4. [12 marks] An advanced uniprocessor von Neumann computer has threads of two sizes, identified by a hardware tag. Heavyweight threads may use the entire register file. Lightweight threads may only use a designated portion of the register file, whose capacity is 1/8 that of the whole file. In this way, we have threads with different-size _thread states_ running on the same machine. A context switch occurs when a thread must temporarily give up the machine. (In a von Neumann computer, one thread runs at a time). Analysis shows that, in time and energy, the cost for a context switch of a lightweight is roughly 1/8 that for a context switch of a heavyweight thread. What is the simplest explanation of this disparity