looks for a boot-loader (MBR) loads the kernel	power on CPU starts up from UEFI looks for a boot-loader (GPT) loads the kernel
CSM mode		UEFI firmware supports booting in legacy BIOS mode -Compatibility Support Module (CSM) - that provides legacy BIOS compatibility. booting legacy BIOS-based systems from GPT disks is also possible
Notes	Intel made the announcement in 2017 that by 2020 all CSM Modes in its Intel Motherboard's BIOSes will be removed.	UEFI classes Class 0: Legacy BIOS Class 1: UEFI with a CSM interface and no external UEFI interface. The only UEFI interfaces are internal to the firmware. Class 2: UEFI with CSM and external UEFI interfaces Class 3: UEFI without a CSM interface and with an external UEFI interface Class 3+: UEFI class 3 that has Secure Boot enabled

How to check which mode of Linux has been installed on the system

# Linux OS can be installed in your system as legacy or EFI mode
# Following command to show which mode is working on your system now 
$[ -d /sys/firmware/efi ] && echo "Installed in EFI mode" || echo "Installed in Legacy mode"