Answer from cs61c-at (soemoe aung 16076861) for Question 4
Memory-mapped IO means there is a special place in memory reserved for controlling interrupts.

loop: lb $t1,28(0xffff0000)
      or $t1,$t1,$0
      beqz $t1,end
      lb $t2,24(0xffff0004)
      add $v0,$t2,$0
      j loop
end:
     ...