$(1)$ What device deals with address translation? In which case we will not use this device? $(3\%)$

$(2)$ Please give two benefits of using paging. $(4\%)$

$(3)$ Does any system use the optimal page replacement method? Why or why not? $(2\%)$

$(4)$ Let average page$-$fault time and memory$-$access time be $70s$ and $250ns,$ respectively. What is the expected page$-$fault rate if we want to get effective access time smaller than $285ns?$ List your calculation. $(4\%)$

$(5)$ What is reentrant code? $(2\%)$