sdram question

Hi, I was recently asked by a friend about an old p3 that uses sdram
pc133. He has two ram modules, a 512mb and a 256mb.The 512mb is in the
slot closest to the processor and the 256mb in the second slot. The
computer is only seeing half of the 256mb or half of the 512mb when
installed in the second slot. At first we thought that the ram was
bad, so after switching the modules around, we have determined that
there must be a problem with the slot itself. If this is correct why
does it only see half of the ram?, would it not just fail to see any
that is installed in that slot? I could use some insight as to why
this is happening. Thanks in advance for any inputs you might have. -J

jinxy wrote:
> Hi, I was recently asked by a friend about an old p3 that uses sdram
> pc133. He has two ram modules, a 512mb and a 256mb.The 512mb is in the
> slot closest to the processor and the 256mb in the second slot. The
> computer is only seeing half of the 256mb or half of the 512mb when
> installed in the second slot. At first we thought that the ram was
> bad, so after switching the modules around, we have determined that
> there must be a problem with the slot itself. If this is correct why
> does it only see half of the ram?, would it not just fail to see any
> that is installed in that slot? I could use some insight as to why
> this is happening. Thanks in advance for any inputs you might have. -J

The memory bus consists of two parts. The address/command bus, which
sends multiplexed RAS and CAS (row address, column address). And the
64 bit data bus, where the data travels.

If the address/command bus does not have enough bits, it cannot
address the larger memory products. Then, only a fraction of the
memory would be detected.

On a 440BX chipset for example, only 256MB "low density" modules
work right. That is a double sided module with 16 chips. If
you were to go to the Best Buy, they'd probably sell a 256MB
stick which was single sided. Due to density problems, it
would only be half detected, and give 128MB. If you bought
the same stick from Crucial.com , they'd ship a 16 chip module
instead. And that would work. If you shoved a 512MB stick into
a 440BX motherboard, then there'd be trouble for sure.

So that is an example of the kind of problem you can run into.

Not all chipsets have problems like that, as the chipset maker
looked far enough ahead, to make sure a decent selection of
memory would work. Sometimes, they cannot test for it, because
when the chip is first made, memory that big doesn't exist.
So sometimes you get lucky, and sometimes not.

To give another example, an AthlonXP motherboard with Nforce2
chipset, can only use certain kinds of 1GB DDR memory sticks.
If the module has x4 chips, one extra (missing) address bit is
needed. Thus, you have to buy a proper module with x8 chips,
to get it to work right.

Generally, if you go to Crucial.com or Kingston.com and look
up the required memory, you won't be steered wrong. The trick
then, is to figure out what is so special about the RAM they
offer, and find similar characteristics in a substitute product.

Paul