You cannot put ram in with mixed rank and density and speed randomly into slots and it works

All the ram should be the same rank, size, density,

It must be installed in 3 channel mode in the slots with white ejector ears

It must be the same for the 1st and 2nd processor

There are 24 ram slots.

So if you use 6 slots on each side. 64gig modules that would be 768G

there is no sequence for having 64 or 128 gig modules in 6 or 12 that will come out to be 1024 GIG

12 128 gig modules would be 1536 Gig

6 256 gig modules would also be 1536 GIG

You MUST INSTALL matched sets of 6 or 12

YOU CANNOT MIX Rank and Density and Speed

You CANNOT MIX 8gig and 32 gig and 64 gig and 128 gig and 256 gig modules randomly.

There is no bug in the bios you are improperly trying to install modules

24 ram slots total must be matching sets of 6

Thanks for your quick reply.

We have 16x64GB LRDIMM at 2666MHz same rank and speed etc. and we've setup the Main Memory to 1024GB according to page 103 of the Precision 7920 Rack manual here

https://topics-cdn.dell.com/pdf/precision-7920r-workstation_owners-manual_en-us.pdf

We've setup 1024GB RAM see table page 103 > second-last Row on the left with "1024" so:

• 2DIMMs per Channel used which makes 8xDIMMS for CPU1.
• 2DIMMs per Channel used which makes 8xDIMMS for CPU2.
• One DIMM = 64GB DDR4 Synchronous LRDIMM 2666 MHz (0,4 ns)

For CPU1 => 512GB RAM with:

• (2xDIMM) for Ch0 => A7 + A1  => 128GB
• (2xDIMM) for Ch1 => A8 + A2  => 128GB
• (2xDIMM) for Ch3 => A4 + A10 => 128GB
• (2xDIMM) for Ch4 => A5 + A11 => 128GB
• Ch2 and Ch5 are not used for CPU1 according to the manual for 1024GB.

For CPU2 => 512GB RAM with:

• (2xDIMM ) for Ch0 => B1 + B7 => 128GB
• (2xDIMM) for Ch1 => B2 + B8 => 128GB
• (2xDIMM) for Ch3 => B4 + B10 => 128GB
• (2xDIMM) for Ch4 => B5 + B11 => 128GB
• Ch2 and Ch5 are not used for CPU2 according to the manual for 1024GB.

Is this correct ?

@gilceldell​ Did you ever solve this? I just added a second CPU and moved 6 of my 12 RAM modules over to the second CPU. Now the system keeps setting the memory mapped IO to 512GB so I'm not getting access to all my RAM. I have 12 64GB modules, total 768GB. I reset the BIOS and reseated every stick of RAM. This RAM was known good and working in the same system, just added a second CPU and moved the sticks. All white slots are being used. No black slots

Hi user_3ffa18,

Unfortunately no, I tried several things months ago but couldn't find a way to have more than > 512GB of RAM. Even with updated BIOS version 2.21.2 it doesn't recognize more than 512GB of RAM.

At first we had 512GB of DELL-RAM and I just added RAM from SAMSUNG (Density, Frequency etc. are equivalent), so I really think it's a BIOS problem  since it doesn't save  the setting of "Setting the MMIO value to 56TB".
Which BIOS version is installed ? The latest BIOS is v2.24.0. Please let me know. 

Latest BIOS here:
/support/home/en-us/drivers/driversdetails?driverid=t4vcn&oscode=wt64a&productcode=precision-7920r-workstation

@gilceldell​ I figured out what my problem was. Sharing this with you in hopes that it might help...

I had an Nvidia Quadro GPU in the system that only supported 40bit memory addressing and could only address up to 1TB. The computer started up and detected this and changed the memory address space to 512GB, since the 2 other options were more than 1TB.

If you happen to have any PCIe cards like GPU's, check to see what address space they support (or just pull them all out and add them back in one at a time) :-)

@gilceldell​ Thank you! I checked your link and validated that I do have the latest BIOS. I had checked for an updated BIOS previously, but I always like to confirm when someone shares a link :-)

I didn't update yet to BIOS v2.24.0 so glad you confirm that it doesn't work with this version either.
Is the installed RAM from DELL or from another brand ? Well DELL RAM is rebranded from Hynix or Samsung but can't remember exactly.

I’m using Samsung LRDIMM. 768GB, 12 64GB sticks.