How rigid are published "Max RAM" guidelines?

Sqrfrk · Mar 30, 2015

ASRock's new X99 itx board has two DIMM slots for a published max RAM limit of 32GB. I have the assumption then that 16GB modules are the largest per slot.

Assuming that the memory controllers (at least for Haswell-EP, among others) are integrated into the processor, is there anything that would prevent this board from using 32GB RDIMM engineering samples floating around right now (along with an E5-2630)?

kanewolf · Mar 30, 2015

Kind of defeats the purpose of the newest CPUs when you cut half the memory bandwidth by only having two memory channels.

Sqrfrk · Mar 30, 2015

kanewolf :

I agree with your general sentiment, but it is what it is. ASRock says 32GB max - will two 32GB modules with a total of 64GB magically work? Is there a limitation in the motherboard (outside of its BIOS) that will keep it from POSTing?

kanewolf · Mar 30, 2015

The BIOS probably would. To be able to address 32GB per slot there has to be that many address lines. The motherboard probably does not have that many address lines per slot.

Pinhedd · Mar 31, 2015

Sqrfrk :

You're absolutely right about the microprocessor containing the memory controller, but it is the platform firmware provided by the motherboard manufacturer that contains the code needed to initialize the SDRAM.

Using engineering sample chips is almost always unsupported; it may work flawlessly, it may work with some issues, or it may not work at all. The only way to be sure is to contact the manufacturer and find out how much tweaking they have done to Intel's memory reference code.

Sqrfrk · Mar 31, 2015

Thanks for the replies. Going along with what both of you are saying, it seems to me that the major roadblocks in getting a higher density RDIMM to work include having the appropriate BIOS (which I'm assuming is what you meant by platform firmware in this context) and the physical layout of the memory addressing bus. The controller (in the chip) appears to be able to support "sky is the limit" amounts of DDR4 out of the box.

How likely is it that the current selection of X99 boards (ATX or otherwise) would physically have the required address lines to support a 32GB module? A lot of the initial motherboards came with a published maximum of 8GB per slot, where newer boards had an increased upper max of 16GB when these DIMMs entered mass production. Have any of these initial boards received a BIOS update that enabled the 16GB modules? That would lead me to believe that the slots are physically capable of supporting a higher amount of RAM, and the manufacturer would essentially need to "unlock" support through a software update.

leeb2013 · Mar 31, 2015

About 49% likelihood

Sqrfrk · Mar 31, 2015

leeb2013 :

Worse than flipping a coin? I like those odds!

Pinhedd · Mar 31, 2015

Sqrfrk :

Hi,

The address signals are not at all a limitation. The limitation usually lies in the firmware's (BIOS/UEFI) ability to interpret the physical layout of the DIMM. For example, many motherboard manufacturers claim support for 8GiB single-rank and 16GiB dual-rank DDR3-DIMMs. However, this support is for DIMMs that use native 8 gigabit DDR3-SDRAM which have never been produced. What have been produced though are stacked 8 gigabit DDR3-SDRAM chips which are constructed from two 4 gigabit DDR3-SDRAM dies stacked on top of each other with a common interface with separate clock enables, separate chip selects, and separate termination. Neither Intel nor AMD support stacked SDRAM, but the MRC can be modified to support them (as far as I know, Asus has done this).

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How rigid are published "Max RAM" guidelines?

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