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BIOS

BIOS 

    BIOS is a software which resides in a small flash EEPROM memory on the motherboard. It is a 
form of read only memory (ROM), but may be rewritten or programmed when the right tools and 
techniques are applied. It works as an interface between the computer hardware and operating system 
and communicates between computer and devices.

     The BIOS is very important part of your computer that makes its home on the motherboard. The BIOS provides an interface between the raw PC hardware and the standard operating system, it is usually stored in ROM. As you can see, BIOS is important; every computer needs it. And like CPU's, it comes in different versions. BIOS versions are based on years. These chips are typically the biggest chips on your motherboard other than the CPU and math-coprocessor. There is usually a big sticker on it that says BIOS. The sticker also says what kind of BIOS it is and what year the BIOS were published. Fig7.1 shows AWARD-BIOS published on 1998. But if you don't have a sticker, you can go to the BIOS screen on your monitor. 
    As the first program executed by the processor at boot time, the BIOS introduces the CPU to the primary components on the motherboard, and instructs the CPU regarding which program to run next when the BIOS code has completed. As a rule, the BIOS then accesses the boot sector on the boot device, which might be a floppy disk, a CD-ROM, a DVD, or a hard disk. The boot sector in turn loads a boot manager program of some kind, which starts up the primary operating system for the machine, such as Windows or Linux. 

    After the start-up process ends, the BIOS still isn't finished with its chores. Many types of hardware access in a PC really involve the operating system accessing the BIOS, which then accesses the hardware on the operating system's behalf 

    Every motherboard has its own unique BIOS designed especially to handle the hardware it contains. The basis for the majority of BIOS Programs is the Phoenix Award BIOS, which occurs in two different formats. You'll also find some computers that run an American Megatrends (AMI) BIOS. 

    We can invoke the BIOS Setup program by pressing a special key on our keyboard. Often, this 
means striking the delete [Del] key, but it might be some other key such as [F2]. Look for an onscreen
message during boot-up for selection of right key.

    It is common in computer design to include a BIOS ROM for major sub systems, such as video and 
drive control. One of the early steps by system initialization is to check for the other valid BIOS ROMs located in upper memory (640KB to 1MB). These are usually referred to as “expansion” or adaptor BIOS. When another BIOS are located it is also checksum-tested and used by the PC. In general, a PC can be fitted with up to five or more BIOS ROMs viz and they are as under.
• System BIOS (Motherboard BIOS) 
• Video BIOS 
• Drive Controller BIOS 
• Network adapter BIOS 
• SCSI adapter BIOS 

Motherboard BIOS 
    The typical BIOS ROM occupies 128KB of space in the Upper Memory Area. BIOS are not a single 
program, but an arsenal of individual programs most of them are quite small. In general BIOS contains 3 sections viz. 
1. POST 
2. CMOS Setup 
3. System Service Routines 
 
POST
Power on Self Test, the “POST” actually manages the entire system startup. It handles virtually all of the initialization activities of a PC. 

    The POST performs a low level diagnostic and reliability test of the main processing components including ROM programs and system RAM. It tests the CPU, initializes the Motherboard’s chipset, 
checks the 128 bytes of CMOS for system configuration data, and sets up an index of interrupt vectors for the CPU. POST then loads the BIOS Data Area and detects any optional equipment (Adapter BIOS ROMs) in the system, and proceeds to boot the operating system.

    All of this happens behind the scenes every time you boot. You may notice the POST, or Power on 
Self Test. When your BIOS test the hard drive, floppy drive, and keyboard, you'll see lights flash on them. Once all of this is done, your BIOS load the operating system. It does this by looking for and reading your various boot up files, such as Config.sys and Autoexec.bat. From there, your operating system takes over. 

SETUP
     The hardware configuration for any given computer is maintained in a small amount of CMOS 
RAM, and a CMOS setup routine is required in order to access the system configuration. Older i286 and i386 systems provided the CMOS setup routine as a separate utility included with the system on a “Floppy disk”. Starting with the late model i386 and later systems the CMOS setup routine has been integrated into the motherboard BIOS itself. 

System Routines 
The system services (also known as BIOS services) are a set of individual functions that form the layer between hardware and the operating system. Services are called through the use of interrupts An interrupt essentially causes the CPU to stop whatever it was working on and sends program control to 
another address in memory. When the interrupt handling routine is complete, the CPUs original state is
restored, and control is returned to where the PC left off before the interrupt occurred.. 
Some of the important BIOS manufacturers are as under 
• American Megatrends Inc (AMI)
• Phoenix Technologies 
• Award 
• Ali 
• Via




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