A: BIOS stands for Basic Input Output System.

All computer hardware has to work with software through an interface. The BIOS gives the computer a little built-in starter kit to run the rest of softwares from floppy disks (FDD) and hard disks (HDD). The BIOS is responsible for booting the computer by providing a basic set of instructions. It performs all the tasks that need to be done at start-up time: POST (Power-On Self Test, booting an operating system from FDD or HDD). Furthermore, it provides an interface to the underlying hardware for the operating system in the form of a library of interrupt handlers. For instance, each time a key is pressed, the CPU (Central Processing Unit) perform an interrupt to read that key. This is similar for other input/output devices (Serial and parallel ports, video cards, sound cards, hard disk controllers, etc...). Some older PC's cannot co-operate with all the modern hardware because their BIOS doesn't support that hardware. The operating system cannot call a BIOS routine to use it; this problem can be solved by replacing your BIOS with an newer one, that does support your new hardware, or by installing a device driver for the hardware.

Q: Well, I see that the BIOS is necessary for the computer, but what can I do with it?

A: You can change hardware configurations that are stored in the CMOS, or Complementary Metal Oxide Semiconductor.

To perform its tasks, the BIOS need to know various parameters (hardware configuration). These are permanently saved in a little piece (64 bytes) of CMOS RAM (short: CMOS). The CMOS power is supplied by a little battery, so its contents will not be lost after the PC is turned off. Therefore, there is a battery and a small RAM memory on board, which never (should...) lose its information. The memory was in earlier times a part of the clock chip, now it's part of such a highly Integrated Circuit (IC). CMOS is the name of a technology which needs very low power so the computer's battery is not too much in use.

Your PC's performance can be highly affected by the CMOS settings. The reason for this is that the CMOS setup allows you to specify how fast your computer reads from memory, whether or not your cache is enabled or disabled, whether or not your CPU's cache is enabled or disabled, how fast your PCI bus communicates with its adaptor cards, plus a lot more.

Additionally, the CMOS setup allows you to specify disk drive and memory configuration. In order for your hard drive to work with your system, it must be configured in the CMOS setup. The exception to that rule is SCSI drives with adaptor cards, as most have their own built in BIOS. Floppy drives can be setup in the CMOS as well; a: can be made to be b: in many systems, and other configuration options can be changed as well.

Q: So how do I change the configuration that is saved in the CMOS?

A: By utilizing a set of menus called the CMOS Setup.

Setup is the set of procedures enabling the configure a computer according to its hardware caracteristics. It allows you to change the parameters with which the BIOS configures your chipset. The original IBM PC was configured by means of DIP switches buried on the motherboard. Setting PC and XT DIP switches properly was something of an arcane art. DIP switches/jumpers are still used for memory configuration and clock speed selection. When the PC-AT was introduced, it included a battery powered CMOS memory which contained configuration information. CMOS was originally set by a program on the Diagnostic Disk, however later clones incorporated routines in the BIOS which allowed the CMOS to be (re)configured if certain magic keystrokes were used.

Unfortunately as the chipsets controlling modern CPUs have become more complex, the variety of parameters specifiable in SETUP has grown. Moreover, there has been little standardization of terminology between the half dozen BIOS vendors, three dozen chipset makers and large number of motherboard vendors. Complaints about poor motherboard documentation of SETUP parameters are very common.

To exacerbate matters, some parameters are defined by BIOS vendors, others bychipset designers, others by motherboard designers, and others by various combinations of the above. Parameters intended for use in Design and Development, are intermixed with parameters intended to be adjusted by technicians -- who are frequently just as baffled by this stuff as everyone else is. No one person or organization seems to understand all the parameters available for any given SETUP.

Q: Now that I know where to edit the CMOS options, how do I access this CMOS setup on my computer?

A: By entering a keystroke combination when the system is first booted.

When the system is powered on, the BIOS will perform diagnostics and initialize system components, including the video system. (This is self-evident when the screen first flicks before the Video Card header is displayed). This is commonly referred as POST (Power-On Self Test). Afterwards, the computer will proceed its final boot-up stage by calling the operating system. Just before that, the user may interrupt to have access to SETUP.

Usually, setup can be entered by pressing a special key combination (DEL, ESC, CTRL-ESC, or CTRL-ALT-ESC) at boot time (Some BIOSes allow you to enter setup at any time by pressing CTRL-ALT-ESC). The AMI BIOS is mostly entered by pressing the DEL key after resetting (CTRL-ALT-DEL) or powering up the computer. You can bypass the extended CMOS settings by holding the key down during boot-up. This is really helpful, especially if you bend the CMOS settings right out of shape and the computer won't boot properly anymore. This is also a handy tip for people who play with the older AMI BIOSes with the XCMOS setup. It allows changes directly to the chip registers with very little technical explanation.