Study On The Functions Of Bios Computer Science Essay

Published: Last Edited:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Every desktop and laptop computer that we use contains a microprocessor as its central processing unit. The Micro-processor is a hardware component. To extract its real value i.e. to get it function, we need a set of instructions known as software. As per the assumption, all of us are familiar with two basic types of software i.e. Operating System and the applications.

So the definition for BIOS : special software used to interface the major hardware components of the computer system with the OS.

The first PC was released on 12 August 1981 named as IBM PC and had 4.77 MHz Intel 8088 microprocessor and 16 kilobytes of memory, expandable to 256k. When PC was first introduced, BIOS software containing all the device drivers for the entire system was collectively burned into one or more non-volatile read-only memory chips. They retain their data even when the power was turned off. The chip was placed on the motherboard.

Storing BIOS

PROM -programmable read-only memory  

The BIOS firmware could be stored on a ROM known as PROM. P stands for Programmable. They store the written instructions permanently which means they are non-volatile in nature. Programming is applied after the construction of the device that distinguishes it from the normal ROM.

Eg: one-time programmable non-volatile memory (OTP NVM)

EPROM -Erasable programmable read-only memory  

Another type of ROM that retains its data when the power is switched off. Generally programmed by an electronic device incorporating use of high voltages. This type can be erased by exposing it to strong ultraviolet light

Flash Memory -

a type of EEPROM (electrically-erasable programmable read- only memory) . As the name suggests, we don't have to work hard for erasing the previous written instructions. The erasing is done electronically and thus, helping us to save the resources. In addition, its more faster and reliable.

Major BIOS manufacturers include:

American Megatrends Inc. (AMI)

Phoenix Technologies



Functions of the BIOS

When we turn on our computer, BIOS performs various tasks. This is its usual sequence:

Checking CMOS Setup for custom system settings

Loading interrupt handlers plus the device drivers

Initializing registers and power management

Performs the power-on self-test (POST)

Displaying system settings

Determining bootable devices

Initiating the bootstrap sequence

The very first thing BIOS does is checking the vital system settings stored on a complementary metal oxide semiconductor (CMOS) chip. The CMOS Setup contains detailed information specifically relevant to the PC system and can be modified later on

Interrupt handlers are defined as small pieces of software acting as translators between the hardware components and the operating system. For example, when you press a key on the keyboard, the signal is directed to the keyboard interrupt handler, the CPU gets the message and verifies what it is and passes it on to the operating system.

The device driver is a piece of software that identifies the base hardware component(s) such as keyboard, mouse, hard drive and floppy drive. BIOS is constantly intercepting signals to and from the hardware and therefore it is more convivial to copy the BIOS into RAM to run faster.

The next thing BIOS does is checking the video card to see if it is operational. Usually, video cards comes with their own BIOS located onboard and if not, there is video driver info on the BIOS chip on the motherboard. Now, importantly, BIOS checks to see if the boot is a cold or a warm boot. This is done by checking the value at memory address  0000:0472. If the value is 1234h then the BIOS realizes that is is a reboot, and the BIOS skips the rest of POST.

So, if it's a cold boot, the BIOS checks the RAM by testing all the memory addresses. Checks the PS/2, USB ports for a keyboard and mouse, PCI cards if any. On encountering errors during the POST, the BIOS notifies by a series of beep codes or a on screen error message. An error at this stage is usually a hardware problem.


POST stands for Power-on Self Test. Whenever the PC is turned on, the first function that the motherboard BIOS initiate is the POST. POST has a sequence of program routines built in the bios chip. These routines tests all the computers components such as the CPU, memory, built in motherboard circuitry and expansion cards. These are short tests and are only meant to find hard errors and not intermittent errors.

When a fault is detected, it halts the test sequence and does not allow the computer to further boot. POST errors can be presented to user in three ways:

Beep codes - Can be heard through the onboard speaker on the motherboard.

POST checkpoint codes - a Hexadecimal checkpoint codes sent to an I/O port address. A special card plugged into either an ISA or a PCI card slot is required to view these codes.

Onscreen messages - Error messages that are displayed on the monitor only after the video card is loaded.

BIOS POST Beep Codes are used for errors that happen during the early process of the POST when the video card is not yet loaded. Since error messages cannot be displayed, the codes are transformed into a series of beeps to indicate a faulty component. On most computers you will hear one short beep when the system goes through the POST check without any errors. If POST finds a problem, a different combination of short and long beeps would be produced depending on the fault. Beep codes can differ depending on the maker of the BIOS. For example, some BIOS generate one or two beeps, while others are short & long, or high & low pitched beeps to indicate the various problems detected during the POST.

BIOS POST Checkpoint Codes are hexadecimal numeric values produced by POST which is sent to I/O port address 80h. These codes can only be viewed by a special adapter card plugged into one of the system's slot, PCI or ISA. Post codes can be used to track the systems progress from the initial power-on right up to the bootstrap loader just before the operating system runs. As the system loads, you can see 2 digit hex numbers flash on the cards display. When the system freezes, you can see the location of the test where the fault was found by looking at the 2 digit hex code. The code identifies the faulty component. As with the beep codes, POST codes also vary depending on the BIOS, motherboard and the manufacturer's requirements.

BIOS POST Onscreen messages are brief messages that are displayed on the computer's monitor. These messages can only be displayed only after the video card adapter and display have been loaded. These error messages, like the Beep Codes and the POST Codes, vary between BIOS manufacturers.


BIOS CHIPThe system configuration program in every PC can be accessed by pressing a single or a combination of buttons just after the power button is turned on. Accessing the configuration program is different for every manufacturer. During startup, a message appears on the screen instructing you what to press to go into setup. Instructions can include Delete, F1, F2, F3, F5, F10, Escape, Insert, Control + Escape, Alt + Escape, Control + Alt + Escape, Control + Alt + Enter and many others

CMOS CHIPPeople commonly believe that the configuration you make is stored in the BIOS chip. However, the Bios is a ROM type memory and cannot store any data. Any data that is changed or configured in the setup is stored in a configuration chip called RTC/NVRAM chip, which stands for Real-Time Clock/Nonvolatile memory. It is usually called the CMOS because it's made using Complementary Metal-oxide semiconductor. This chip requires a battery to keep its memory.

Inside the setup, you can configure the Basic setup, Advance setup, boot drive sequence, PCI/Plug and Play setup, Power Management, Password and CPU setup and many more.


Bootstrap Loader is a small program stored in the BIOS. After the POST completes performing its test and finds no errors, the bootstrap loader is sent into RAM. Its main function is to read the first physical sector of various disk drives, working its way through the boot sequence, looking for a Master Boot Record (MBR). The MBR code, located on Cylinder 0, Head 0 & Sector 1, contains a partition table that determines which partition has the operating system and where it should start. Once it finds the bootable partition, called the Volume Boot Record, it checks if the VBR ends with the signature bytes 55AAh. When all the criteria are met it loads the first operating system files, which is usually OI.SYS for Windows 9x/Me, ntldr for Windows XP/2000/NT, or bootmgr for Windows Vista/7. At this point, the operating system takes over and continues the boot process.

BIOS (Basic input/output system)

In essence, the main function of the BIOS is to create a link between the hardware and software in a PC. In able to create the link, low-level software, built in the BIOS, control the systems hardware. It also acts as interference for the operating system to communicate with the hardware.

The BIOS can come from 3 possible sources:

Motherboard ROM

Adapter card ROM (eg. video card, network card, scsi)

From hard disk which are loaded into RAM

Early PCs had all the BIOS software (also called device drivers) contained in a single ROM chip. Drivers such as keyboard, serial/parallel ports, floppy/hard disk controller, and joystick. As computers evolved, more and more hardware were added (eg. video card, network card, SCSI). The mother board's BIOS chip however, does not have the device drivers preinstalled to be able to communicate with these new hardware. In able for the new hardware to communicate with the rest of the pc system, it either has to come with its own BIOS rom or software that can be installed to the hard disk and loaded into ram. The BIOS had grown from one entity located in the motherboard ROM chip, to having additional device drivers coming from adapter card ROMs, to having even more coming from storage devices which are loaded into RAM. All 3 different locations act as a whole that make up the BIOS. BIOS firmware are linked together via BIOS subroutine calling system of software interrupt. When an operating system or application needs to talk to a component, it would call to a specific software interrupt and the interrupt vector table would transfer the message to the specific BIOS/device driver.

Updating BIOS

There is always a need to keep the system update to get it functioning effectively and efficiently.  But its not as easy it sounds. Some of the Bios firmware is written on the ROM. Its little difficult to upgrade such BIOS. We can download the upgrade and the utility to install the upgrade from the manufacturer's website. The BIOS Wizard can check our BIOS at BIOS Upgrades.



Be aware when upgrading BIOS and make sure to upgrade to a version that is compatible with your computer system. Otherwise, it may corrupt the BIOS, which means computer wont boot. In case of a doubt, don't hesitate to consult with your computer manufacturer to be sure you need to upgrade.


BIOS initiates the actual processing of our PC's or any computing device. Most importantly it load the Operating System.

Responsible for the hardware and the Operating System Communication. Interrupt handlers and the device drivers stored in the BIOS firmware facilitates the communication.

Performs POST. Notifies the user on encountering system errors and faulty components

( beep codes, checkpoint codes and on screen messages )

Bootstrap : detects the bootable devices, locates the MBR and then the Volume Boot Record Finally,loads the OS.

Stands on alert for any hardware interrupt. Basic inputs and Outputs.

There is always a need to check if the system is performing well and efficiently. Updating BIOS definitely helps a lot to achieve that goal.