What the BIOS Does

As Chapter 5 explained, the BIOS performs several functions.

First, it reads the settings on a CMOS (complementary metal oxide semiconductor) memory chip that contains details about the computer's current configuration settings, including the current date and time, whether or not to use certain optional features (such as the audio and graphics controllers that are on many motherboards), and other options specified with the BIOS Settings Utility described later in this chapter. The computer uses a coin-sized battery to maintain a constant charge to the CMOS chip.

Based on the information in CMOS memory, the BIOS configures the chipset to work with the computer's hardware. Among other things, this includes the timing and latency of the RAM modules, and the order in which the BIOS searches disk drives and other storage devices for the boot loader software. These and other configuration settings are described later in this chapter, in the "Changing BIOS Settings" section.

Next, the BIOS loads interrupt handlers and device drivers. The computer's central processor (the CPU) uses these to exchange data with a limited number of I/O devices, such as the keyboard and the disk drives. When the CPU receives a request for access (an interrupt), it suspends and saves whatever it is doing and turns its attention to an interrupt handler that contains the instructions for responding to that type of interrupt. For example, when the processor receives an interrupt request from the keyboard, the interrupt handler converts keystrokes to code that the CPU can understand. After the interrupt is complete, the CPU returns to the task it was handling before it received the interrupt request.

Device drivers are similar blocks of software that supply details about I/O devices to the chipset and the CPU. Between the interrupt handlers and the device drivers, the BIOS provides the chipset with enough information to recognize and handle input and output signals from essential peripheral devices before the operating system loads.

After the device drivers and interrupt handlers have loaded, the BIOS starts the graphics controller that controls the video monitor. Many modern video controllers have their own BIOSes that they use to start the graphics processor in response to a command from the system BIOS. If the computer has more than one video card and monitor, the system BIOS loads only the one designated in the CMOS code as the primary; additional controllers wait for the operating system to start them.

Next, the BIOS runs the Power-On Self Test. The details of this test are described in the next section of this chapter.

The next step depends on the way the computer was turned on. There are two ways to start your computer. One is by simply turning on power (or pressing the Reset button on the front panel, which tells the processor, chipset, and other hardware to reinitialize themselves at a very basic level, and then restarts the computer), and the other is by entering a software command (the famous Ctrl+Alt+Del combination or a Restart command in Windows). Turning on power is a cold boot; restarting with a command is a warm boot. During startup, the BIOS looks for a flag in RAM (at address 0000:0472h) that identifies a warm boot. If it's a warm boot, the BIOS skips the memory portion of the POST. If it's a cold boot, the BIOS runs a read/write test on every memory address and displays the result on the monitor screen. When the memory test is complete, the BIOS tests the expansion cards connected to the motherboard.

At the end of the POST, the BIOS starts the graphics controller, which immediately displays some details about itself. Next, the BIOS reads some information about your computer from the chipset and displays it on the monitor. In most cases, this information scrolls up the screen faster than you can read it, but if you press the Pause/Break key, the BIOS routine stops and whatever information is visible on your screen at that moment remains on your screen. To restart, press the Reset button and follow any instructions that appear on the screen.

Finally, the BIOS searches for the boot loader program on a disk drive in the order listed in the Boot Sequence on the CMOS chip. If the first items in the Boot Sequence are a floppy disk drive or a CD/DVD drive (or some other drive with removable media), and there's no disk in the drive, the BIOS moves on to the next drive in the list. When the BIOS finds the boot loader, it starts the sequence of programs that leads to loading the operating system.

If a boot drive has a disk without a boot loader program, or if the BIOS can't read the disk, the BIOS displays a message like this one:

Note 

Non-System disk or disk error Replace and strike any key when ready

     Non-System disk or disk error     Replace and strike any key when ready 

Or:

     BOOT: I/O error reading disk     Please insert another disk 

When you see one of these messages, make sure there are no disks in the diskette drive or the CD/DVD drive and push the Reset button. If the message appears again, there's a problem with the hard disk drive that holds the operating system software.

What Kind of BIOS Do I Have?

During day-to-day operation, it doesn't really matter which brand of BIOS your computer contains. But if you're trying to interpret the POST beep codes or use the POST diagnostic codes described later in this chapter, it may become necessary to know who produced the BIOS.

Here are several ways to identify the BIOS in your computer:

Before Windows Loads:

1. Turn on the computer, or if Windows has already started, select Start Turn Off Computer Restart.

2. As soon as you see the splash screen that appears while the BIOS starts to load, or the first lines of text on your screen, press the key that opens the BIOS Settings Utility. On most computers, this is either the Delete key or the F1 key, but it might be a different key on your computer-it's usually identified on the screen, and always in the manual. The first screen of the utility usually includes the name of the BIOS.

3. If your computer does not show the type of BIOS in the Settings Utility, close the Settings program. Allow Windows to load and use the "When Windows Is Running" method described below.

This computer uses an AMI BIOS …

When Windows Is Running:

1. From the Start menu, select All Programs Accessories System Tools System Information.

2. When the System Information window appears, look for the BIOS Version about halfway down the list of items in the System Summary.

… and this computer contains a Phoenix BIOS.