What Happens When You Start’er UP? –

The Power-On-Self-Test (POST)

Now that you know about the 10…or 11 main parts of a modern computer system, you need to know how they work together.

When any computer first boots up, it performs a POST (Power On Self Test) or system check. Basically, a POST check allows a computer to count all its “fingers and toes” and make sure all the ten parts listed earlier are online and active.

If anything goes wrong during the POST check, the computer will signal an error (a possible part failure) via a series of “beeping” sounds or with a text message on the monitor if the video/graphic card is working.

The Power on Self Test (POST)

  1. The Power Supply is turned on via the power-on button on the case.
  2. The BIOS is triggered. The BIOS (basic input/output system) is pronounced “bye-ose”. It is a very basic software program (most are mutations from the first IBM BIOS from 1981) and it is installed at the factory on chip on the motherboard.
  3. Imagine if you will, the BIOS as a traffic cop manually directing how each device comes up and making sure that all the other chips, drives, ports and CPU all flow together without crashing.
  4. The BIOS tests itself. (If this fails the game is pretty much over).
  5. The BIOS tests the Power Supply Unit and makes sure it’s online and able to send a steady energy signal to the rest of the motherboard.
  6. The CPU is powered on and made active.
  7. Next, the CMOS is started and “read”. Pronounced “see-moss”, these CMOS chips store the date, time, any system passwords and other system settings. They are powered by a battery when the computer is turned off.
  8. The BIOS tests all memory chips and RAM sticks. If the first 64K of RAM is bad or unreadable, the BIOS will create an error message and the POST will stop with an error beep.
  9. Find, initialize and organize all devices that are available for boot up. The BIOS signals that all is well internally with a final “All Systems Go” beep.
  10. The BIOS then turns on the video card. This displays the motherboard & memory test information on screen. You may also see a manufacturer’s logo flash on the screen.
  11. It then does a keyboard and mouse test. This is one test where you can watch as your keyboard’s lights blink on or off. A keyboard failure will be shown on screen.
  12. Next the hard drive, floppy and CD/DVDs are tested and booted up. You will hear and see the drive lights wink on and off. A hardware (hard drive or CD/DVD) failure will be shown on screen.
  13. The POST check is complete and the BIOS hands everything off the main Operating System software.

Okay so how does this POST cycle help you?

During any part of the testing cycle (which takes only a couple of seconds by the way) if something doesn’t work, the computer will give you an audible error signal. In other words, if something bad happens your computer will “beep” at you.

The original BIOS or POST beep codes were first developed by IBM. Today most beep codes are installed by each motherboard manufacturer. While each manufacturer may modify codes for their own testing purposes, most vendors maintain a fairly standard set of codes.

Here are some of the most common PC beep codes:

  • 1 short beep – Normal POST – No problems detected
  • No beep – no power, power supply, motherboard problem, disconnected CPU, loose card or disconnected speaker
  • Continuous beep – Power supply, motherboard, or keyboard problem
  • Repeating short beeps – Power supply or motherboard or keyboard
  • Repeating long beeps – RAM (memory stick) is unseated
  • 1 long, 1 short beep – Motherboard problem
  • 1 long, 2 short beeps – Video Card, Video Card Slot
  • 1 long, 3 short beeps – Video Card, Video Card Slot
  • 3 long Beeps – keyboard or keyboard port (motherboard problem)
  • 1 beep, Blank or Incorrect Display – Monitor, Video Card, Video Card Slot

See by watching and “listening” to what your computer is telling you, you will have a jump on any major hardware problems just by powering on your PC.