|Well that's a strange question, akin to asking why a car can only be on one road at a time, or why a toaster can only hold a single piece of bread per slot. |
Perhaps a better method would be to tell us how you believe computers do what they do, and why you believe that. From there, we can explain the basic concepts.
EDIT: I should specify I'm assuming a single core without hyperthreading or complex instruction pipelining. Basically, I'm going back to the . . . let's say i386. It's a good foundation, and everything else after that just adds layers of complexity that this forum typically doesn't go into.
How To Ask Questions The Smart Way
message edited by Razor2.3