The Power On Self Test: Using BIOS (Basic Input/Output System) For Computer Diagnostics

Topics: BIOS Pages: 7 (1641 words) Published: October 8, 1999
The Power On Self Test

When the system is powered on, the BIOS will perform diagnostics and initialize system components, including the video system. (This is self-evident when the screen first flicks before the Video Card header is displayed). This is commonly referred as POST (Power-On Self Test).

Afterwards, the computer will proceed its final boot-up stage by calling the operating system. Just before that, the user may interrupt to have access to SETUP.

To allow the user to alter the CMOS settings, the BIOS provides a little program, SETUP. Usually, setup can be entered by pressing a special key combination (DEL, ESC, CTRL-ESC, or CRTL-ALT-ESC) at boot time (Some BIOSes allow you to enter setup at any time by pressing CTRL-ALT-ESC). The AMI BIOS is mostly entered by pressing the DEL key after resetting (CTRL-ALT-DEL) or powering up the computer. You can bypass the extended CMOS settings by holding the key down during boot-up. This is really helpful, especially if you bend the CMOS settings right out of shape and the computer won't boot properly anymore. This is also a handy tip for people who play with the older AMI BIOSes with the XCMOS setup. It allows changes directly to the chip registers with very little technical explanation.

A Typical BIOS POST Sequence

Most BIOS POST sequences occur along four stages:

1.Display some basic information about the video card like its brand, video BIOS version and video memory available.

2.Display the BIOS version and copyright notice in upper middle screen. You will see a large sequence of numbers at the bottom of the screen. This sequence is the .

3.Display memory count. You will also hear tick sounds if you have enabled it (see Memory Test Tick Sound section).

4.Once the POST have succeeded and the BIOS is ready to call the operating system (DOS, OS/2, NT, WIN95, etc.) you will see a basic table of the system's configurations:

·Main Processor: The type of CPU identified by the BIOS. Usually Cx386DX, Cx486DX, etc..

·Numeric Processor: Present if you have a FPU or None on the contrary. If you have a FPU and the BIOS does not recognize it, see section Numeric Processor Test in Advanced CMOS Setup.

·Floppy Drive A: The drive A type. See section Floppy drive A in Standard CMOS Setup to alter this setting.

·Floppy Drive B: Idem.

·Display Type: See section Primary display in Standard CMOS Setup.

·AMI or Award BIOS Date: The revision date of your BIOS. Useful to mention when you have compatibility problems with adaptor cards (notably fancy ones).

·Base Memory Size: The number of KB of base memory. Usually 640.

·Ext. Memory Size: The number of KB of extended memory.

In the majority of cases, the summation of base memory and extended memory does not equal the total system memory.

For instance in a 4096 KB (4MB) system, you will have 640KB of base memory and 3072KB of extended memory, a total of 3712KB.

The missing 384KB is reserved by the BIOS, mainly as shadow memory (see Advanced CMOS Setup).

·Hard Disk C: Type: The master HDD number. See Hard disk C: type section in Standard CMOS Setup.

·Hard Disk D: Type: The slave HDD number. See Hard disk D: type section in Standard CMOS Setup.

·Serial Port(s): The hex numbers of your COM ports. 3F8 and 2F8 for COM1 and COM2.

·Parallel Port(s): The hex number of your LTP ports. 378 for LPT1.

·Other information: Right under the table, BIOS usually displays the size of cache memory.

Common sizes are 64KB, 128KB or 256KB. See External Cache Memory section in Advanced CMOS Setup.


During the POST routines, which are performed each time the system is powered on, errors may occur. Non-fatal errors are those which, in most cases, allow the system to continue the boot up process. The error messages normally appear on the screen.

Fatal errors are those which will not allow the system to continue the boot-up procedure. If a fatal...
Continue Reading

Please join StudyMode to read the full document

You May Also Find These Documents Helpful

  • Computer system Essay
  • Input and Output Devices Essay
  • Different types of computer systems in Essay
  • Ups System- Inputs Processing and Outputs Essay
  • Computer Basics Essay
  • Input and Output Devices Essay
  • Essay on Input and Output
  • Computer System Information System Paper

Become a StudyMode Member

Sign Up - It's Free