OS functions

BIOS

• this stands for basic input/output system.when it is first switched in it looks into the bios to get it up and running and so the processors program counter points to the Bios memory
• The Bios will check that the computer is functional , memory is installed and accessible the processor is working.This is called the power- on-self-test (POST). Once this is done it can use a boot loader program to load the operating systems kernel into memory.
• The BIOS is usually stored on flash memory so that it can be updated, this also allows settings such as boot order of disks to be changed  

Virtual machines

it is possible to write a program that has the same functionality as a physical computer.A common use of virtual machines is to run operating systems within another operating system.This might be because a program is needed that will not run on the host operating system or it might be because it offers a convenient way to test a program being developed.

system virtualisation 

Because virtual machines are just programs and data , they have advantages over physical machines.They can be backed up and duplicated and more than once can be run at one time on physical machine.It is for these reasons that many organisations are virtualising their network infrastructure , making their servers a group of virtual machines running from a cluster of physical machines 

Process virtualisation
Another common of virtual machine is for interpreting intermediate code. intermediate code offers a compromise between these two approaches.A complier converts the source code into something called byte code.This isn't  machine code but is a much more efficient representation than  the original source code.Because it isn't machine code it can be run directly on a processor. instead a virtual machine is used to read the code.this means code can be highly portable , as hardware becomes cheaper and more powerful , virtual machines are likley to become more common place