Tech Tip:
Chapter 15 PC Troubleshooting 667
Windows 95, 98, Windows 2000 and XP Windows Vista
and Me
POST POST POST
io.sys
Initial startup phase Initial startup phase
msdos.sys
Boot loader phase Windows Boot Manager phase
config.sys
Detect and configure Windows Boot Loader phase
hardware phase
command.com
Kernel loading phase Kernel loading phase
autoexec.bat
Logon phase Logon phase
Power-On-Self-Test (POST)
When a computer has power first applied to the motherboard by pressing
the on power switch, the BIOS or EFI will start the boot process by performing
a quick check of hardware components and verifying that all hardware devices
listed in the BIOS configuration database are present and appear to be in working
order. The BIOS configuration settings are typically automatically detected or
manually modified when the computer is first assembled and started the very
first time. The BIOS typically has a default configuration that will usually start
most computers without a problem, but not always. Some BIOS configurations
require technician modification. The hard disk drive is automatically detected
and configured by the BIOS and typically does not need to be modified by the
technician. All configuration data is then stored in the CMOS memory.
The BIOS is independent from the operating system. All systems today use
either BIOS or EFI as the first computer software routine to run on the computer.
Since the POST is independent of the operating system, it is safe to assume
that you have a hardware problem if the computer fails during the POST or the
POST generates an error message or a series of beeps. You can research the error
message or beep codes at the BIOS or motherboard manufacturer Web site. You can
also do an Internet search using the contents of the error message as the key terms.
The following is a partial list of the system hardware checked during the POST:
✔ CPU system clock.
✔ CPU registers.
✔ Keyboard controller.
✔ Video controller.
✔ RAM.
✔ Disk controllers.
✔ Motherboard bus.
✔ Adapter card ROM.
The POST can only display error messages after the video
has been tested and verified. The system can fail or lock
up before the video has been verified and thus give no
screen error message.