computer organization- pipe lining

What is Pipelining

Pipelining is an implementation technique that overlaps

multiple instruction execution.

MIPS instructions takes 5 steps

1.Fetch instruction from memory [ IF ]

2.Read insrtuction while decoding instruction. [ ID ]

3.Execute operation or calculate an address. [EX]

4.Access an operand in data memory. [ MEM ]

5.Write the result into registers. [ WB ]

Example (without pipelining)

Five steps are involved in the lw fetch and execution.

Time taken to complete each step is as follows:

Instruction fetch: 200 psRegister read: 100 ps (for base value)ALU: 200 ps (for memory address)Memory read: 200 ps (for reading data from memory)Register write: 100 ps (for register write)

Execution time for lw instruction = 800 psExecution time for a sequence of 3 lw instructions = 2400 ps

Example (with pipelining)

Since the lw instruction is divided into five steps, a 5 stage pipeline isemployed.

Each pipeline stage takes one clock cycle. Clock cycle for a pipeline stage must be long enough to

accommodate the slowest operation (200 ps in our example).

Figure 6.3 Nonpipelined versus pipelined execution of 3 lw instructions

Tthe total execution time for the sequence of 3 lw instructions is 1400 ps

Figure 6.3 Nonpipelined versus pipelined execution of 3 lw instructions

Pipeline Performance - Summary

Pipeline does not change the individual instruction execution time

Pipeline improves performance by increasing the instruction throughput

The pipelined processor has a lower average CPI when compared to a multicycle implementation with the same clock rate.

The pipelined processor has a lower product of clock rate and CPI when compared to the single cycle implementation

Ideal speedup is proportional to the number of stages

Pipeline Hazards

Hazard: A situation in pipelining when the next

instruction cannot execute in the next clock

cycle

Three types of hazards: Structural hazard Data hazard Control (branch) hazard

Structural Hazard

The hardware cannot support the combination of

instructions that we want to execute in the same

clock cycle.

S. Barua – CPSC 440 [email protected] http://sbarua.ecs.fullerton.edu

Data Hazards

Data hazard can occur when one or more of the instructions in the

pipeline are data dependent.

Consider the following sequence of instructions:

add $s0, $t0, $t1

sub $t2, $s0, $t3

The sub instruction is dependent on the result in register $s0 of the first

instruction.

Consider the following sequence of instructions:

lw $s0, 20 ($t1)

sub $t2, $s0, $t3

The data required by the sub instruction is available only after the fourth

stage of the first instruction.

Data Hazard - Solutions

Two methods are used to resolve a data hazard.

Forwarding or bypassingRetrieves the missing data element from internal buffersinstead of waiting for it to come from the registers or memory location specified by the instruction (Figure 6.5)

Pipeline stall (bubble)

Stall the pipeline by the required number of stages. This guarantees correct execution, but could result in a lower performance.

In our example (lw followed by sub), we would have to stall

by one stage (Figure 6.6).

Figure 6.5 Forwarding or bypassing

Figure 6.6 Pipeline stall (bubble)

Control (Branch) Hazards

In a pipeline, an instruction is fetched at every clock cycle to

sustain the pipeline.

If the instruction fetched is a “branch” instruction, the decision

about whether to branch does not occur until the execution pipeline

stage.

The delay in determining the proper instruction to fetch is called a

“control hazard” or “branch hazard”.

Resolving Branch Hazards

Techniques employed are:

Always stall Pipeline is stalled until the pipeline determines the outcome of the branch and knows what instruction address to fetch from. The penalty will be several clock cycles.

Assume branch not taken Execution of the branch instruction is continued in

the pipeline assuming that the branch is not likely to take place.

If the branch is taken, the instructions that are being fetched and decoded are discarded (flushed).

Performance of Pipelined Systems

Pipelining reduces the average execution time per

instruction, thereby improving the system performance.

Hazards limit the performance improvement, but

appropriate hardware/software techniques can be

devised to circumvent these limits.

S. Barua – CPSC 440 [email protected] htt

S. Barua – CPSC 440 sbarufullerton.edu http://sbarua.ecs.fullerton.edu

computer organization- pipe lining

Documents