Number 260 - January 2005

THE BIOS
by Dick Maybach, Brookdale Computer Users Group, Inc.
The "brains" of your PC is its central Processing Unit (CPU), but by itself it is more helpless than a new-born infant. It can't access a disk, read from a keyboard, write to a screen, or perform any other complex task. The CPU has only one built-in ability--when it's first turned on or reset, it can read and execute one instruction from a fixed location in memory. Your PC contains two types of memory. Most of it is Random Access Memory (RAM), which clears itself when you turn off the power, but a small amount is a form of Read Only Memory (ROM), which holds its contents even if power is removed. All PCs locate ROM so that when the CPU performs the only trick it knows, it reads the instruction from ROM.    Computer manufacturers load this ROM with a set of programs called the Basic Input/Output System (BIOS). The first instruction that is executed is the start of a program that teaches your PC how to perform its basic tasks (reading from the keyboard, writing to the screen, reading from and writing to hard disks, diskettes, and CD-ROMs, and making simple sounds on the speaker). The BIOS ROM can be one of several types of semiconductor. Older PCs used Complementary Metal Oxide Semiconductor (CMOS), which required a battery to hold its contents when the main power was off. Modern PCs may use Electrically Erasable Programmable Read Only Memory (EEPROM), which does not require a battery.    After completing this basic education, the BIOS checks the health of your PC with a program called the Power-On Self-Test (POST). Here it initializes the CPU, checks the CMOS battery (if there is one), checks that the BIOS contents are valid, performs a simple test of RAM, tests the video system and the keyboard, initializes and tests the serial and parallel ports, and checks that the hard and removable disks are accessible. These tests are quite simple and can miss many serious problems, but if they fail, your PC is certainly inoperable. Normally, the POST is successful and the PC indicates this with a single long "beep" from the speaker. Any other sound means serious hardware trouble.	Your PC's manufacturer has told its BIOS about the hardware, in particular, the sizes of its disks and the configuration of its external ports. You can access this data with the BIOS configuration program, usually called the CMOS set-up program (whether or not the BIOS uses CMOS).    On most PCs, instructions on how to start this program appear briefly on the screen when the machine boots. If you run this program, be careful; a wrong key-stroke can disable your PC. The program will show you many parameters you can change, usually several screens full. It is worthwhile to go through these and write down every parameter value. (Use the help screens and your PC manual to help you understand them.) When you exit the program, it will ask you if you want to save the changes. Unless you have made hardware changes, such as adding a new disk, the answer is "No."    Before Windows 95, most computer programs accessed the PC hardware through the BIOS. For example, to write a character to the screen, they would send the character to the BIOS and ask that it be displayed. This is no longer true; instead, the main task of the BIOS is to start the PC. After it has initialized the PC and run the POST, the BIOS loads Windows into RAM and turns over all control to it.    Loading Windows is not a simple process, as you probably know from the time it requires. To find which disk holds the Windows program and where on the disk it is, the BIOS runs a short program in the Master Boot Record (MBR), which is located in the first part of the disk. This short program works much like the BIOS when you first boot the PC. It executes an instruction located on the disk, which is the first instruction in the program that begins the Windows loading process. The BIOS's work is now done, and Windows assumes control of the computer.    Dick Maybach is a member of the Brookdale Computer Users Group, Inc., Lincroft, NJ, and can be reached at, n2nd@att.net.
Number 260 - January 2005