3:
Booting the Computer Whenever you turn on your
computer, the first thing you see is the BIOS software doing its
thing. On many machines, the BIOS displays text describing things
like the amount of memory installed in your computer, the type of
hard disk and so on. It turns out that, during this boot sequence,
the BIOS is doing a remarkable amount of work to get your computer
ready to run. This section briefly describes some of those
activities for a typical PC.
After checking the CMOS Setup and loading the interrupt handlers,
the BIOS determines whether the video card
is operational. Most video cards have a miniature BIOS of their own
that initializes the memory and graphics processor on the card. If
they do not, there is usually video driver information on another
ROM on the motherboard that the BIOS can load.
Next, the BIOS checks to see if this is a cold boot or a
reboot. It does this by checking the value at memory address
0000:0472. A value of 1234h indicates a reboot, and the BIOS skips
the rest of POST. Anything else is considered a cold boot.
If it is a cold boot, the BIOS verifies RAM by performing a
read/write test of each memory address. It checks the PS/2 ports
or USB ports for a keyboard and a mouse. It
looks for a peripheral component interconnect (PCI) bus and, if it finds one, checks all the PCI
cards. If the BIOS finds any errors during the POST, it will notify
you by a series of beeps or a text message displayed on the screen.
An error at this point is almost always a hardware problem.
The BIOS then displays some details about your system. This
typically includes information about:
- The processor
- The floppy drive and
hard drive
- Memory
- BIOS revision and date
- Display
Any special drivers, such as the ones for small computer
system interface (SCSI) adapters, are
loaded from the adapter, and the BIOS displays the information. The
BIOS then looks at the sequence of storage devices identified as
boot devices in the CMOS Setup. "Boot" is short for
"bootstrap," as in the old phrase, "Lift yourself up by your
bootstraps." Boot refers to the process of launching the operating system. The BIOS will try to initiate
the boot sequence from the first device. If the BIOS does not find a
device, it will try the next device in the list. If it does not find
the proper files on a device, the startup process will halt. If you
have ever left a floppy disk in the drive when you restarted your
computer, you have probably seen this message.
This is the message you get if
a floppy disk is in the drive when you restart your
computer.
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The BIOS has tried to boot the computer off of the floppy disk
left in the drive. Since it did not find the correct system files,
it could not continue. Of course, this is an easy fix. Simply pop
out the disk and press a key to continue.
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