Tim Anderson's ITWriting

May 10, 2005

Intel steals my memory

The other day I put together a new Intel EM64T system for testing 64-bit Windows and Linux. The motherboard is an Intel D915PBL, supposedly with full EM64T compatibility.

Why "supposedly"? Well, everything runs fine just as I expected it would, except that I installed 4GB and I'm only getting the use of 3GB. Sounds like a 32-bit problem, doesn't it? The problem appears to be that this chipset reserves approximately 1GB at the top of the 4GB address space for system functions - things like BIOS firmware, digital media interface, PCI Express ports, and memory-mapped I/O. Of these, the PCI Express element is by far the biggest, and might vary according to how much RAM is on your graphics card. The result is that when you boot, the board declares:

Memory consumed by system resources: 1024MB

Let me put this another way. This memory is not really being "consumed". It is just that the board can't really address it, as its address locations are already used to point to other things.

This is not a limitation of the processor, which can address vast amounts of RAM; and it is nothing to do with any operating system. Intel is capable of making boards which don't suffer this problem, and does so for the Xeon. I consider it a bug. After all, why would anyone want an EM64T CPU, if not to address more RAM? Why are there slots for 4GB, if the memory can't all be used? Why does the user-friendly installation manual not mention that you might as well save your money and only install 3GB?

Of course 3GB is still a lot, and the price difference between a pair of 512MB sticks and a pair of 1GB sticks is not vast. I still reckon it's poor. Should have gone with AMD after all...

Postscript: I have to add that there are AMD motherboards that suffer from this same problem. So it is not really an AMD vs Intel issue. I am still researching this; if anyone has a clear explanation of why this limit exists and how the high-end boards get round it please let me know or comment here. Losing 25% of your RAM is careless to say the least and this problem deserves to be more prominent; equally, now that 64-bit is hitting the mainstream we need mass-market boards that can accept more RAM.

Postscript 2: I initiated a lengthy discussion of this issue on some of the hardware newsgroups. The conclusion seems to be that some of the address space between 3GB and 4GB will always be reserved for non-memory functions on a PC. This part of the installed RAM might be lost even if you have a board that supports much more - say 24GB. However, a well-designed board can remap the memory, so that all the installed RAM can be used. It appears that the Asus A8N-SLI Deluxe is an example, in the AMD world. The BIOS offers a "DRAM Over 4G remapping function". So it can be done.

One further correction. The Intel D915PBL printed guide does actually make reference to this problem. However, I missed it for two reasons. First, it is not mentioned in the section on "Installing and removing memory", which is one part of a board manual I always tend to read as it gives the DIMM configuration options. Second, where it is noted (eg. in the list of features), it doesn't indicate the scale of the issue. To be specific, it says "...this may result in less than 4GB of memory being available to the operating system." Even if you read this warning, I doubt you will expect a full 1GB to be lost.

If you run into this problem, please complain to the board vendor. As 64-bit installations become more common, I expect most boards will in future be designed to workaround this issue.