Robert Hyatt Computer and Information Sciences
[EMAIL PROTECTED] University of Alabama at Birmingham
(205) 934-2213 115A Campbell Hall, UAB Station
(205) 934-5473 FAX Birmingham, AL 35294-1170
On Tue, 7 Sep 1999, John F. Nixon wrote:
> Robert M. Hyatt writes:
>
> > How are you going to have _two different threads_ access the same cpu at
> > the same time?
>
> Modern CPUs can have many instructions executing in parallel. In
> particular, the Alpha 21264 can have up to 4 integer instructions
> executing per CPU cycle, and two floating point instructions per
> cycle. It is possible for different threads to have instructions in
> process at the same time on the same CPU.
No it isn't. Superscalar processors execute multiple instructions in
one cycle... but they have exactly _one_ program counter. So there is
_no_ way for two threads to issue instructions to the same cpu at the same
time.
And yes, I know about the superscalar stuff in the alpha 21x64, but the
intel Pentium and beyond do _exactly_ the same thing (superscalar
execution). In fact, _every_ processor being made today does super-
scalar execution, unless you count the ones put in blenders and so
forth...
>
> > I read his question to be "can one thread use the FPU
> > while the other thread uses the normal integer processing CPU. the
> > answer is no. Because the cpu/fpu combination has one program counter,
> > one cpu state. Unless I have overlooked some remarkable CPU breakthrough
> > from Intel, that is...
>
> Check out the design of the Alpha 21264 at
> http://www.digital.com/hpc/ref/ref_systems.html
>
> Basically, with out of order and speculative execution, the idea that
> a CPU has only one instruction in process at any time, and hence only
> one PC, is incorrect. The CPU maintains that view from the outside,
> today at least 8^) but internally that isn't the case.
Oh but it is... because while they execute instructions out of order,
they then have to re-assemble the results into what would have happened
had the instructions exected in order. That was the novel feature of the
original pentium-pro cpu... No way to execute multiple threads on one cpu
because there is just one program counter...
>
> --
> cheers, Fred
> -
> Linux SMP list: FIRST see FAQ at http://www.irisa.fr/prive/mentre/smp-faq/
> To Unsubscribe: send "unsubscribe linux-smp" to [EMAIL PROTECTED]
>
-
Linux SMP list: FIRST see FAQ at http://www.irisa.fr/prive/mentre/smp-faq/
To Unsubscribe: send "unsubscribe linux-smp" to [EMAIL PROTECTED]
