Initially a computer is a hardware device for any software program to be loaded into it we need to integrate this hardware with software. So , every hardware device is equipped with ROM by manufacturer himself. ROM is non volatile memory in pc hardware which stores some configuration details of the hardware and It cannot be edited usually and set of programs that are stored in this ROM are called BIOS ( basic input output system).
How does your system points to this bios memory location ?
There is a special hardware circuitry in the personal computer that raises the logical value of RESET pin of CPU. When this pin is set some set of registers in CPU will be assigned with some fixed default values and code found at physical address 0xfffffff0 begins to execute . This is the ROM location.
What does this BIOS contain ?
BIOS contains essentially following operations
POST Stands for Power On Self Test , During this phase series of test’s are planned on hardware to determine whether they are active and performing well and this is the phase when bios banners and messages are displayed.
In modern PCI based architecture it is also important to perform this active hardware tests to determine whether all hardware devices are up and able to perform without any conflicts on IRQ lines and Input/Output ports.
Once BIOS determines if all hardware devices are running properly then it probes for operating system before going to that I should say you’ve heard of CPU beeps an alarm sound is generated if any hardware malfunction , This alarm beep also has its own importance to determine which hardware device is malfunctioned , I don’t remember exactly of this beeps and errors I will publish it in next post after going through some references .
Boot Sequence , Probing will take place base on the order you gave it in BIOS , It probes devices for boot sector. This boot sector is called Master Boot Record in hard disk . as soon as it is found it will load this address into RAM physical address starting from 0x00007c00.
So that is it all about , Lets Cut it short and describe the procedure of booting.
when you power on your PC , BIOS takes control of PC and handles the operating system selection and once operating system is selected it takes control of the hardware and starts loading it.
