Computer BIOS Technology

BIOS or the Basic Input Output System, is a set of programs stored inside a ROM chip and put on the Motherboard. The main job of the program stored into this ROM is as it's name suggestion is to provide the computer user with a set of standard routines to take care of input/output form different input, output and storage devices connected to the computer.

This BIOS ROM is always available in the computer, so any user program ca access these routines for their input/output requirements.

The first PC BIOS was made by the IBM for their PC range of computer. The BIOS program being the copyright property of the IBM cop. PC compatible makers use the BIOS made by AMI, Award, Phoenix etc.

BIOS BASICS:
What is the BIOS?
BIOS is the acronym for basic Input Output System and it pronounced "bye-OS". It is a program stored inside a PROM chip on your computer.
This program lets your operating system and application programs communicate with the computer hardware and peripheral devices such as floppy disk, hard disk, printer, display adapter etc.
Reading the keystroke, displaying on the screen, reading/writing of the serial & parallel port, reading/writing to and from the floppy & hard disk etc. is done under the control of the BIOS.
The BIOS is a set of instructions or software stored inside a PROM (programmable Read Only Memory), PROM is a hardware, so a BIOS is not complete software nor a complete hardware that is the reason a new name "firmware" is used to describe the BIOS.
The term "firmware" is used to describe a software stored in red only memory. When the user program asks the BIOS to red the keyboard it is the job of the BIOS to read the keyboard in a proper manner, so as the data form the keyboard can be made available to the user program.
If you have two different computers, one having keyboard connection at I/O port 60 and the other having keyboard connection at I/O port 62, you don't have to purchase a different program for each of them.
Your program can just give the "read keyboard" command to the BIOS and it will be the job of the BIOS to read proper I/O port:

• Port 60 on the first computer and
  

• Port 62 on the second computer.

PC-BIOS Communication:
The computer or user program and the BIOS communicates with each other using something called interrupts.
Whenever the computer has to request for some information to the BIOS, computer generated an interrupt, and the BIOS will carry out a specific work such as reading the keyboard or displaying something on screen etc. according to the interrupt number.These interrupts are software interrupts and are different from the hardware interrupt generated by some devices to communicate to CPU. Just like hardware interrupt, when a software interrupt is received by the processor, the processor stops whatever it was doing and takes care of the interrupt. When an interrupt is received, the BIOS will search the interrupt vector table (IVT) for the address of the program that should provide the required services to the interrupting program. Once the address is found, BIOS starts execution of the interrupt service routine a program that provides the requested service to the interrupting program. After finishing the interrupt service routine the BIOS will routine back to the old process that has initiated or called the interrupt routine. The Interrupt vector table is located at the lowest RAM memory address that is at the address 0000 at segment 0000 and contains 4byte for each interrupt number. This interrupt Vector Table (IVT) is loaded from the BIOS ROM to the RAM at the tome of system startup. Loading the address from the ROM to the RAM is done so that if the user wants to change the interrupt service routine's provided by the BIOS, he can do so by changing the routine's address in the RAM vector table.
Other BIOS Functions
other than the I/O function explained so far the BIOS contains two more modules

• Power on self test and
  

• Bootstrap loader.

POST (Power On Self Test):
What is POST?
This is an abbreviation of Power On Self Test. It is a set of diagnostic programs loaded from the BIOS ROM during the system power on time. This program ensures that in a computer all the major system components are present and are working properly. This POST program checks the processor, memory, support circuits and other devices connected to the computer when the computer is first powered on. If this routine finds any error or fault during this POST, the error is informed to the user as an error beep or as some error message on the screen. Some errors called nonfatal errors allow user to continue, whereas fatal errors will not allow the POST to continue until the problem is rectified. Once the POST routine is executed successfully, BIOS goes to the boot process.
Bootstrap Loader:

After the POST routine, BIOS continues with a program called bootstrap loader. It is the job of bootstrap loader to load Operating system (OS) from the floppy disk, hard disk or CD-ROM into the computers main memory (RAM). Unless the boot process is completed successfully i.e the OS is copied from storage device to the main memory, user cannot use the computer.
A successful transfer of the OS into the RAM or the boot process is indicated by displaying the DOS prompt such as A:\> or C:\> on the computer monitor. If you have windows 95/98/ ME etc. installed in your system then instead of DOS prompt, you will directly go to the Windows screen.