Question

In this exercise, we examine how data dependences affect execution in the basic 5-stage pipeline described in Section 4.5. Problems in this exercise refer to th following sequence of instructions: addi $2,$2,22 SW $3,20($2) OR $4,$2,$3 Also, assume the following cycle times for each of the options related to forwarding: Without Forwarding With Full Forwarding 300ps With ALU-ALU Forwarding Only 250ps 290ps 4.9.1 [10] <4.5> Indicate dependences and their type. 4.9.2 [10] <S4.5> Assume there is no forwarding in this pipelined processor. Indicate hazards and add nop instructions to eliminate them 4.9.3 [10] <S4.5> Assume there is full forwarding. Indicate hazards and add NOP instructions to eliminate them. 4.9.4 [10] <S4.5> What is the total execution time of this instruction sequence without forwarding and with full forwarding? What is the speedup achieved by adding full forwarding to a pipeline that had no forwarding? 4.9.5 [10] <S4.5> Add nop instructions to this code to eliminate hazards if there is ALU-ALU forwarding only (no forwarding from the MEM to the EX stage) 4.9.6 [10] <$4.5> What is the total execution time of this instruction sequence with only ALU-ALU forwarding? What is the speedup over a no-forwarding pipeline?

Answer 1

Given that how data dependences affect execution in the basic - 5 stage pipeline described without forwarding - 250 ps with full forwarding - 300 ps with ALU - ALO forwarding only - 290 ps 4.9.1 Instruction sequence I_1 = oR R_1, R_2, R_3 I_2 = OR R_2, R_7, R_4 I_3 = OR R_1, R_1, R_2 Dependencies RA m R_1 from I_1 to I_2 and I_3 RAu m R_2 from I_2 to I CaA_2 m R_2 from I_1 to I_2 WAR m R_1 from I_2 to I_3 WAW on R_1 from I_1 to I_3 4.9.2 There is no forwarding in this piplined processor Indicates hazards and add instructions to eleminete them implies In the basic five state pipeline WAR x WAW dependencies do not cause any hazard without forwarding, any RAw dependence b/w an instruction and the next two instructions The Code that eliminates these hazard by inserting NOP instructions is: OR R_1, R_2, R_3 NOP NOP