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By Root 1321 0

an older system? First of all, read the error carefully. Let’s say that on that same system you got an “8042—gate A20 failure” beep code. What will you do? Assuming you know (and you should!) that the “8042” refers to the keyboard, a quick peek at the keyboard and its connection would be a good first step. Beyond that specific example, here is a good general rule: If you don’t know what the error means or the bad part isn’t replaceable, replace the motherboard. Clearly, you will stumble across exceptions to this rule, but more often than not, the rule stands.

The Boot Process

All PCs need a process to begin their operations. Once you feed power to the PC, the tight interrelation of hardware, firmware, and software enables the PC to start itself, to “pull itself up by the bootstraps” or boot itself.

When you first power on the PC, the power supply circuitry tests for proper voltage and then sends a signal down a special wire called the power good wire to awaken the CPU. The moment the power good wire wakes it up, every Intel and clone CPU immediately sends a built-in memory address via its address bus. This special address is the same on every Intel and clone CPU, from the oldest 8086 to the most recent microprocessor. This address is the first line of the POST program on the system ROM! That’s how the system starts the POST. After the POST has finished, there must be a way for the computer to find the programs on the hard drive to start the operating system. The POST passes control to the last BIOS function: the bootstrap loader. The bootstrap loader is little more than a few dozen lines of BIOS code tacked to the end of the POST program. Its job is to find the operating system. The bootstrap loader reads CMOS information to tell it where to look first for an operating system. Your PC’s CMOS setup utility has an option that you configure to tell the bootstrap loader which devices to check for an operating system and in which order (Figure 7-31).

Figure 7-31 CMOS boot order

Almost all storage devices—floppy disks, hard disks, CDs, DVDs, and even USB thumb drives—can be configured to boot an operating system by setting aside a specific location called the boot sector. (Later chapters show you how to do this.) If the device is bootable, its boot sector contains special programming designed to tell the system where to locate the operating system. Any device with a functional operating system is called a bootable disk or a system disk. If the bootstrap loader locates a good boot sector, it passes control to the operating system and removes itself from memory. If it doesn’t, it goes to the next device in the boot order you set in the CMOS setup utility. Boot order is an important tool for techs, because you can set it to load in special bootable devices so you can run utilities to maintain PCs without using the primary operating system.

Care and Feeding of BIOS and CMOS

BIOS and CMOS are areas in your PC that you don’t go to very often. BIOS itself is invisible. The only real clue you have that it even exists is the POST. The CMOS setup utility, on the other hand, is very visible if you start it. Most CMOS setup utilities today work acceptably well without ever being touched. You’re an aspiring tech, however, and all self-respecting techs start up the CMOS setup utility and make changes. That’s when most CMOS setup utility problems take place.

If you mess with the CMOS setup utility, remember to make only as many changes at one time as you can remember. Document the original settings and the changes on a piece of paper so you can put things back if necessary. Don’t make changes unless you know what they mean! It’s easy to screw up a computer fairly seriously by playing with CMOS settings you don’t understand.

Losing CMOS Settings

Your CMOS needs a continuous trickle charge to retain its data. Motherboards use some type of battery, usually a coin battery like those in wrist watches, to give the CMOS the charge it needs when the computer is turned off (Figure 7-32). This battery also keeps track