64 bit maximum address space

32 bit processeor have 4GB address space. 2 pow 32 = 4 GB (4294967296)

Is that mean 64 bit can access can access 2 pow 64 = 16 exabytes ??

not really

1. 64 bit does not mean all the bits of those points are actually used.
x86_64 -> 48 bit address lines. -> theoreitically 256 terabytes of RAM.

2. mother board have their own limits how much RAM can be supported.

Physically we can only have few slots ;)

3. OS may have internal limitation on how much it can support.

Linux allows 128 TB of virtual address space per process on x86_64.

Source: http://superuser.com/questions/168114/how-much-memory-can-a-64bit-machine-address-at-a-time
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