What's a BIOS?

Summary: BIOS, for Basic Input Output System, is the software already built in to you computer. It's primary job? Load other software.

I keep hearing people talk about something called a BIOS in my computer. What is it?

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Your computer's BIOS is perhaps one of the oldest legacies of PC computers. It's special software that's on your computer before you take it out of the box, and before you even turn it on.

Even before the computer has a hard disk installed, the BIOS is there.

It's software that has a critical role in getting your computer started.

It's a little like my morning coffee that way.

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The BIOS, for "Basic Input Output System" is software (or more properly, "firmware") that resides in a special memory chip on your computer's motherboard. The BIOS begins running the instant that your computer is turned on, before any other software is loaded. It runs before your hard disk is even touched, or for that matter before your computer even knows that there is a hard disk.

"Your computer's BIOS is completely separate from (and unrelated to) Windows ..."

Your computer's BIOS is completely separate from (and unrelated to) Windows or whatever other software you might have installed on your hard drive.

The BIOS has three primary purposes:

• When you first turn on your machine the BIOS performs various tests - called the "Power On Self Test" or POST - to ensure that your hardware is operating properly at some basic level. It'll perform tests such as ensuring that memory is working, a keyboard is present, and that a hard drive can be found. The tests are not exhaustive (so as not to delay the next step), but often detect basic problems that would impact your ability to use the computer.
• After completing the POST it's the BIOS that boots your machine. It figures out what device (Floppy? CD/DVD? Which of several hard disks perhaps?) to boot from, and then loads and runs the software that it finds on the boot device. It's likely that on your computer this is where Windows starts to load.
• After the operating system is loaded, the BIOS is still available and can provide a common software interface to some of your computer's hardware. It's not uncommon for Windows (or other operating systems) to continue to use the software in the BIOS to access your hard disk or other common hardware.

The BIOS originally was truly software in hardware - it was placed in unalterable read-only memory (ROM) and could be replaced or updated only by physically opening the computer and replacing the chip that contained it.

In later years, ROM's were replaced with "Flash ROM's", which are similar in some ways to the Flash memory used in USB memory sticks and memory cards. The contents of the Flash ROM could be replaced by a upgrade process that required only special software. Typically this involved booting from a floppy disk and running a utility specific to that particular motherboard and ROM that would perform the magic sequence to replace the Flash ROM contents.

Unfortunately, if that failed, and the BIOS was incompletely updated, the result was often a dead motherboard. While all the hardware might be in fine working order, without a working BIOS there is no way to boot - not from floppy (to update the BIOS) or from a hard disk or from anything else for that matter. Initially, that meant physically replacing the chip once again.

Fortunately, memory got cheaper and many machines now include a backup copy of the "factory original" BIOS on the motherboard which can be reset - typically by opening the computer and setting a special jumper on the motherboard or some other special sequence. (The specific technique varies based on your motherboard.)

Normally your BIOS is not something you really need to think about. In fact, unlike other software on your machine I actually recommend updating it only when there's an identified need. Since it is possible for a BIOS update to fail, and recovering from that failure can often be quite painful, it's often just not worth it. When I've checked, most BIOS updates available for my equipment actually have nothing relevant to my machines or usage.

On occasion, however, updating a BIOS can be just the thing to do for specific problems. If that's the case, research on that specific problem will lead you to a BIOS update.