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What is the maximum amount of ram a 64bit machine can theoretically address?

What is the maximum amount of ram a 64bit machine can theoretically address?

I'm reading through my computer architecture book and I see that in an x86, 32bit CPU, the program counter is 32 bit.

Asked by: Guest | Views: 123
Total answers/comments: 4
Guest [Entry]

Effectively, yes - processes could, in theory, address 2^64 bytes of memory. But as you pointed out, there are ways around this limit.
Guest [Entry]

"Would be also good to note that the operating system has its own limitation about memory in a 64-bit architecture.

For example, see what wikipedia sais about Windows Vista 64:

All 64-bit versions of Microsoft
operating systems currently impose a
16 TB limit on address space.
Processes created on the 64-bit
editions of Windows Vista can have 8
TB in virtual memory for user
processes and 8 TB for kernel
processes to create a virtual memory
of 16 TB.[29] In terms of physical
memory Windows Vista 64-Bit Basic
supports up to 8 GB of RAM, Windows
Vista 64-Bit Home Premium supports up
to 16 GB of RAM, and Windows Vista
64-Bit Business/Enterprise/Ultimate
supports up to 128 GB of RAM.[8]"
Guest [Entry]

The biggest advantage to 64 bits is not the RAM it can address, but everything else. You can define an address for every byte on a disk, for example, and increasing disk capacities will not invalidate this for decades.
Guest [Entry]

Most of today's current processors have some sort of artificial limit on their address size. For example, the AMD64 architecture has a 52-bit limit on physical memory and currently only supports a 48-bit virtual address space. (Via Wikipedia). However yes, physically ~16.4 million terabytes is possible.