Instruction Pipeline Design

Jun 21, 2024

Instruction Pipeline Design

Introduction

  • Instruction Pipeline: A technique used in computer design to increase instruction throughput (number of instructions executed per unit time).
  • Allows a stream of instructions to be executed in an overlapping manner to achieve instruction-level parallelism within a single processor.

Key Concepts

  • Throughput: Number of instructions executed per unit time.
  • Parallelism: Multiple instructions executed in a pipelined manner to improve performance.
  • Phases/Stages: Instructions are divided into multiple phases or stages which are executed sequentially. Different phases can be processed simultaneously for different instructions.

Instruction Cycle Phases

  1. Instruction Fetch (IF): Fetch the instruction from memory into a temporary buffer (instruction register).
  2. Instruction Decode (ID): Decode the instruction to determine the necessary operations and operands.
  3. Calculate Operand (CO): Determine whether operands are directly provided or need to be fetched based on addressing schemes.
  4. Fetch Operand (FO): Fetch the operands from the address calculated in CO phase.
  5. Execute Instruction (EX): Carry out the operation specified by the instruction.
  6. Write Back (WB): Store the result of the execution back in memory or a register.

Benefits of Instruction Pipelining

  • Reduction in Execution Time: Enables the execution of multiple instruction phases simultaneously, leading to reduced overall execution time.
  • Increased Throughput: More stages in the pipeline lead to higher throughput.

Pipeline Stages Example

  • 4 Stage Pipeline: IF, ID, FO, EX
  • 5 Stage Pipeline: IF, ID, CO, FO, EX, WB
  • 6 Stage Pipeline: IF, ID, CO, FO, Execute, Write Back

Timing Diagram

  • Each stage takes one clock cycle to complete.
  • Example for 6-stage pipeline with 4 instructions:
    1. First clock period: IF for instruction 1
    2. Second clock period: ID for instruction 1, IF for instruction 2
    3. Third clock period: CO for instruction 1, ID for instruction 2, IF for instruction 3
    4. Follow this overlapping pattern for all clock periods.
    5. Total execution time for 4 instructions in a 6-stage pipeline is 9 clock periods.
  • Without Pipelining: Same 4 instructions would take 24 clock periods (6 stages * 4 instructions).

Summary

  • Instruction pipelining effectively reduces the execution time and increases CPU throughput by allowing simultaneous execution of different instruction phases.
  • The success of pipelining depends on the design and number of pipeline stages.