The Power-On Self Test

The POST is a series of hardware tests that run on startup to confirm that the computer's essential components and processes are working properly. If the system fails any portion of the test, the BIOS stops before it loads the boot loader.

The POST includes these tests:

  1. The BIOS checks that the power supply voltages are within specification.

  2. The BIOS inspects its own checksum to confirm that the BIOS code has not been corrupted.

  3. The BIOS confirms that the CMOS checksum is correct.

  4. The BIOS instructs the CPU to read and write every address in the computer's memory. Many BIOSes have a fast memory test mode option in which they just check a few addresses per bank to verify the module is there.

  5. The BIOS tests the I/O controller.

  6. The BIOS tests the video controller.

When the POST memory test does not find any problems, it displays the amount of RAM it has tested as one of those lines of white-on-black text that appears for a few seconds before the computer begins to load Windows. If you concentrate, you might be able to read it, but it's not visible for long. If you're using a monitor with a cathode ray tube rather than a flat-panel display, the whole thing might be long gone by the time the monitor's picture tube has warmed up enough to display anything at all.

Don't worry about missing this information. If Windows starts, it's a safe bet that the memory passed the test. If there's a memory failure, the BIOS doesn't try to load Windows, and it either sounds a beep code or displays text that identifies the cause of the failure.

Beep codes

When the system fails any of these tests, the POST issues an error code and sounds an alarm. Because part of the POST runs before the BIOS has started the video controller, the alarms are a series of audible beeps in a specific sequence of long and short sounds (a series of beeps). If the system passes all of the POST tests, it may beep once or twice.


If the POST sounds a beep code to tell you that it has found a problem, it might catch you by surprise.

If you do hear a beep code, try to replay the sound in your mind, and count the number of long and short beeps. If you can't remember the sequence, push the Reset button and count the beeps as they sound.

Each manufacturer uses a different set of beep codes, but there's no overlap among the codes, so you can't confuse them. Just count the number of long and short beeps and look up that code on one of the following Web sites in Table 8.1.

Table 8.1: BIOS POST Codes
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Web Address




Other Brands

POST diagnostic codes

In addition to the beep codes, the BIOS also produces a series of two-digit POST status codes that identify the active process in the BIOS routine. Some motherboards include an onboard LED readout that displays the POST code, but if your motherboard does not have a built-in display, you need a special tool called a POST Code Diagnostic Card to read these codes. The diagnostic card plugs into one of the computer's expansion slots on the motherboard.

Each BIOS maker uses a slightly different set of codes. If there's a display on your motherboard, the motherboard manual includes the specific list that applies to your system. If you're using a plug-in diagnostic card, consult the BIOS maker's Web site at one of the addresses in Table 8.1.

Analyzing POST codes is definitely a form of advanced troubleshooting that goes beyond anything that a typical user ever needs. Most of the time, the beep code tells you enough to fix the problem. But if you do a lot of repair work, or if you're just curious about what's going on inside your computer's tiny brain during startup, the cost of a diagnostic card (typically in the US$50–$35 range) might be worth the expense.

PC User's Bible
PC Users Bible
ISBN: 0470088974
EAN: 2147483647
Year: 2007
Pages: 372

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