Answer from cs61c-au (Robert Webb 14343043) for Question 4
Memory-mapped io means that the input device puts it's data into a space that the processor can access like memory.

loop:
lw $t0 0(0xffff0000)
sll $t0 $t0 31
srl $t0 $t0 31
bne $t0 $0 loop
lw $t1 0(0xffff0004)
sll $t1 $t1 24
srl $t1 $t1 24
add $v0 $t1 $0