1. Annotate the following MIPS instructions to indicate source registers and destination registers. A source register is read during the instruction's execution, while a destination register is written during the instruction's execution. (12 points)
   1. add $s1, $t2, $t3
   2. lw $s3, 8($gp)
   3. sw $s4, 12($s5)
   4. addi $s1, $zero, 100
   5. bne $t1, $t2, else
   6. add $s1, $s1, $s1