64-bit CPU

[Lennart Sorensen]

On Wed, Sep 15, 2004 at 12:16:50PM -0400, Andrej Marjan wrote:
> >Of course
> >memory wise most applications don't need to be 64bit, while some do gain
> >in speed by running 64bit (apparently from what I have read, up to 50%
> >speed gain on the athlon 64 in some cases when running 64bit mode.  I
> >guess some code really likes being run in 64bit mode.)
> > 
> >
> x86-64 has many more registers, hence the performance improvement for 
> programs that don't need the extra addess space or 64 bit integer 
> computations.

Then why have people seen the Xeon em64t (nacoma?) slow down slightly
instead?

There has to be more to it than that.  I guess for one, 64bit integer
math can be done natively rather than whatever x86 used to do (I don't
think x86 32bit chips have done 64bit addition and such natively,
I figure they had some way around it that chopped it up and manipulated
it a bit in the compiler.)  Then again maybe it is just having a few
more registers helping out, although the P6 architecture's register
renaming (which has a lot more than 8 registers inside the chip) used
in the speculative execution and out of order execution already helps many
such programs run faster than they should on an 8 register architecture.

Lennart Sorensen
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