the hardware (especially the chipset and processor) and the operating
system. The BIOS provides access to the system hardware and
enables the creation of the higher -level operating systems (DOS, Windows 95, etc.)
that you use to run your applications. The BIOS is also responsible f or allowing you
to control your computer's hardware settings, for booting up the machine when you
turn on the power or hit the reset button, and various other system functions.
System Boot Sequence
The system BIOS is what starts the computer running when you turn it on.
The following are the steps that a typical boot sequence involves. Of course this will
vary by the manufacturer of your hardware, BIOS, etc., and especially by what
peripherals you have in the PC. Here is what generally happens when you tu rn on
your system power:
The internal power supply turns on and initializes. The power supply takes
some time until it can generate reliable power for the rest of the computer, and
having it turn on prematurely could potentially lead to damage. Therefore , the
chipset will generate a reset signal to the processor (the same as if you held the
reset button down for a while on your case) until it receives the Power Good signal
from the power supply.
When the reset button is released, the processor will be r eady to start
executing. When the processor first starts up, it is suffering from amnesia; there is
nothing at all in the memory to execute. Of course processor makers know this will
happen, so they pre-program the processor to always look at the same plac e in the
system BIOS ROM for the start of the BIOS boot program. This is normally location
FFFF0h, right at the end of the system memory. They put it there so that the size of
the ROM can be changed without creating compatibility problems. Since there are
only 16 bytes left from there to the end of conventional memory, this location just
contains a "jump" instruction telling the processor where to go to find the real BIOS
startup program.
The BIOS performs the power -on self test (POST). If there are any f atal errors, the
boot process stops.
The BIOS looks for the video card. In particular, it looks for the video card's
built in BIOS program and runs it. This BIOS is normally found at location C000h in
memory. The system BIOS executes the video card BIOS, which initializes the video
card. Most modern cards will display information on the screen about the video card.
(This is why on a modern PC you usually see something on the screen about the
video card before you see the messages from the system BIOS itsel f).
The BIOS then looks for other devices' ROMs to see if any of them have
BIOSes. Normally, the IDE/ATA hard disk BIOS will be found at C8000h and
executed. If any other device BIOSes are found, they are executed as well. The
BIOS displays its startup screen.