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

Question

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

So, the number of bytes it can address is 2^32 bytes, or 4GB. So it makes sense to me that most 32 bit machines limit the amount of ram to 4gb (ignoring PAE).

Am I right in assuming that a 64bit machine could theoretically address 2^64 bytes, or 16 exabytes of ram?!

Answer 1

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

Answer 2

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

Answer 3

You would be correct. You can address up to 16 exabytes of RAM. Now.. whether the operating system can handle it would be another question....

Answer 4

Your question already contains the answer. The underlying logic also has been revealed.

So, yes, you are right.

Answer 5

2^64 addresses, yes you are correct

Answer 6

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.

Answer 7

Am I right in assuming that a 64bit machine could theoretically address 2^64 bytes

Yes, you're right!

Answer 8

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.

Answer 9

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]