Pipelining Andreas Klappenecker CPSC321 Computer Architecture

Basic Idea

Time Required for Load Word Assume that a lw instruction needs 2 ns for instruction fetch 1 ns for register read 2 ns for ALU operation 2 ns for data access 1 ns for register write Total time = 8 ns

Non-Pipelined vs. Pipelined Execution

Question What is the average speed-up for pipelined versus non-pipelined execution in case of load word instructions? Average speed-up is 4-fold!

Reason Assuming ideal conditions time between instructions (pipelined) = time between instructions (nonpipelined) number of pipe stages

MIPS Appreciation Day All MIPS instructions have the same length => simplifies the pipeline design fetch in first stage and decode in second stage Compare with 80x86 Instructions 1 byte to 17 bytes Pipelining is much more challenging

Obstacles to Pipelining Structural Hazards hardware cannot support the combination of instructions in the same clock cycle Control Hazards need to make decision based on results of one instruction while other is still executing Data Hazards instruction depends on results of instruction still in pipeline

Structural Hazards Laundry examples if you have a washer-dryer combination instead of a separate washer and dryer,… separate washer and dryer, but roommate is busy doing something else and does not put clothes away [sic!] Computer architecture competition in accessing hardware resources, e.g., access memory at the same time

Control Hazards Control hazards arise from the need to make a decision based on results of an instruction in the pipeline Branches: What is the next instruction? How can we resolve the problem? Stall the pipeline until computations done or predict the result delayed decision

Stall on Branch Assume that all branch computations are done in stage 2 Delay by one cycle to wait for the result

Branch Prediction Predict branch result For example, predict always that branch is not taken (e.g. reasonable for while instructions) if choice is correct, then pipeline runs at full speed if choice is incorrect, then pipeline stalls

Branch Prediction

Delayed Branch

Data Hazards A data hazard results if an instruction depends on the result of a previous instruction add $s0, $t0, $t1 sub $t2, $s0, $t3 // $s0 to be determined These dependencies happen often, so it is not possible to avoid them completely Use forwarding to get missing data from internal resources once available

Forwarding add $s0, $t0, $t1 sub $t2, $s0, $t3

Single Cycle Datapath

Pipelined Version