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How much RAM can a 64-bit Windows 8 Ultimate Operating System use?

I have read on the Microsoft website that you can place up to 512 gigabytes (GB), but to my understanding, I thought 64-bit implies a maximum usage of 16 gigabytes of RAM for any executable program.

So in other words: Can Windows 8 use all 512 GB of ram?

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Theoretically, 256 terabytes of physical RAM. [SuperUser Answer][1] [1]: superuser.com/questions/168114/… –  Ryan E Feb 25 '13 at 20:37
    
There is a difference between how much RAM the OS can recognize and the maximum size 'chunks' it can allocate for actual programs. 64-bit allows for bigger 'chunks' which basically equates to more bandwidth (which means better performance despite bigger file sizes). –  ioSamurai Feb 25 '13 at 20:44
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@op: I am not sure where you got the 16 gigabyte figure from. 2^64 is a lot more. --- For easy comparisions: 32 bits allow up to 4 GB, 33 bit up to 2*4 GB (8GB), 34 bit (34, not 64) up to 64GB, ... etc etc. –  Hennes Feb 26 '13 at 1:47
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I'd note that the MOST you can go with a desktop grade motherboard is 32 or maybe 64 gb. Your motherboard/processor is somewhat more likely to be the bottleneck. I think the per process limitiation on ram is the more interesting part of this question, and probably the main reason this should be kept open –  Journeyman Geek Feb 26 '13 at 2:03
    
@JourneymanGeek there is no per-process limitation on RAM. The limitation of 8TB is a limitation on virtual memory size. The whole of this question and its answers shows a misunderstanding of binary arithmetic and the difference between physical and virtual memory. –  David Marshall Feb 26 '13 at 10:39

5 Answers 5

The theoretical 64-bit address space is 2^64 or 16 exabytes (18,446,744,073,709,551,616 bytes)

Microsoft limits the maximum physical memory to 512 GB for Windows 8 Pro and Enterprise. (reference)

This is mostly for commercial reasons although I have heard that Microsoft will only authorise up to the memory size for which there exists equipment they can test.

As far as the per-process user mode virtual address space, this was limited to 8192 GB prior to WIndows 8.1. Windows 8.1 has increased this to 128TB. (reference) (reference)

The way the virtual address space is laid out is a result of the way that current processors only implement 256TB of virtual memory.

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While it could address much more RAM in theory, Microsoft limits the possible RAM for Windows 8 to 512GB in the Pro and Enterprise editions, and even less in the others.

Windows Server 2012 is required to get to several terabytes of memory, which is still below the limit of appropriate hardware.

It's probably safe to assume a non-technical reason, i.e. they want you to buy a real server OS for server (or related) tasks.


Note that CPUs have their own limit for addressable memory as well. Intel e.g. specifies a limit of 750GB for current Xeon E5-2600 (designed for dual socket boards) CPUs. To get to >1TB, you need the (more expensive) ones designed for 4+ CPU boards.

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Well, your sources are right even if they aren't Microsoft. The NT kernel will map this to different address regions, but even current processors only implement 48/52 Bits, which limits the amount to something below the theoretical maximum of the 64 Bit architecture.

Also, don't confuse the idea of virtual memory a program can use with the physical RAM the operating system can access through the CPU. You might want to read up on concepts such as Nonpaged and Paged Pool in Russinovich's "Windows Internals" as well as about how Paged Pool actually gets used to represent the process space for usermode processes.

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Theoretically, 16.8 million terabytes. In practice, your computer case is a little too small to fit all that RAM.

http://en.wikipedia.org/wiki/64-bit#Limitations_of_practical_processors

Answer taken from: What is the maximum amount of ram a 64bit machine can theoretically address?

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True for a theoretical answer. Not correct for win 8 which has human set limits. –  Hennes Feb 25 '13 at 20:48
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I think you mean exabytes. –  David Marshall Feb 25 '13 at 21:16

In a 64bit OS your system board will be the limiting factor in how much memory you can use, not the OS. You will bump up against your hardware limit long before you can max out Windows.

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Thats a server board. I guess should have been more specific. I'm assuming he was talking desktop PC since he asked about Win8 not server 2012. –  BroScience Feb 25 '13 at 20:57
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I don't see anything about him using a server board anywhere? I don't think you deserve a -1 even the high end x79 consumer boards top out at 64 GB. Your answer is correct as far as I'm concerned... –  Kyle Feb 25 '13 at 21:45
    
Kyle, someone commented on my post and gave an example of a board that can take 512GB of RAM, but then they deleted their post, so now I look crazy. lol –  BroScience Feb 26 '13 at 14:45

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