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Half of RAM missing after installing PCIe SSD

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Zbig:
It's been a while since the last time I needed to mess with PC components and I haven't seen such behavior before. My PC's motherboard has the usual arrangement of four DDR RAM slots color coded for dual-channel operation. The factory setup was two 8 GB sticks in the black bank for a total of 16 GB of RAM. After I added an Intel PCIe SSD card (x4 IIRC), half of the RAM went AWOL with both UEFI and Windows 10 now only reporting 2*4=8 GB of RAM. It wasn't a case of bad electrical contact or anything like that as I've tried reseating both the DIMMs and PCIe card and the RAM halving was consistently correlated with the presence of the SSD in the PCIe slot. Any (of only two as it's an SFF PC) PCIe slot for that matter. The issue went away after I moved the DIMMs from the black slots (bank 0) to the blue ones (bank 1). The trouble is, I want to add another two 8 GB sticks and would very much like to get whole 32 GB instead of 24 or other, even weirder, behavior.

What's going on there? One naive explanation would be the SSD "stealing" PCIe lanes but I don't think modern PCs use PCI Express as a memory bus, right? Or is it a case of the PCIe SSD device getting memory-mapped into an address range normally used by the RAM bank 0? Is there anything I could try to fix it?

The PC in question is a Fujitsu Esprimo D738 and it's using Intel Q370 chipset. The SSD is an Intel P3520.

GlennSprigg:
Don't know if it's related, but a look-up shows that the "Intel P3520" SSD had been discontinued & is out of date/support... ?
Such findings/links also suggest/recommend usage of an "Intel Memory & Storage Tool"...  for example...
https://downloadcenter.intel.com/download/30380/Intel-Memory-and-Storage-Tool-GUI-
Maybe that will help with the 'addressing' etc ?   >:(

rsjsouza:
Perhaps contact the manufacturer's support line? Certainly the DDR memory is not affected by PCIx lanes, unless deliberately by some sort of artifically imposed limitation (BIOS, etc.) Its specs say it supports up to 64GB, thus this ceiling is well above your specs.

magic:

--- Quote from: Zbig on May 29, 2021, 08:52:07 pm ---What's going on there? One naive explanation would be the SSD "stealing" PCIe lanes but I don't think modern PCs use PCI Express as a memory bus, right? Or is it a case of the PCIe SSD device getting memory-mapped into an address range normally used by the RAM bank 0? Is there anything I could try to fix it?

--- End quote ---
No idea.
I don't think PCI had ever been used as the memory bus in anything and it surely isn't today.
Addresses of DRAM are programmable and somewhat flexible, memory is mapped from zero to whatever it takes, with a hole below 4GB for PCI devices. It shouldn't matter which slot you plug it into.

Some kind of BIOS bug? :-//
See if there are updates to that.

R-star:
Zbig,

I am assuming you have already:
- updated to the latest BIOS version
    modern computers are extremely buggy, these days we literally must update.
- reset to defaults you updated BIOS, and then rebooted and re-configured all settings
- examined various BIOS settings that possibly might have something to do with your issue, such as:
    Slot n Non-Compliance
    Slot n Link Speed
    Above 4G Decoding
    ... and so on

...and it still is not working. Then like before, what you need to do is to assume the SSD card is bad, and try another type/model.
If that works, then try same model but known good instance.

best of luck

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