[Pc_Support] Memory DIMMs--PC66/PC100/PC133

[Bryan J. Smith <b.j.smith at ieee.org>]

From: Mike Webb <GrokWeeble at HotPOP.com>
> 1) What happens if you plug a PC66 DIMM into a PC100 or PC133 motherboard?

Depends on the mainboard.  The jumper, soft-select or possibly CPU generated
clock will attempt to synchronize to the synchronous dynamic random access
memory (SDRAM).  If the integrated circuit (IC) SDRAM chips on the DIMM are
incapable of supporting the synchronous rate, the system will fail to power-on
self-test (POST).

Now that's assuming the DIMM is compatible in the first place.

You see, the synchronous clock rate is the _least_ of your concerns.  You need
to verify the DIMM is registered or unregistered, buffered or unbuffered, what
size of buffer, what size of SDRAM IC technology, width, etc...  You can have
two completely _incompatible_ PC100 DIMMs that are designed for different
chipsets.

The synchronous clock rate is actually a very minor detail.

> 2) What happens if you plug a PC100 DIMM into a PC66 motherboard?
> 3) Same as #2, but the mobo is PC133?
> 4) What happens if you plug a PC133 DIMM into a PC66 or PC100 mobo?

Once again, the clock is the least concern.

> And while I'm talking about deceased hardware, who has a Pentium II 
> motherboard (one that will accept a PII/450 chip) they would be willing 
> to donate to a project that will help out someone who can't afford a 
> computer? (BTW, if it makes a difference, I'll be wanting to put it into 
> a Dell Dimension case.) Feel free to email me offlist on that, just put 
> something like "Weeble" or "Motherboard" in the subject line so I don't 
> nuke your message as spam (I do all my nuking manually, so you don't have 
> to worry about going awry of a spam nuker).

Intel has created several SDRAM chipsets that use different types of memory
IC size, width, etc...  The i440FX/LX/BX, i810/815 and i845 take very different
types -- with only a little overlap.

And none of it has to do with PC66 v. PC100 v. PC133.  The only way you
know what is compatible is to become familiar with what IC sizes and widths
each chipset takes.

Typically you won't blow anything unless you attempt to use a non-ECC
memory in an ECC requiring mainboard.  If you put in the wrong size/width
IC DIMMs, they will typically not POST, or maybe show up as 1/2 or 1/4th.


--
Bryan J. Smith   mailto:b.j.smith at ieee.org