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.
> 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.
--
cheers, Fred
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