Hi,
Yesterday I put an extra stick of RAM (PC 2100 256MB 184 pin DIMM) into a friend's PC.
It already had 256 and it needed to be upgraded.
However, upon booting with the new stick installed the screen became fuzzy and full of artefacts.
It returned to normal when I took out the upgrade RAM.

I'm wondering what caused this and what exactly is happening?

Thanks!

There's multiple possibilities.

What type is the existing module? What chipset? Does the module work by itself? Does it work if the modules are switched around? Did you try a memtest? Is the BIOS configured for automatic clock/latency detection?

Tried everything you said and as usual you were right on the money.
I configured the BIOS for automatic CAS latency detection and it fixed the problem right away!
Thank you again.

While I'm on the subject though can I ask why this fixed the problem?

Glad to help, as always.

This may go over things you already know, but it should answer your question.

Memory latencies are one of the most interesting PC configurations options, because they don't dictate how fast a particular action is performed, like say, a clock cycle, but how long the system will wait for a certain thing to occur.

SDRAM (of which DDR is a type; DDR SDRAM) is, as the acronym says, Synchronous Dynamic. The Synchronous part means that every transaction must be done as a single discrete operation; once a transaction is in motion, there can be no moving onto another transaction while things happen. Storing or retrieving things from memory is rather complicated on a physical level, and involves lots of little wait stages, such as waiting for the Column Address Strobe (CAS) signal to stablise. Because the operation has to be synchronous, the entire process has to be held up for however long it will take for the CAS to stablise, but it's impossible to know precisely how long this will take. If we try to proceed to the next stage before the signal is stable, we may select the wrong column and that would be writing or reading to the wrong location in memory.

If that makes sense to you, then you can see that a CAS latency that is just a little too tight might result in one error per thousand. Any errors at all are unrecoverable, so 1/1000 is completely unacceptable and will result in a system that might be able to POST, but do little else. Higher quality memory is faster, takes less time to do things so the wait time is shorter, and latency values are smaller. Another issue can be the voltage supply to memory; increasing voltage to memory can protect against energy leaking out of transistors as power, but over voltage can damage a module. Generally, I do not recommend manual DRAM voltage selection unless you know the exact capabilities of your modules. The high end modules that you see, that can do CAS 2 and even lower, usually do so close or over the 3V mark.

Contrary to general belief, tightening your memory latencies is not overclocking, because you're not actually increasing the speed of anything, you're just giving memory modules less time to do things. DDR SD modules have Serial Presence Detect (SDP) chips that hold default setup values, which are usually very conservative (CAS 3 for example). It's extremely rare that a module wont post on any given system when configured to auto. This is so that you can always be assured of a good POST on auto.

Thanks for the detailed reply.
It makes alot more sense now!

Always a pleasure, to impart a little of my rather limited knowledge of things PC.

HEY! long time.

Finally, my Ram arrived after some payment altrications!

Alas, as all upgrades go, it seems to be incompatible, the computer monitor sits in stand-by mode as well. The only thing i can think of is that the mixture of ram modules, has diferences of ECC. but my motherboard says that it is variable??!!

what could i do? and how do i check the bios settings as described above?

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