limits to memory on amd64

Julian Elischer julian at
Tue Nov 9 17:20:41 UTC 2010

On 11/9/10 9:04 AM, Bakul Shah wrote:
> On Tue, 09 Nov 2010 08:45:14 PST Julian Elischer<julian at>   wrote:
>> During the discussion at MeetBSD the question came up as to what the real
>> limiting factors were with regard to how much RAM a system could have.
>> it was put to us that the limit was currently around 512 GB, though no-one
>> at teh discussion knew what the mechanism of the limitation was or
>> what might ligh beyond it.
>> Could anyone who knows, pipe upt and let use know what the factors are,
>> and if the current limit is overcome, what the next one after that will be?
> You mean beyond architectural limits?

no, though of course they are relevant.
I was thinking more of details like limits to the KVM space or
any limitations there may be on the size of the direct-map region,
or maybe some limit on some data structure size in the kernel.
Since I don 't know the details, this is exactly the question..
what IS the limit?

> > From Wikipedia:
>      Larger physical address space: The original
>      implementation of the AMD64 architecture implemented
>      40-bit physical addresses and so could address up to 1 TB
>      (2^40 bytes) of RAM. Current implementations of the AMD64
>      architecture (starting from AMD 10h microarchitecture)
>      extend this to 48-bit physical addresses and therefore
>      can address up to 256 TB of RAM. The architecture permits
>      extending this to 52 bits in the future (limited by the
>      page table entry format); this would allow addressing of
>      up to 4 PB of RAM.

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