Question

how can multiple threads run simultaneously on a single-processor system?  

Answer 1

Answer:---------

It can be done on a single-processor system, and in fact it's much easier that way. It works the same way as running multiple processes -- the kernel, via a timer interrupt or other similar mechanism, suspends one, saving its machine state, and replacing that by the previously-saved state of another -- the only difference being that two threads of the same process share the same virtual memory space, making the task-switch much more efficient.

Multi-threading on multi-processor systems is actually much more difficult, since you have issues of simultaneous access to memory from multiple CPUs / cores, and all the nasty memory synchronization issues that arise out of that.