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What's a BIOS?


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

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.


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.

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9 comments on “What's a BIOS?”

  1. Nice explaination.. ( as usual):-)
    In layman’s term, we can say that, BIOS acts as a Bridge for the communication between the Hardware parts in the computer & the Operating System..

  2. This article was helpful. Before now, I had heard of BIOS, but had no idea what it was or what it did. It’s good to know more.

    You should be aware that the links in this article, which I tried to use to amplify my understanding of the topic, led to ADS, not the info I had expected. There didn’t seem any way to get rid of the ads and get to the content.

    If the links have double underlines, then they are ads. More here: How do I turn off those ads that look like links on your site? (Remember that without advertising, Ask Leo! probably wouldn’t exist.)


  3. Also, the battery in the motherboard is used for keeping the bios.
    If the battery is removed the bios loses all it’s data.

  4. Banyarola: You are partiallly correct. The battery does help keep the BIOS _settings_ but not the BIOS itself. If you remove the battery, BIOS settings (including date and time) are reset to factory defaults but no harm is done to the BIOS.

  5. I have a related question I hope someone would address.

    In the early/mid-90s when I had my first computer, there seemed to be many instances of dead BIOS batteries causing the BIOS to lose its preferred settings; I believe a dead battery would then cause the BIOS to revert to the factory defaults which may NOT have been compatible or ideal for your computer’s hardware. The number of settings were relatively few and as a precaution, I noted all my BIOS’s settings on paper.

    Now, though, BIOS settings seem to be a more complex affair. There can be scores of settings, I believe. From time to time I keep wondering: Is the problem I described above likely to occur on modern systems? Is there a way (short of using paper and pencil) to save all your BIOS settings to a text file by using a special utility? If so, you’d be able to print the settings and save them in case you ever needed to manually re-apply your correct BIOS settings.

    Thoughts, anyone? Thanks … And many thanks to Leo for such great articles, as usual!

    BIOSes have two parts: a program, and the data or settings stored by that program. The battery is what keeps the settings “remembered” when your computer is unplugged. If that battery dies, the settings go with it. The BIOS *program* however – the instructions that it has that know how to do things – is NOT affected by the battery. Only flashing a new BIOS will update that. That’s why even without a battery at all the BIOS still comes up does its tests – even though it may have lost track of the date and time and all of its settings.


  6. Saving bios setting other than writing them down .?..a photo of each page might work ok.

    Obviously prtscn function would not work until windows had actually booted.


  7. I did just that late last year. I took a digital photo of each of the BIOS setting screens.

    I had bought a new 1 TB USB disk for my old PC, only to find that it would not boot if the new disk was connected. Turns out that the BIOS would not recognise the newer high-capacity USB drives. I found that there was a much later version of my BIOS which I downloaded and flashed, hoping that it would resolve the problem (it didn’t – I ended up buying a new machine for other reasons). But by taking the photos, I was able to ensure that the new BIOS had all the correct settings before I booted to it.

  8. my system some times present a blue screen and restarts again whts the reason behind
    what makes it to be like that

  9. As I mentioned in, probably, some other thread, USB devices in general, not just large disks, seem to cause random occasional problems with many machines I have seen. Typically, I have seen them prevent POST from running at all. Unplug the device, turn the machine off, and back on. All is well.


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