The 8051 has no hardware notion of multitasking. Note however that a lot of modern microprocessors do support some notion of hardware multitasking.

A great example of hardware multitasking support is Intel's hyperthreading technology. The Pentium 4 processor, which has a very deep pipeline and plethora of hardware resources, improves its performance using this technique. The fundamental idea behind hyperthreading is to duplicate enough hardware required to make maintaining state for two active threads of execution in hardware physically possible. Then, on-chip logic can schedule execution from the two threads to be largely concurrent, except in situations where there is contention for a particular resource (e.g., memory access). The interface to the hyperthreading technology implemented by the Pentium 4 is that of two logical processors. Thus, software targeting the Pentium 4 need not take into account the existence of this functionality. Any application which is written to use multiple processors can benefit from it. Even applications written for a single processor can benefit, as the second thread of control can be used for a process belonging to the operating system or another application, thus improving the overall performance of the system.

--Sasha Jevtic