The Computer's Basic Input Output System

The pages contained within this section are devoted to helping you understand the complexities of a Basic Input / Output System in a computer.

Introductions And Overviews

   The BIOS is a very crucial component of any personal computer since its the BIOS's primary responsibility to control how hardware works with the operating system.

The BIOS controls the proper setup and configuration of hard disk drives, floppy drives, memory, and other related devices at the hardware layer. It also has some control over how an operating system interacts during boot up.

If you add a new hard disk drive, it’s the BIOS that will see the drive first. It will then attempt to configure it according to the manufacturer's specifications ( for example, the number of tracks, cylinders, and sector size will all be configured automatically by the BIOS so that the operating system can properly recognize and use the drive).

If you add additional memory to a system, the BIOS handles all updates and counts the quantity of memory installed and will return an error code, if a problem exists. The BIOS also sets all configuration items required for the memory to function properly in the system, including speed and voltage requirements. 

The BIOS is also in total control of the system boot up process.  When you first turn on the computer, the BIOS performs what is known as a Power On Self Test (POST) routine which consists of a series of tests tests to ensure devices are working properly. You can think of this test as a sort of computer hardware inventory system each time the computer boots up ensuring that all devices in the system are functioning properly. The BIOS also checks to see whether other devices, such as a video graphics adapter has its own BIOS that needs to be run during boot up. It will then attempt to configure it according to the manufacturer's specifications (for example, the number of tracks, cylinders, and sector size will all be configured automatically by the BIOS so that the operating system can properly recognize and use the drive).

How A BIOS Recognizes Memory In A PC

    When you first turn on the computer, the BIOS performs what is known as a Power On Self Test (POST) routine which consists of a series of tests to ensure devices are working properly.

    During the POST routine, the BIOS counts handles a pre-test of memory to determine the quantity installed and may return an error code letting you know a problem exists. The BIOS also sets configuration items required for the memory to function properly in the system, including speed and voltage requirements.

    You can think of these tests as a sort of computer hardware inventory system each time the computer boots up. make sure all devices in the system are functioning properly and checks to see whether other devices, such as a video graphics adapter has its own BIOS that needs to be run during boot up.

The Power On Self Test (P.O.S.T) Routine

    The Power On Self Test (P.O.S.T) is a routine test invoked by the system BIOS each time the computer is powered on or rebooted. Several key devices are checked by the BIOS to ensure they are functioning properly. Some of the items tested during the P.O.S.T routine include a basic keyboard and mouse test, a memory count, floppy, hard disk drive, and optical storage drive initializations.

    The fundamental purpose of these tests are to determine if the system is properly recognizing each device installed in the machine.
   
    The BIOS also performs a check on all expansion card devices including sound and ethernet controllers and a monochrome based black and white screen displaying results are shown directly before it calls for the operating system files to load. Various errors might be noted on this screen usually requiring some type of user intervention to proceed properly.

How The Power On Self Test Works

The Video Card P.O.S.T

The first test that the BIOS needs to complete in its series of P.O.S.T tests is a video card test. This performs a basic initialization of a video card to determine its functionality.

If the screen shows nothing when powered up then a problem is with the the video card. A video card P.O.S.T routine carries out its own set of tests and instructions that need to be run during the boot-up process. The system BIOS will yield to the video BIOS and wait until the video BIOS completes its duties. After that process completes, the system BIOS will kick back in and continue to check other elements in the system.

BIOS Version Test

This test will determine the exact version of the BIOS being used. It will also display the manufacturer of the ROM chip where the BIOS settings are stored in addition to a year the BIOS file was created.

Mainboard Identification

This test would immediately follow the video card test normally displaying a string of characters known as an identification string used to accurately identify the model and make of mainboard installed in a PC.

The string indicates that a Gigabyte brand of mainboard is being used and the model number of the mainboard is a 693A which translates into an Apollo based mainboard manufactured by Gigabyte.

CPU Test

The most important P.O.S.T routine completed by a system BIOS is a CPU test. This test determines the model and speed of the installed processor. It may also show basic information about the processor including an id representing the family of processor being used. The speed at which the processor is running is normally displayed in either Mhz or Ghz.

The Memory Count Test

This test determines how much physical memory is installed in the computer. The test will display the quantity of Random Access Memory as a value in K.

If a system is equipped with 2 sticks of RAM each totaling 128 Megabytes, the P.O.S.T report would show 294912K of physical ram.
      
One example to demonstrate how a P.O.S.T routine could fail is during a the keyboard and mouse check. If the keyboard fails to respond upon boot time, the system BIOs will prompt the user with an error message similar to the following: "Keyboard Error Press F1 to continue”.

How Operating Systems Work

As soon as the POST routine completes testing all devices control is given to the operating system. The operating system further initializes system files and hardware components so that once a graphical or text based user interface loads, the computer and its respective devices can be accessed. In its most basic terminology, an operating system is a suite of software applications and utilities that perform basic computer tasks such as accepting input from a keyboard or mouse, sending output to a computer display, and organizing files and directories on hard disk drives, floppy diskettes, and other removable based storage mediums. The operating system during boot time can control and initialize a wide variety of devices including the keyboard, mouse, game controllers, sound and network cards, and other input/output devices. There are wide variety of free and commercial operating systems but most fall into two categories: linux based operating systems, and Windows based operating systems. The most popular single user operating systems include Windows XP and OS/2. Some popular multi-user operating systems include UNIX, Linux, and Windows Server 2003. Operating systems are a huge topic that will not be discussed in great detail on this page.

Overview Of System Files

To understand the boot up process, the first thing to discuss is the location of various start up files.

After the POST procedure has finished, the BIOS instructs several files to be loaded. These files are listed below

COMMAND.COM

• Root directory of the hard drive.  The root directory on a hard drive is usually C:
• The basic command interpreter available in most operating systems such as DOS and Windows.  Command.com includes basic commands for copying & renaming files, making directories, and is a neccessary file in any operating system.

AUTOEXEC.BAT -

• Root directory of the hard drive.
• A batch file containing various device drivers and shortcuts to applications that execute every time the system is booted up.

CONFIG.SYS -

• Root directory of the hard drive.     
• The file that loads various device drivers each time the system boots up.  It is a configuration file system for DOS system and executes any commands found within the system.  Common commands within the config.sys file include BUFFERS=x, FILES=x.

MSDOS.SYS (a hidden file) -

You can’t see this file but it is typically stored in the root directory of C:\

•     Standard MSDOS operating system file used to control various devices within a computer system.
•     IO.SYS (a hidden file)
•     Hidden file.  You can’t see this file but is typically stored in the root directory of C:\
•     Standard operating system file that contains various input/output configuration commands.
•     Table 1.1 Location and purpose of various MSDOS startup files.

    If the COMMAND.COM file is missing or corrupted your system will be halted as it contains key commands for interacting with files and data stored with the personal computer.  Additionally, Windows 95 requires that you have the CONFIG.SYS file in the root directory of your hard drive because there are certain required elements needed by Windows 95 to boot efficiently. One further element that should be understood about BIOS is the CMOS (Complimentary Metal Oxide Semiconductor) element.  The CMOS stores valuable information regarding the various elements found within a PC.