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I have a Toshiba Portege M780-S7240. It's an i7, 64-bit and currently has 4GB RAM. I'd like to upgrade to 16GB. (If it matters, I'm running 64-bit Ubuntu.)

I checked with a RAM vendor who said it wasn't possible with my config, and saw the user guide says:

"Configured with 4GB (2GB+2GB) DDR3 1066MHz (max 8GB)"

What causes this hard limit? What exactly is the limitation or combination of limitations that keeps it from supporting 16GB? Is it the motherboard? Is there a firmware upgrade I can do to get beyond this?

EDIT

Here's a link to the specs from the manufacturer. (It's a PDF.) I've been told that these PDFS are created before release, and usually not updated as firmware changes.

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It is the motherboard. Since you won't be able to replace it your stuck with 8GB. –  Ramhound Apr 16 '12 at 18:29
    
Why? Is there a BIOS upgrade around that? What causes this limitation? The OS supports it, it fits in the slots, the processor theoretically supports it---what causes this limit? –  Mittenchops Apr 16 '12 at 18:38
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Chipsets frequently have memory limits, e.g. in my ThinkPad R60 I can not use more then 3 GiB of memory. There is no way around it short of replacing the motherboard. Keep in mind that there are plenty more components involved than the RAM slots, the processor and the OS. –  Јοеу Apr 17 '12 at 21:17
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3 Answers

up vote 6 down vote accepted

I believe the memory controller of the Intel HM55 chipset has this limit. Looks like a design fetaure/limit related to Intel's design. The exact model of your Protege 780 might result in some additional feedback from others

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Thanks, @Dave M. I updated the model information to reflect that. Portege M780-S7240. –  Mittenchops Apr 16 '12 at 18:22
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Kingston knows about 10 versions of protege m780 accepting from 6GB (2+4) in single channel mode to 16GB (8+8) dual-channel. Probably you need to figure out modification of "Protege M780" before approaching the vendor.

Memory supported depends on how pins are connected on motherboard to actual RAM sockets. From description of kingston i'd guess Toshiba wanted to have two 4GB options i.e 4 or 2+2, gladly they designed BIOS well and maximum module in each socket works just fine.

I'd guess actual CPU might go minimally with 36-bit addressing i.e 64GB, mine (Acer, Athlon 64 of 2006) for example could handle 40bit physical addresses - 1TB, but motherboard has two slots where DMI says each is 4GB (and motherboard has logic present to support it) but they accept in fact only 2GB modules, ignoring 4GB ones even if plugged, probably to save annoyance for XP users which max out with /3GB switch

It is merely wire not drawn from motherboard controller (aka northbridge) to memory slot, so there is no household way around it (i.e even good soldering skills will not help)

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Thanks ZaB. I've updated this above. Is there a resource for checking this with Kingston, or just email? –  Mittenchops Apr 16 '12 at 18:26
    
Like on the right side there on their website there is a dropdown where you can search memory for specific system? –  ZaB Apr 17 '12 at 7:01
    
Got it. See it now, thanks, ZaB. –  Mittenchops Apr 17 '12 at 18:46
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I won't get into detail about your specific machine, actually it seems you want a more general answer. My following answer is mostly backed by this article, read up on it if you want more specific knowledge.

This limit is caused by your motherboards hardware. A recent 64bit processor is limited to access of 64GB, this limit is a hard limit caused by the available pins on the processor. The theoretical limit would be 2^64. (But there is no current need for this much memory, so the pins are not built into the processors, yet)

The northbridge manages a so called memory map which maps areas of the memory to be read and written to by certain devices, the northbridge has a hard limit too. Remember that every pin and every connection on the motherboard makes designing it harder and the chips places on them more expensive. So this a sheer cost-factor, the manufacturer just assumes that most people will not use more than the limit given by the hardware. Hardware supporting more, is more expensive.

The Bios has nothing to do with anything on your computer as soon as your kernel is loaded. Note that recent processors embed the northbridge, I am not sure how the limit is defined on motherboards without a northbridge. (But I think the limit still is not only defined by the processor)

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crawl through the comment section of the linked article, to get some more answers. there are some (i think) spanish comments, but the author mostly replies in english and gives additional information. –  Baarn Apr 16 '12 at 19:45
    
Cpu-Z and PC wizard from www.cpuid.com will show correct address size of CPU, most likely 32 or 44bits (4GB (atom)-> 1TB(server xeon)) for modern 64bit CPUs (with 36bit aka 64GB already present in Pentium Pro/Pentium II in 1999) –  ZaB Apr 18 '12 at 6:32
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