While discussing Intel desktops with DDR2 memory using 2 Gbit technology (4 GB UDIMMs), the question of Intel’s next generation and 4 Gbit DDR3 (8 GB UDIMMs) came up. It’s more or less the next iteration of exactly the same problem.
I decided to focus on the LGA1366 platform which was the first post-Core 2 generation and Intel’s first platform that used exclusively DDR3 memory (recap: later Core 2 chipsets supported both DDR2 and DDR3). Reviewing Intel’s documentation turned out to be… confusing.
The datasheet for the first generation Bloomfield Core i7 (45nm Core i7 LGA1366, Nehalem microarchitecture) says that DDR3 “512Mb, 1Gb, 2Gb, technologies/densities are supported”, 24 GB RAM maximum. The memory controller in the CPUs could handle 6 DIMMs, and with 2 Gbit technology/4 GB UDIMMs, that obviously results in 24 GB maximum.
The datasheet for the follow-up Gulftown processors (32nm Core i7 LGA1366, Westmere microarchitecture) does not state what technologies or memory limits are supported. It was not hard to find reports of people using Westmere generation LGA1366 CPUs with 48 GB RAM so I started digging further.
I only have one desktop LGA1366 board that could possibly take 48 GB RAM, Intel’s own DX58SO2 (Smack Over 2). That’s where it gets interesting. The DX58SO2 Technical Product Specification clearly says 24 GB maximum. But the DX58SO2 Product Brief just as clearly says “Maximum system memory up to 48 GB using 8 GB double-sided DIMMs.” Who do you trust, Intel or Intel? Could they be both both wrong?
No, the DX58SO2 product brief is right and the board indeed supports 48 GB RAM.
First of all, the BIOS release notes include an interesting tidbit for BIOS Version 0876: “Fixed the memory Maximum Capacity value reported in SMBIOS Type 16 to support 48GB.” That wouldn’t affect what memory sizes were actually supported but it implies that the BIOS was originally meant to handle only up to 24 GB.
My initial assumption was that Westmere CPUs would support 8 GB DIMMs and the older Nehalem generation would probably not. That assumption turned out to be wrong.
My DX58SO2 happened to have a Xeon W3690 in it. That CPU is, depending on who you ask, either Gulftown or Westmere EP (Intel’s ark is obviously very inconsistent in that Xeon W3680 is supposedly Westmere EP and W3690 is Gulftown). Some people also call it Westmere-WS, perhaps just to prove the point that the code names are meaningless.
At any rate, the DX58SO2 with a W3690 does support 8 GB DIMMs, and although it is picky and initializing newly added memory modules can take rather long time, I was able to stuff the board with 8 GB DIMMs:
The memory works just fine, I had no trouble booting Linux and using all the memory. In fact with 48 GB RAM and six cores, the now antique platform (LGA1366 came out in 2008!) is surprisingly usable.
Okay, so Intel’s product brief was not lying and the board does support 48 GB RAM. But does it actually work with other CPUs? The next test was a Nehalem generation Xeon W3580. That processor might be a Nehalem-WS or a Bloomfield… thank goodness Intel reduced the insane number of code names when Sandy Bridge came out.
And lo and behold, the W3580 which supposedly supports up to 24 GB physical memory, does this:
So the Intel datasheets are wrong and the claim on Intel’s ark that this CPU supports up to 24 GB physical memory is wrong too. The memory is not just detected, it really works:
I did not run a full test but I let memtest86 run long enough that it should have failed if it couldn’t address all the memory. And yes, I have a funny mix of memory modules there.
The next test was an i7-965 (Core i7-965 Processor Extreme Edition, which people generally agree is a Bloomfield CPU). The CPU I have is from late 2008, one of the first batch of LGA1366 CPUs.
At first I was convinced that it can’t work with 8 GB modules. The board would not boot and kept getting stuck at various POST codes. But apparently the real problem is that this Intel board, like some other Intel boards (looking at you, DX79SR) is not very good at changing CPUs.
That is to say, if you plug in a different CPU, the results depend on what was in the board before. It also matters whether the BIOS jumper is in the “config” position or whether the B2B button on the back is pressed. It is… not helpful.
After some more CPU swapping, and messing with jumpers and buttons, the i7-965 somehow decided to boot up after all:
And memtest86 was happy enough with the memory, too:
Again, I was able to boot into Linux and use 48 GB RAM without any apparent trouble. It is worth repeating that this is a 2008 desktop CPU, albeit a high-end one. When I bought a regular Sandy Bridge desktop in 2011, it did not support that much memory. And a Haswell based Intel DZ87KLT from 2013 also maxes out at 32 GB. It’s really unfair.
Just for the sake of sanity I checked that a Core i7-975 also works with 48 GB RAM (it does).
Next I wanted to test if the “real” E and X series Xeons also work. Those are well known to have no trouble with 8 GB registered DIMMs, so the chances were good they’d work with 8 GB unbuffered DIMMs too. First I tried a low-end Xeon E5504 (Nehalem), and found that it can handle 48 GB as well. Next I went to the very opposite end and tried a Xeon X5690 (Westmere), which of course worked too.
I should note that if the DX58SO2 board didn’t have the super useful POST code display, I would have almost certainly concluded that it can’t support 48 GB RAM. It can take surprisingly long time (perhaps 30 seconds) for anything to show up on the screen when the RAM is fully populated. But from the POST code display it’s apparent that the board is not, in fact, stuck.
After testing a broad selection of LGA1366 processors in a desktop board, it appears that these CPUs can generally handle 48 GB RAM using 8 GB UDIMMs, despite Intel’s documentation stating that they only support 24 GB RAM. This is true for Core i7, Xeon W, and Xeon E/X LGA1366 processors of the Nehalem and Westmere generations. A prerequisite is obviously a desktop board which has six memory slots and can properly configure six 8 GB UDIMMs, but such boards do exist.