[wellylug] 32 Bit vs. 64 Bit

[Daniel Pittman]

Rob Giltrap <rob at kiwihq.com> writes:
> Gerald Roehrbein wrote:
>
>> what's faster 32 Bit or 64 Bit applications?
>
> Both, neither and it depends!
>
> Here's some big generalizations but...
>
> If you're running a 32bit program on a 32bit OS then it will generally
> run a fraction faster than something that uses the full memory
> addressing available in a 64bit system (we are only talking like 1%
> difference).
>
> If you are using an AMD64 processor there are a number of additional
> registers available in 64bit mode (particularly floating point
> registers) so if your application does complex FP operations then it
> will likely run a good chunk faster in 64bit mode than 32bit mode. Not
> sure about the new Intel processors.
>
> If you are using more than 4GB of physical ram then memory operations
> are going to be in general faster in 64bit mode.

The one final difference: 64-bit code is larger.  That means it uses up
more space in the L1 and L2 cache than 32-bit code.

If that makes the difference between your inner loop (for something CPU
or data intensive) running from L1 or not (or L2 or not) you can see a
huge performance /loss/ with the larger code.

	Daniel
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