Question 9798 – Pipelining

Consider a 5-stage pipeline – IF (Instruction Fetch), ID (Instruction Decode and register read), EX (Execute), MEM (memory), and WB (Write Back). All (memory or register) reads take place in the second phase of a clock cycle and writes occur in the first phase of the clock cycle. Consider the execution of the following instruction sequence:

      11:      sub r2, r3, r4;         /*   r2 ← r3 – r4    */
      12:      sub r4, r2, r3;         /*   r4 ← r2 – r3    */
      13:      sw r2, 100(r1)          /*   M[r1+100] ← r2  */
      14:      sub r3, r4, r2;         /*   r3 ← r4 – r2    */  

(a) Show all data dependencies between the four instructions.
(b) Identify the data hazards.
(c) Can all hazards be avoided by forwarding in this case?

Correct Answer: A