Answer from cs61c-bd (Dae-hyeon Wi 17782249) for Question 4
Memory-mapped io is a method of addressing the device. In memory mapped io, some part of address space are allocated to the device. We can control(I think communication will be better!) the device by reading & writing to this allocated memory space.

li $t0, 0xffff0000
li $t2, 1;
loop:
lb $t1, 0($t0)
andi $t1, $t1, 1;
bne $t1,$t2, loop
beq $t2, $0, read
li $t2, 0;
j loop
read:
lb $v0, 4($t0)