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Memory Ranks!?

Eschaton

54 months ago

I've been farting around with an old HP xw6400 workstation - exclusively uses a special kind of RAM called FB-DIMMs. They run very hot, have mandatory heat spreaders, and feature a little asic right the center to help them (I assume with some kind of error correction, since FB-DIMMs are classed as a kind of ECC).

To get to the point, I've upgraded the machine now to an 8 core (2 CPU) configuration, but upgrading the RAM is proving very difficult. I bought a 2x4gb set off ebay that met all the listed specifications required by HP for this workstation: 4gb PC2-5300F, CAS 555-11, 1.8V. The system supports up to 16gb RAM across 4 slots, so 4gb must be supported. This RAM is not supported. Turns out, it's "4RX8" RAM, while this xw6400 ONLY accepts " 1RX4" or MAYBE "2RX4" RAM.

What are these bizarre numbers, you ask? Well, I was baffled, so I did some research. I've found that at least the first half of this number refers to something called "rank," which apparently refers to the method of adding RAM caches to the stick - single rank RAM is faster because of its construction, dual rank RAM is slower, and quadruple rank RAM is slowest of all - and my xw6400 doesn't support quad rank RAM at all in the BIOS - just doesn't recognize the memory (if I have 1RX4 - single channel - RAM in slot 1, and 4RX8 RAM in any other slot, it boots with an error, but I can get into BIOS, where it shows me that the total system memory detected is only the sum of the 1RX4 RAM, even though the 4RX8 RAM is detected and listed in the slot that it's in!).

So, obviously there are real differences here, but I'm intrigued by the suggestion that lower rank construction methods yield RAM that is somehow faster. Is this still true for everyday consumer RAM, or even modern ECC RAM, or is this just something that used to be important but no longer really is?

Comments

  • 54 months ago
  • 1 point

Where I'm getting my information:

http://www.oempcworld.com/support/SinglevsDualRAM.html

  • 54 months ago
  • 1 point

Isn't Wikipedia awesome? http://en.wikipedia.org/wiki/Memory_rank http://en.wikipedia.org/wiki/Memory_geometry
So it seems that ranking is quite similar to having a single/dual/quad channel RAM setup on a single ram stick. Or not, as the next few links seem to indicate...

http://www.dataram.com/blog/?p=32 Hard to explain; seems like the rank is a way for the RAM chip to be sized.

http://www.simmtester.com/page/news/showpubnews.asp?num=128

http://www.anandtech.com/show/3851/everything-you-always-wanted-to-know-about-sdram-memory-but-were-afraid-to-ask/2

Right, my head is spinning... I'm going to rest and see if this all makes sense afterwards...

  • 54 months ago
  • 1 point

Yeah, it at least partially has to do with the size of the chips they use to reach the specified capacity for a stick.

It doesn't do the same things as memory channels, though. Rather, having more ranks seems to be a limitation for the chip, as only a single rank can normally be accessed at any given time on one channel; therefore you'd basically be accessing only part of the capacity of your stick if you had a single-channel configuration using a 2+ rank stick. Obviously it can probably switch to the other rank really, really fast, but that would still constitute an extra step and therefore theoretically be a little slower than a stick that had all its modules arranged together on a single rank.

I think. Stuff's whack.

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