BIOS and CMOS

[Devin2k]

How does the BIOS and the CMOS memory work? I know the BIOS contains Assembly language routines related to I/O and that it contains enough code to load the operating system from secondary memory to RAM.

I have never seen a clear, knowledgable explanation of how these two work together. Can anyone explain or give me references to other sources?

Thx, Devin

[geebee76]

Hi Devin2k, welcome to The PC Guide Forums.

This one is probably best left to the professionals. Have a look at our very own PC Guide (http://www.pcguide.com/ref/mbsys/bios/index.htm) BIOS section. There is a great deal of good information in there that should answer your question. If not, post back any specific questions.
http://www.PCGuide.com/ubb/smile.gif

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[Steve]

Hi Devin,

Resources? I think so. You can start here. (http://pcguide.com/ref/mbsys/bios/index.htm)

Or here. (http://www.howstuffworks.com/bios.htm)

Or maybe here. (http://www.webopedia.com/TERM/B/BIOS.html)

That should give you a good start. http://www.PCGuide.com/ubb/smile.gif

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[jahn]

Devin,
I am an Electrical Engineer so I understand the CMOS memeory part and I will try and explain what I think happens with BIOS.

Basically in older machines the CMOS memory chip also included a RTC (clock) and a ROM section. When the power was turned off a battery (usually a small silver oxide type) retained the contents of the memory section and kept the clock running. The contents of the CMOS ROM section was a small program (probably written in assembler) that laoded the values of the BIOS into the computer's RAM and the ram section held the BIOS values. When you entered the BIOS setup you were given axis to these memory locations on the CMOS memeory chip and you could change them. (the default values were normlly stored as well in ROM). The problem is that the batteries had a finite life and sooner or later the clock stopped keeping accurate time and eventually the contents of the CMOS memory erased and only the defaults were avalable.

The newer boards now use Electrically Erasable Programmable Read Only Memory (EEPROM) and the clock is not nessesarily even on this chip. The battery on the motherboard is for the clock and can be for RAM for the BIOS values or they can be stored in the EEPROM section. The EEPROM stores the program and is not suseptible to erasure when power is removed. The other advantage is that they can be updated to newer versions of BIOS by simply loading a program on a floppy for example. The floppy then overwrites the contents with a newer version. The older CMOS needed the chip to be swapped. The downfall is that some viruses can write to the BIOS and overwrite it as well.

This is as I understand it. I hope it helps