The BIOS is the basic input/output system that is built into a computer. It determines what the computer can do without an operating system and is the first software run when a computer is powered on. When turned on, the BIOS initializes hardware components, performs self-tests, and determines which devices can be used to boot the operating system. Modern BIOS also include user interfaces to configure hardware settings and select boot devices.

2. What is BIOS?
• It is built in software that determines what
your computer can do without accessing
programs.
• It is first software run by a PC when it is
powered on.
• It is typically placed in ROM chip and often
known as a ROM-BIOS.

3. Initialization…
• The BIOS is special software that interfaces the major hardware
components of your computer with the operating system
• 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.

4. P.O.S.T.
• Power On Self Test
• Post tests and identifies system devices such as CPU, RAM,
interrupt, keyboards, DMA controllers, hard disk.
• The principal duties of the main BIOS during POST are as follows:
 verify CPU registers
 verify the integrity of the BIOS code itself
 verify some basic components like DMA, timer, interrupt controller
 find, size, and verify system main memory
 initialize BIOS
 identify, organize, and select which devices are available for booting

5. BIOS boot process
• If system has just been powered up (cold boot)the full
POST will run.
• But if reset button was pressed (warm boot)specific flag
value is detected in memory and BIOS does not run POST.
• This saves the time otherwise used to test all memory.
• The BIOS begins its POST when the CPU is reset.
• A little-known feature of the original IBM BIOS versions is
that before beginning the normal boot process they
would attempt to load a program through the keyboard
port.

6. User Interface…
• The BIOS in the IBM PC and XT had no built-in user-
interface.
• A modern BIOS has a menu-based user interface(UI)
accessed by pressing a certain key on the keyboard
when the PC starts. In the BIOS UI, a user can:
– configure hardware
– set the system clock
– enable or disable system components
– select which devices are potential boot devices
– set various password prompts, such as a password for
securing access to the BIOS user interface functions itself
and preventing malicious users

7. Virus Detected…
• There are four known BIOS attack viruses.
• The first BIOS virus was CIH, and able to erase flash ROM BIOS content.
• The second BIOS virus was a technique presented by John Heisman.
• He shown how to elevate privileges and read physical memory, using
malicious procedures that replaced normal functions stored in flash
memory.
• The third BIOS virus was a technique called "Persistent BIOS infection.
• Mebromi is a Trojan which targets computers with Award-
BIOS, Microsoft Windows, and antivirus software from two Chinese
companies.
• For boot sector virus, you can simply fix it using Fixboot
or use a utility to fix it.

8. Update BIOS
• Computer will need to its BIOS updated especially for older
machines.
• Download the upgrade and utility program needed to install it.
• Major BIOS manufactures include:-
– American Megatrends
– Phoenix Tech.
– ALi
– Winbond
• As we changes to the CMOS setup be careful when upgrading your
BIOS,otherwise you could corrupt the BIOS.