BIOS vs UEFI
BIOS bios stands for basic input output service is basic or lowest level firmware or software of any system basically the term bios name from its working it self . as we know the bios is responsible for all the lowest level operation of any computer system whether it is x86 of AMD64 based or any other . bios is similar to boot code in embedded devices. bios are mostly flashed on a permanent memory chips (Rom: Read only memory) in older days but now bios can be updated without any data-loss. so now we know what is bios but what is this lowest level software means and how it work . so the lowest level software means in any computer software architecture the hierarchy of software can be seen as below fig.1 where the OS is Top level and the BIOS is the Lowest level there may be other levels but i didn't shown them here for ease of understanding.
Fig .1
BIOS and UEFI Functional Classification
The Bios handle all the i/o operation of hard-disk, keyboards and VGA to processors and other devices . the bios are flashed or stored on a ROM which are physically attached to the main board so when we power on the the PC the Bios will check all the hardware connected to the main board properly if the bios found all the hardware are fine and working than it may generate a massage called as POST( power on self test) if the post is successful no the computer system is ready for Load the OS which may load in many parts including boot-loader and kernel load and user space init process load . but we are not talking about OS process here.now we know the BIOS and its working. but if bios are fine why we are now shifting to new UEFI .
UEFI: unified Extensible Firmware interface is successor of bios and solve many issues which are occurred with bios as we know bios have many limitation like memory addressing of 1 MB, not able to boot the system from hardisk with more than 2 TB , it runs in 16 bit mode and it may not able to initialize multiple hardware concurrently which may slow the boot process. where as in uefi has many advantages like it may support unlimited partitions and any on partition is larger than 2 TB which was not possible in Bios .UEFI also are very advance security features like secure boot .
UEFI has also a capability for remote repairing of system whether OS is installed or not . so UEFI is now gradually replacing the BIOS from computer but it is also backward compatible and called as Legacy support for old disk formatted in MBR(master Boot Record) whereas the UEFI use the GUID partition table known as GPT .
The boot record in UEFI is stored ESP or EFI system Partition which may be searched by uefi application on boot in each partition and if the EFI file were not found the UEFI try to boot in legacy mode which is the same earlier BIOS. there are many application are now available that may run on UEFI shell directly without any OS. UEFI also provides processor independent device drivers. and GOP support for graphic console output and now the device vendors have choice to design a graphic interface in UEFI with HID support.
Security BIOS Vs UEFI
Security in Bios is not very vulnerable because the hacker may plant any malware or torjan in BIOS by update but any antivirus or anti malware software may not able to detect it as they are not intended to scan bios of any system so the security may compromised when a bios is attacked by hackers or any security threat. and when we talk about UEFI the security is prime concern for the developers. so they introduced secure boot which may prevent malware attacks when they are configured to secure boot.