Jim Higson wrote:
> A 64bit CPU will be slower running 32bit porgrams than the
> same speed 32bit CPU.
It ain't necessarily true. There's a lot more to it than that.
Firstly, the concept of "same speed" is -- dodgy. If you mean "MHz",
then it's definitely false: there's a lot more to performance than that.
The best measure of performance is "how fast it runs your programs": if
the 64 bit processor is slower running 32 bit programs than a 32 bit
processor, then they can't be the same speed.
All Athlon 64s and Opterons are inherently 64 bit, yet they are largely
sold on their 32 bit performance. They do have a few -- fixes -- to
limitations in the 386 instruction set, which means that they *normally*
run programs faster if they are compiled in 64 bit mode.
One might expect similar speed running a 32 bit program on a 32 bit OS
or on a 64 bit OS, and (with the AMD designs) this is more or less true.
There may be a small amount of performance increase if the rest of the
system is running faster (especially if the program needs the kernel to
do a lot of work for it).
However, if the program is dynamically linked, it will need 32 bit
versions of the libraries. Now some of these libraries will probably
already be loaded, but on a 64 bit OS, they might well be 64 bit
versions. So the kernel will probably have to keep 32 bit and 64 bit
versions of the same libraries loaded. This reduces the efficiency of
the CPU's caches, and makes less RAM available for hard disk caching.
Intel's Itanium CPUs were not designed with 32 bit *performance* as a
priority, and it shows. They *are* much slower when running 386 code.
Other 64 bit chips tend to have 32 bit variants (PowerPC, Sparc) or
32 bit modes (Alpha), and are designed to have good performance when
running 32 bit binaries (compiled for the appropriate instruction set).
James.
--
E-mail address: james | Practically any car advert, for example, shows you
@westexe.demon.co.uk | that if you buy this car you will get so lost that you
| end up parked (well, no. The word here is "stuck") on
| a mountain in Monument Valley. -- Telsa Gwynne
