« Gotcha with Outlook 2003 with RPC-over-HTTPS | Main | What is "stopfaxnow.com"? »

August 30, 2006

Excellent overview of memory technologies

I remember the good old days of buying RAM, where the only two parameters were:

Those days are long gone.

Every time I have to spec memory for a server, I start to wallow in all the various attributes: DDR, ECC, Registered, PC1600, Dual-Ranked, etc. and even when looking at a full list of required specs, there's always the fear that I'm somehow missing a parameter that matters.

For instance: I once bought a stack of RAM upgrades for some Dell Optiplex workstations, but found that only DIMMs with chips on both sides were supported, not DIMMs with chips on only one side. Huh? I assume this has something to do with rank or bank, but they flat-out did not work. This was not listed anywhere in the specs for the RAM, and it contributes to this "I don't know how to order RAM" mentality.

While researching this today, I ran across an outstanding Flash presentation from Corsair that covers almost all of this. It's quite technical but doesn't require a BSEE to follow, and it's by far the most informative reference I've ever seen.

I learned, for one thing, what "Registered" memory is, and it made perfect sense. In unregistered RAM, the address lines from the memory controller go to each individual chip on each DIMM, and this can be 4, 8, or even 32 chips on a single stick. As more DIMMs are added, this presents a growing electrical load on the output of the memory controllers: how can the memory controller drive all those inputs?

By adding a register - kind of like a buffer - at the edge of the DIMM, the memory controller sees one load per DIMM, not 4 or 8 or more, and this allows for much electrically cleaner address signals.

They even covered how to read a timing spec such as 2.5-3-3-7-1T, and how it's even possible that half a clock can be involved (in DDR RAM, data is transferred on the both the rising and falling edges of the clock).

It's just an outstanding presentation:

Corsair - Memory Basics

The only real missing area is a discussion on single- versus dual- ranked memory and the cost/performance tradeoffs involved. I think this presentation is slightly dated, so this area of ranking (not banking!) may not have settled down yet.

Posted by steve at August 30, 2006 07:52 AM

Comments