COMPUTER ORGANIZATION AND DESIGN. 5 th Edition. The Hardware/Software Interface. Chapter 4. The Processor. Dr. Randa Mohamed

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The Hardware/Software Interface

5th

Edition

Chapter 4

The Processor

Agenda

◼

Performance Issues

MIPS Pipeline:

Performance Issues

◼

Longest delay determines clock period

◼ Critical path: load instruction

◼ Instruction memory → register file → ALU →

data memory → register file

Performance Issues

◼

Not feasible to vary period for different

instructions

Pipelining Analogy

◼

Pipelined laundry: overlapping execution

◼ Parallelism improves performance

MIPS Pipeline

◼

Five stages, one step per stage

1. IF: Instruction fetch from memory

2. ID: Instruction decode & register read

3. EX: Execute operation or calculate address

4. MEM: Access memory operand

5. WB: Write result back to register

Pipeline Performance

◼ Assume time for stages is

◼ 100ps for register read or write ◼ 200ps for other stages

◼ Compare pipelined datapath with single-cycle

datapath

Pipeline Performance

Single-cycle (T_c= 800ps)

Pipeline Speedup

◼

Speedup = CPUtime

CPUtime

◼ CPUtime_N= IC × T_N

Pipeline Speedup

◼

Speedup = CPUtime

CPUtime

◼ CPUtime_N= IC × T_{N 3×800=2400}

Pipeline Speedup

◼

Speedup = CPUtime

CPUtime

◼ CPUtime_P= No. Stages × T_P + (IC-1) × T_P

Pipelined (T = 200ps)

◼ Speedup = IC × T_N

No. Stages × T_P + (IC-1) × T_P = IC × T_N

T_P (No. Stages + IC-1)

Pipeline Speedup

◼

Speedup = CPUtime

CPUtime

◼ CPUtime_N= IC × T_N

◼ Speedup = IC × T_N

No. Stages × T_P + (IC-1) × T_P = IC × T_N

T_P (No. Stages -1+ IC)

Pipeline Speedup

◼

Speedup = CPUtime

CPUtime

◼ CPUtime_N= IC × T_N

◼ CPUtime_P= No. Stages × T_P + (IC-1) × T_P

◼ Speedup = IC × T_N

No. Stages × T_P + (IC-1) × T_P

= IC × T_N = T_N = No. Stages × T_P Tp× IC T_P T_P

Pipeline Speedup

Speedup = CPUtime

CPUtime

◼ CPUtime_P= No. Stages × T_P + (IC-1) × T_P

= No. Stages

3×800=2400 (5×200)+ (2×200)=1400

1.7

◼ Speedup = IC × T_N

No. Stages × T_P + (IC-1) × T_P

= IC × T_N = T_N = No. Stages × T_P Tp× IC T_P T_P

Pipeline Speedup

Speedup = CPUtime

CPUtime

◼ CPUtime_P= No. Stages × T_P + (IC-1) × T_P

= No. Stages

1000×800=800000 (5×200)+ (999×200)=200800

3.9

Pipelined Datapath

◼

Need registers between stages

Pipeline Operation

◼

Cycle-by-cycle flow of instructions through the

pipelined datapath

◼ “Single-clock-cycle” pipeline diagram ◼ Shows pipeline usage in a single cycle ◼ Highlight resources used

◼ c.f. “multi-clock-cycle” diagram ◼ Graph of operation over time

WB for Load

Multi-Cycle Pipeline Diagram

Multi-Cycle Pipeline Diagram

Single-Cycle Pipeline Diagram

Pipelined Control

◼

Control signals derived from instruction

Concluding Remarks

◼ ISA influences design of datapath and control ◼ Datapath and control influence design of ISA ◼ Pipelining improves instruction throughput

using parallelism

◼ More instructions completed per second ◼ Latency for each instruction not reduced

Problems to solve