by Dinesh Thakur

BIOS stand for Basic Input Output System (pronounced "by ose," that's "ose" as in comatose). BIOS are a set of instructions that tell the computer how to handle the flow of information between the computer and its peripherals, such as the keyboard (input) or the printer (output). . The BIOS is firmware, meaning it is a program built into the read-only memory (ROM) in your computer, rather than stored on a disk (because The BIOS is stored in a ROM chip and automatically executed whenever the power is switched on, it's sometimes called the ROMBIOS). Since the ROM BIOS instructions are read-only memory, they cannot be changed. 

Modern PCs employ FLASH MEMORY rather than ROM to store the BIOS routines so that they can be updated from a floppy disk whenever a new version is released, say to support some newly invented device.  These chips are normally divided into blocks. Each block can be erased and programmed independently. Blocks can also be locked to prevent accidental reprogramming. This ability to program the BIOS after it has been installed forestalls the obsolescence of BIOS chips as new hardware features are installed. In this way the BIOS can be updated by modemor directly from a disketteto bring the code in line with new hardware capabilities.

When you turn on your computer, the BIOS is responsible for checking all the hardware, including memory; it will display an error message if it finds a problem. The BIOS then loads the operating system-whether it's DOS, os/2, Unix, or what have you-into memory from disk. Even after the operating system is running, the BIOS handles many essential chores, putting characters on the screen, getting characters from the keyboard, reading and writing sectors to the floppy or hard disk. You'll see this as your ROMBIOS chip on your computer that works with your software.

One function that BIOS performs happens only when the computer is turned on or restarted (booted). ROM BIOS checks out the computer by performing the power on self test (POST). The computer reads these instructions each time it is turned on and performs a self check of the computer and its components.

The BIOS keeps a store of crucial parameters, such as the number and nature of disks present and the type of PROCESSOR fitted, in a small, separate writeable memory area called the CMOS- one of these settings determines on which disk to look for an operating system. The user can inspect and alter these CMOS settings by holding down certain combination of keys (e.g. function key Fl for some makes of PC) to interrupt the computer's BOOT-UP sequence.

How Does It Work?

When the computer is booted, the CPU activates the ROM BIOS chips. ROM BIOS then begins a series of system checks, called the power on self test (POST). The POST tells the CPU to check the bus (a series of connections that link all of the PC's components), the memory (RAM), the peripherals (keyboard, mouse, etc.), and the disk drives. This system check is fast and not very thorough. The POST determines whether everything is connected properly, but it does not check to see if everything is functioning perfectly. After the POST check is complete, the computer is ready to load the computer's operating system. At this point, a user may notice that the light in drive A comes on again as the CPU checks to see whether a bootable disk has been placed in the drive. If it does not find the operating system software there, the CPU continues to the hard drive, where it copies the operating system into memory so it is ready to go.