Answer from cs61c-cq (Shijie Li 15959617) for Question 4
contain addresses are not regular memory. Instead, they correspond to register in I/O device.
     la $s0, receiver
loop: lb $t0, 0($s0)
      addi $s0, $s0, 1
      beqz $t0, receive
      j loop
receive: mov $v0, $t0
         jr $ra