addi    $s1,    $zero,  1
addi    $s2,    $zero,  2
add     $s3,    $s1,    $s1 # MEM/WB-EX
add     $s3,    $s3,    $s1 # EX/MEM-EX
addi    $s4,    $zero,  3
add     $s3,    $s3,    $s2 # MEM/WB-EX