Why does 64-bit address space equal 2^64 bytes of RAM?

[bob]

Because individual bits aren't addressible.

But why not?

By definition of the term "byte". A byte is defined as the unit of memory addressing. These days people tend to believe that a "byte" means 8 bits, because most widely used CPUs today have 8-bit bytes. In the past, there used to be computers with smaller or bigger bytes.

That the standard byte size has ended up being exactly 8 bits is probably just a historical accident.

Just to drive you all further down the bitroad of insanity I hear that AMD is coming out with a new chip called Bulldozer which will be 128bits!

There have been machines that addressed 6 bit values, 8 bit values, 9 bit values, 16 bit values, 18 bit values, 30 bit values, 32 bit values, 36 bit values, 60 bit values and 64 bit values. There were early machines that addressed 4 bit decimal value, not hex. The list goes on and on. Most everything made today is based on 8 bit bytes however, for lots of reasons.