 The magazine of the Melbourne PC User Group Taking the mystery out of CMOS Tom Coleman |
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The woes that plague members' computers seem to come in waves. Recently I seem to have had a lot of calls from members with CMOS problems. Depending on how well prepared the caller has been, the problem was fixed in a couple of minutes or a couple of hours (and sometimes not over the phone)
Let's take a look at CMOS and see where it fits into the overall scheme of things. A look at the first part of the computer's start-up sequence would be a logical place to start.
When you turn on your computer the 240 V AC electricity powers up the computer's power supply. This is a rather clever bit of electronics that converts the incoming power into +5 V, -5 V, +12 V and -12 V DC.
The output of an ordinary transformer/ rectifier is usually pretty lumpy but the output of your computer's power supply has been electronically massaged so it comes out pretty smooth.
It's not only smooth but it is pretty spot on for voltage too. You see all of these different voltages are continuously sampled by a sort of quality control chip that generates a special "Power Good" signal - provided of course everything is up to expectations.
This "Power Good" signal is used by most motherboards as an "Okay to Start" signal. No "Power Good" signal, no start.
When you press the "Reset" button on the front of your computer all you've done is remove "Power Good". The motherboard thinks that it's had a power failure and shuts down. When you take your finger off the button the motherboard receives an "Okay to Start" signal and it does, from the beginning of course.
Once the motherboard receives the "Okay to Start" signal it starts to run the ROM BIOS. That sounds pretty impressive, doesn't it? A good bit of computer voodoo.
The ROM BIOS program checks the computer to see that it is turned on and working properly. You can see the memory count at startup. The program also checks the video and most other things and then it sets up low-level routines for doing some fundamental computer functions. When it has completed the check it usually gives a single beep. If things do not check out then you'll hear a coded series of beeps.
Once the ROM BIOS has run out of things to do, it passes activity to the CMOS. CMOS stands for Complimentary Metal Oxide Semiconductor. Don't ask me why. It could have stood for Computing's Most Obscure Secret, or perhaps Coleman's Mostly Old and Senile. We could run a competition on this couldn't we?
Anyway the CMOS is a sort of memory chip that stores a description of the computer. Well some parts of the computer anyway. Note: CMOS is not BIOS, some people confuse the two or think that they are the same thing.
Check it out
You can usually take a look at your computer's CMOS by "Pressing Del to enter setup" during the startup process. Some computers use a different key and some require you run a special setup program. You've probably seen and ignored such a message each time you have started your computer.
When the computer reads the CMOS it learns what kind of floppy and hard disks it has. It reads the date and time from an on-board clock and sets up some aspects of how it is expected to go about working.
Exactly what is contained in the CMOS varies from one version to another. Some have passwords, virus checking, some memory allocation and so on. All versions however, have information about hard and floppy drives and the date and time. Virtually all have information about the keyboard, the video and the NumLock status.
Go ahead, have a look
You cannot do any harm by looking at your computer's CMOS. Just don't change anything unless you know what you are doing. If you think you might have changed something while you were having a sticky-beak, do not panic. And do not save the changes. (Respond "No" when you are asked whether you wish to save your changes when you exit the CMOS setup menu.)
Most CMOS menus offer you the choice of "BIOS defaults." It's a kind offer, that I urge you to reject, unless you know what you are doing or you want to have a pleasant evening sobbing into your keyboard.
The BIOS defaults are how CMOS is set if it hasn't been told differently. The defaults are usually a 5.25-inch drive A, no drive B, and no hard drive. (Unless this describes your computer, you are probably starting to realise why you don't want to select BIOS defaults). Most of the other settings are set to "Disable" or "Not Installed."
While not all CMOS setups default to a 5.25-inch drive, you can be sure that whatever your BIOS A drive default is, it won't coincide with your actual A drive.
So what? When you try to boot from drive A you will get an error message. You won't be able to book from drive C either. Why? Because according to your standard BIOS defaults, you CMOS has been told that you don't have a C drive!
Do you have the time?
For the computer's date and time functions to work, it needs to run, even when the computer is turned off. So computers are supplied with a small battery that keeps the CMOS working between sessions. This is usually a rechargeable battery and will last many years. Some computers come with regular torch cells and these will run out of enthusiasm every six to twelve months and you will have to replace them.
Almost all computers come with pins into which you can plug in an external battery. This comes in handy if your rechargeable battery dies or the charging circuit fails, because it is usually more convenient to convert to external batteries than to try and install new rechargeable, internal batteries. The batteries and the battery holder cost less than a replacement NiCad battery and you don't have to pay someone to pull out your motherboard and solder in the new, internal, rechargeable battery.
| How to tell if your battery is dying, but not yet dead BIOS defaults to a date of 1-1-80 and a time of 01:00. An almost flat battery usually restarts with the day and time at which it was last turned off. A totally dead or disconnected battery will go to "BIOS defaults." |
Why do you need to know this? Because I have heard stories of shonky dealers who try to sell replacement motherboard to users who only need a $10 external battery and holder when their CMOS battery died.
Of course if your CMOS battery dies a slow, agonising death, you will have some warning and presumably will have the opportunity to make arrangements well in advance of any disaster. On the other hand there have been plenty of instances of CMOS batteries dropping dead without warning. Actually there probably was a warning, but it wasn't recognised.
Of course you are sure that this will not happen to you. So you can safely ignore the rest of this article. Oh, by the way, Bill Clinton is secretly being controlled by telepathic commands from beings on Alpha Centuri.
Now, let's see, your computer relies on CMOS
That sounds to me like some pretty compelling arguments for knowing what information is in your CMOS.
But, first, while we are on the subject of CMOS induced amnesia, there are other ways your CMOS can get trashed - besides a dead battery or mistakenly setting your CMOS to BIOS defaults. Sometimes a program goes berserk and stomps on the CMOS information. And if you remove a card or unplug a drive the CMOS may reset itself to the dreaded "BIOS defaults."
Now a show of hands, please
Raise your right hand if you know your CMOS hard drive settings. Pretty close to none, which is what I expected.
Sometimes lightning strikes twice
My second line of defense is not quite as high-tech as my first, but it works. Why have a second line of defense anyway? Well, I know that it's possible that my boot floppy will be corrupt or lost on the day that I need it. So I have cunningly made a printed copy of each screen of my CMOS settings.
Different computers will do this different ways, some will not print with the "Print Screen" at CMOS time. I am lucky, mine does. If yours doesn't you may have to make a hand-written copy or find one of the diagnostic utilities (available on the BBS) that will display CMOS settings after boot up and from which "Print Screen" will work.
How do you do a Print Screen?
The Print Screen key is set up when ROM BIOS runs. It works under most circumstances. But here's is an inside tip for success; Print Screen will fail every time if the printer is turned off or out of paper.
My A drive is a 3.5-inch disk drive, but my BIOS default is for a 5.25-inch disk drive. So how do I get started after I have crashed and burned?
Simple. I press Del and enter Setup. Then adjust the setting for drive A, press Esc, Write to disk and exit. Then I boot from Drive A. (Ah, would that life was always so simple.)
Shoot yourself in the foot, and save time too!
Now here is a good way to shoot yourself in the foot (just in case you are looking for one). While rummaging around in your CMOS setup you may notice that you have the option of altering the boot sequence, the usual sequence is A drive then C drive.
What this means is that your computer tries to start using the floppy in drive A: and only if that fails will it try to boot from drive C. If you take the option of changing the sequence the computer will go directly to drive C. In doing so you will have saved a half a second at bootup time and avoided that annoying "Non System Disk" error message (which you get when you boot your computer with a data disk still in your A drive).
Sounds great, but it ain't
What if you develop a problem with your C drive? It's not unheard of, believe me. Some people say "Oh it's no problem. You just press Del and enter Setup". (I have even heard computer salesmen say this very thing.)
Sorry, big problem! Because the BIOS detects the problem and you never get the option to press Del. It just times out and hangs. And now you can't start your computer at all so you don't even know if you have a problem with your hard disk. The symptom is simply, it doesn't go.
This does not happen every time, of course, but I have struck it about half-a-dozen times, more than enough times for me to have an almost religious belief in the following:
make sure that your computer boot sequence is A drive then C drive.
If you don't, be prepared for my jeering, scornful, sarcastic, sympathetic laughter.
There is a lot more to CMOS than what I've covered here. Come to think of it there a lot more to start-up too. If I was to have an article in the next issue on more of the same, you would have had a month to get this part under your belt. It will take you much less than half-an-hour to get a copy of your CMOS.
If you have don't already have a copy of your CMOS settings, Go and Do it Now!
Reprinted from the July 1996 issue of PC Update, the magazine of Melbourne PC User Group, Australia
