Answer from cs61c-ao (Roger Tseng 16507336) for Question 4
memory-mapped io means that portions of teh address space in memory are assigned to i/o devices.

li $t0, 0xffff0000
li $t1, 0xffff0004
li $t2, 0xffff0008
li $t3, 0xffff000c

L1: lw $t7, 0($t1)
    lw $t4, 0($t2)
    and $t5, $t4, 1
L2: bne $0, $t5, exit
    j L1
exit: sw $t7, 0($t3)
      j L1