You're right that in the past machines have been built that supported memory
addresses that were longer than the length of an integer register in the
processor. But systems built more recently prefer to have a single integer
register contain an entire virtual memory address. Normally, "32-bit" or
"64-bit" refer to the length of the integer registers, so 64-bit machines
can address a much larger virtual memory space.
Right, 2^64 isn't *really* infinity, but it's certainly more than all the
bytes of RAM in all the computers in all the world. And that isn't likely to
change anytime soon.
Robert Swier
Toronto
On 11/3/05, Mark Dapoz <md@xxxxxxxxxxxx> wrote:
>
> On Thu, 3 Nov 2005, Robert Swier wrote:
> >
> > No, the 4GB limit comes from the fact that a 32 bit machine names each
> > memory location with a string of 32 bits. There are only 2^32 (or
> > 4294967296) different bit strings of length 32. So, that is the maximum
> > number of memory locations that the system can use in total.
> > A 64 bit machine names each memory location with a string of 64 bits, so
> > there are 2^64 possible names. 2^64 is practically infinity.
>
> Those are logical addresses that programs use. The memory management units
> can map those addresses to a larger physical address which doesn't have to
> be
> limited to 32 bits. Some 32 bit PowerPC's have been doing this for years.
> -mark
>
>
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