Is it possible to brick a computer due to corrupted CMOS?

Does the BIOS do error checking before booting with the CMOS configurations? Or does it blindly take whatever inside the CMOS and try to boot with it?

myopenid

Posted 2014-07-12T07:27:01.947

Reputation: 1

Does it matter? Either way if the CMOS contents is corrupted then the system probably won't boot. Clearing the CMOS settings should fix that though. Can you describe the problem that you are having instead? – zelanix – 2014-07-12T12:11:04.550

Yes, there's a checksum on the data. No. – sawdust – 2014-07-12T19:07:32.767

@zelanix I have come across a lot of laptops which are perfectly healthy but will not boot, with symptoms of being stuck with black screen on startup and nothing more. If the CMOS is verified before being used I suppose there is a chance the BIOS can load a default from the ROM instead of booting ahead with a corrupted config? – myopenid – 2014-07-17T14:29:12.517

A few things to try, first take the main battery out and hold down the power button for 30 seconds. If that doesn't work, you could try to find the CMOS battery and remove it for 30 seconds. I've had a few laptops with similar symptoms where removing the CPU and reseating it helped. Also try reseating all the RAM, in fact, reseat all removable components, one at a time! Given the symptoms, I would not expect a corrupted CMOS to be the cause. Good luck! – zelanix – 2014-07-17T20:48:10.113

1"laptops which are perfectly healthy but will not boot" -- Your criteria for "perfectly healthy" needs to be more stringent. – sawdust – 2014-08-13T02:00:56.717

Answers

Every time computer boots, BIOS calculates checksum of CMOS memory. So if something is corrupted (even one bit) BIOS will end up with error message.

CMOS checksum is old good parity byte. Here is simple demonstration:

Let's take some bytes, for example $4E 61 54 69. In binary form, they are

01001110
01100001
01010100
01101001

We got some kind of table. We should look at columns in that table and write zero below each column if it has even number of ones and one if number of ones is odd. After performing this simple operation we get one more byte:

01001110
01100001
01010100
01101001
00010010

Last byte is parity byte, our checksum. And checksum of these five bytes is eight zeroes, it indicates the fact that everything is alright.

Time to try to change something, for example rightmost bit in the middle row, and compute parity byte again.

01001110
01100001
01010101
01101001
00010010

00000001

We didn't get eight zeroes, so we can be one hundred percent sure: something is changed.

P.S. Also BIOS checks its own ROM (actually flash) memory in the same way.

Resurrexisse

Posted 2014-07-12T07:27:01.947

Reputation: 19

Actually most checksum algorithms will give false negative results (i.e. fail to report corruption) for some multiple bit errors. Hence the use of more sophisticated cyclic redundency checks, e.g. CRC32. – sawdust – 2014-08-13T02:03:38.553

All you have to do is remove the CMOS battery and the BIOS will reset to default. Bricking the BIOS does nothing to brick the computer.

Christian Chapman

Posted 2014-07-12T07:27:01.947

Reputation: 1

Since the PC is made out of lot components - the term "brick the computer" is slightly incorrect. The BIOS does something called "P.O.S.T" - Power On Self Test when the PC starts.
It looks at the available hardware for "all good" to start.
If the BIOS is bricked, then there is no way out rather than changing the physical BIOS chip from the PC's motherboard and putting an equivalent one in.
I do not have enough knowledge background on flashing the bad BIOS with good one... U can google it if u have time...

Prasanna

Posted 2014-07-12T07:27:01.947

Reputation: 3 554