Answer from cs61c-ba (Zhou Zhou 16250641) for Question 4
Memory mapped IO mean that a portion of the address space is assigned to I/O device.

assume receiver control at $a0 and receiver data at $a1.

Loop:
andi $t0,a1,0x00000001
beq $t0, $0, end
lbu $v0,0(a1)
j Loop

End: