BIOS

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BIOS (basic input/output
system)
BIOS (basic input/output system) is the program a personal
computer's microprocessor uses to get the computer system started after you turn it on.
It also manages data flow between the computer's operating system and attached
devices such as the hard disk, video adapter, keyboard, mouse and printer.
BIOS is an integral part of your computer and comes with it when you bring it home. (In
contrast, the operating system can either be pre-installed by the manufacturer or vendor
or installed by the user.) BIOS is a program that is made accessible to the
microprocessor on an erasable programmable read-only memory (EPROM) chip. When
you turn on your computer, the microprocessor passes control to the BIOS program,
which is always located at the same place on EPROM.
When BIOS boots up (starts up) your computer, it first determines whether all of the
attachments are in place and operational and then it loads the operating system (or key
parts of it) into your computer's random access memory (RAM) from your hard disk or
diskette drive.
With BIOS, your operating system and its applications are freed from having to
understand exact details (such as hardware addresses) about the attached input/output
devices. When device details change, only the BIOS program needs to be changed.
Sometimes this change can be made during your system setup. In any case, neither
your operating system or any applications you use need to be changed.
Although BIOS is theoretically always the intermediary between the microprocessor and
I/O device control information and data flow, in some cases, BIOS can arrange for data
to flow directly to memory from devices (such as video cards) that require faster data
flow to be effective.

BIOS may refer to any of the following:
1. Short for Basic Input/Output System, the BIOS (pronounced bye-oss) is a ROM
chip found on motherboards that allows you to access and set up your computer
system at the most basic level. The picture below is an example of what a BIOS
chip may look like on a computer motherboard. The BIOS pictured is of an early
AMIBIOS, a type of BIOS manufactured by AMI.
What is included in a BIOS?
The BIOS includes instructions on how to load basic computer hardware. It also
includes a test referred to as a POST (Power-On Self-Test) that helps verify the
computer meets requirements to bootup properly. If the computer does not pass
the POST, you will receive a combination of beeps indicating what is
malfunctioning in the computer.
What are the four main functions of a PC BIOS?
POST - Test the computer hardware and make sure no errors exist before loading
the operating system. Additional information on the POST is available on our
POST and beep codes page.
Bootstrap Loader - Locate the operating system. If a capable operating system is
located, the BIOS will pass control to it.
BIOS drivers - Low-level drivers that give the computer basic operational control
over your computer's hardware.
BIOS setup or CMOS setup - Configuration program that allows you to configure
hardware settings including system settings such as computer passwords, time,
and date.

Can a BIOS chip be upgraded or updated?
Adding additional memory to a BIOS chip, as an upgrade, can only be done by
physically removing it from the motherboard and replacing it with a new, more
advanced BIOS chip.
The data on a BIOS chip can be updated if it's a flash BIOS. Using specially
designed software, the BIOS can be updated to fix problems or add new features
for the motherboard.
What BIOS is on my computer?
To determine the type of version of BIOS on your computer, you need to access
the BIOS setup. On the main BIOS screen, it should display the BIOS type, like AMI,
Award, or Phoenix, and the version of the BIOS.
What was the first BIOS?
The first BIOS was developed by Gary Kildall and included in the CP/M operating
system in 1974. Kildall coined the term "BIOS" to describe a part of a machine
that loads at boot up and communicates with other hardware in the machine.

How BIOS Works
The BIOS software has a number of different roles, but its most important role is
to load the operating system. When you turn on your computer and the
microprocessor tries to execute its first instruction, it has to get that instruction
from somewhere. It cannot get it from the operating system because the
operating system is located on a hard disk, and the microprocessor cannot get to
it without someinstructions that tell it how. The BIOS provides thoseinstructions.
Some of the other common tasks that the BIOS performs include:
A power-on self-test (POST) for all of the different hardware components in the
system to make sure everything is working properly
Activating other BIOS chips on different cards installed in the computer - For
example, SCSI and graphics cards often have their own BIOS chips.
Providing a set of low-level routines that the operating systemuses to interface to
different hardware devices - It is these routines that give the BIOS its name. They
manage things like the keyboard, the screen, and the serial and parallel ports,
especially when the computer is booting.
Managing a collection of settings for the hard disks, clock, etc.
The BIOS is special software that interfaces the major hardware components of
your computer with the operating system. It is usually stored on a Flash memory
chip on the motherboard, but sometimes the chip is another type of ROM.
When you turn on your computer, the BIOS does several things. This is its usual
sequence:

Check the CMOS Setup for custom settings
Load the interrupt handlers and device drivers
Initialize registers and power management
Perform the power-on self-test (POST)
Display system settings
Determine which devices are bootable
Initiate the bootstrap sequence
The first thing the BIOS does is check the information stored in a tiny (64 bytes)
amount of RAM located on a complementary metal oxide semiconductor (CMOS)
chip. The CMOS Setup provides detailed information particular to your system
and can be altered as your system changes. The BIOS uses this information to
modify or supplement its default programming as needed. We will talk more
about these settings later.
Interrupt handlers are small pieces of software that act as translators between
the hardware components and the operating system. For example, when you
press a key on your keyboard, thesignal is sent to the keyboard interrupt handler,
which tells the CPU what it is and passes it on to the operating system. The device
drivers are other pieces of software that identify the base hardware components
such as keyboard, mouse, hard driveand floppy drive. Since the BIOS is constantly
intercepting signals to and from the hardware, it is usually copied, or shadowed,
into RAM to run faster.

Whenever you turn on your computer, the firstthing you see is the BIOS software
doing its thing. On many machines, the BIOS displays text describing things like
the amountof memory installed in your computer, the type of hard disk and so
on. Itturns out that, during this boot sequence, the BIOS is doing a remarkable
amount of work to get your computer ready to run. This section briefly describes
some of those activities for a typical PC.
After checking the CMOS Setup and loading the interrupt handlers, the BIOS
determines whether the video card is operational. Most video cards have a
miniature BIOS of their own that initializes the memory and graphics processor on
the card. If they do not, there is usually video driver information on another ROM
on the motherboard that the BIOS can load.
Next, the BIOS checks to see if this is a cold boot or a reboot. Itdoes this by
checking the value at memory address 0000:0472. A valueof 1234h indicates a
reboot, and the BIOS skips therestof POST. Anything else is considered a cold
boot.
If it is a cold boot, the BIOS verifies RAMby performing a read/writetest of each
memory address. Itchecks the PS/2 ports or USB ports for a keyboard and a
mouse. Itlooks for a peripheral componentinterconnect (PCI) bus and, if it finds
one, checks all the PCI cards. If theBIOS finds any errors during the POST, it will
notify you by a series of beeps or a text messagedisplayed on the screen. An
error at this point is almost always a hardwareproblem.

The BIOS then displays somedetails about your system. This typically includes
information about:
The processor
The floppy driveand hard drive
Memory
BIOS revision and date
Display
Any special drivers, such as the ones for small computer systeminterface (SCSI)
adapters, are loaded fromthe adapter, and the BIOS displays theinformation. The
BIOS then looks at the sequence of storagedevices identified as boot devices in
the CMOS Setup. "Boot" is shortfor "bootstrap," as in the old phrase, "Lift
yourself up by your bootstraps." Bootrefers to the process of launching the
operating system. The BIOS willtry to initiate the boot sequence fromthe first
device. If the BIOS does not find a device, it will try the next device in the list. If it
does not find the proper files on a device, the startup process willhalt. If you have
ever left a disk when you restarted your computer, you haveprobably seen this
message.
The BIOS has tried to boot the computer off of the disk left in the drive. Since it
did not find the correctsystemfiles, it could not continue. Of course, this is an
easy fix. Simply pop out the disk and press a key to continue.

Modern PCs employ FLASH MEMORY rather than ROMto storethe BIOS routines
so that they can be updated froma floppy disk whenever a new version is
released, say to supportsomenewly 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
programthe BIOS after it has been installed forestalls the obsolescenceof BIOS
chips as new hardwarefeatures are installed. In this way the BIOS can be updated
by modemor directly froma disketteto bring the code in line with new hardware
capabilities.
When you turn on your computer, the BIOS is responsiblefor checking all the
hardware, including memory; it will display an error messageif it finds a problem.
The BIOS then loads the operating system-whether it's DOS, os/2, Unix, or what
have you-into memory fromdisk. Even after the operating systemis running, the
BIOS handles many essential chores, putting characters on the screen, getting
characters fromthe 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 outthe computer by performing the
power on self test (POST). Thecomputer reads these instructions each time it is
turned on and performs a self check of the computer and its components.
The BIOS keeps a storeof crucial parameters, such as the number and nature of
disks presentand the type of PROCESSORfitted, in a small, separatewriteable
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 interruptthe computer's BOOT-UP sequence.
How Does ItWork?
When the computer is booted, the CPU activates the ROM BIOS chips. ROMBIOS
then begins a series of systemchecks, called the power on self test (POST). The
POSTtells 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 systemcheck is fastand not very thorough. The POST
determines whether everything is connected properly, butit does not check to
see if everything is functioning perfectly. After the POSTcheck 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 softwarethere, the CPUcontinues to the hard drive, where it
copies the operating systeminto memory so it is ready to go.

BIOS

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