Pipelining and Instruction Execution Overview

Overview of Class Structure

  • Final class session after five weeks.
  • Focus on topics: Pipelining and Cash Analysis.
  • Upcoming events:
    • Exam scheduled for April 9 at 08:20 PM covering material after Exam 3.
    • DiffiQ exam on the same day to be accommodated for students as needed.

Class Assignments

  • Last assignment: Cash Analysis.
  • Encouragement to utilize resources available for understanding material and finding out remaining assignments.

Pipelining

  • Pipelining Definition: Technique in computer architecture that allows overlapping execution of multiple instructions, enhancing performance by executing instruction in stages.
  • Stages of Pipelining:
    • Cycle 1: Fetch instruction from memory.
    • Cycle 2: Decode instruction and read required registers.
    • Cycle 3: Execute the instruction.
    • Cycle 4: Read/Write to memory.
    • Cycle 5: Write results back to registers.

Instruction Execution Path

  • Details on how instructions are executed step by step through a simple data path:
    • Program Counter: Holds the address of the instruction to be executed.
    • Instruction fetched from Instruction Memory and passed to Instruction Register.
    • Control Unit determines instruction type and manages register accesses.
    • Calculation phase with ALU (Arithmetic Logic Unit).
    • Memory access phase for load/store instructions.
    • Writing results to register.

Pipelining Hazards

  • Definition: Anything that prevents the pipeline from executing instructions smoothly, causing delays or stalls.
Types of Hazards:

  • Structural Hazards:

    • Occurs when two instructions need the same hardware resource at the same time, such as instruction memory and data memory.
    • Solution: Separate paths for data and instruction memory (Harvard architecture).
    • Example: Handling register file access concurrently during instruction decode and write back stages.

  • Data Hazards:

    • Occurs when an instruction requires data that is not yet available (e.g., a register that has not been written yet).
    • Example: A subsequent instruction needs the result from a previous instruction's ALU operation.
    • Forwarding: Technique to mitigate data hazards by allowing subsequent instructions to use results that are not yet written back to registers.

  • Control Hazards:

    • Occurs when the pipeline doesn't know which instruction to fetch next due to branches or jumps.
    • Solutions include branch prediction methods.
    • Discussion on the impact of branch prediction on pipelining discussed for the next class.

Summary of Key Learnings

  • Understanding the flow of instruction through the pipelining stages.
  • Recognizing different types of hazards and their implications on instruction execution efficiency.
  • Practical implications of pipelining in computer architecture applications.
  • Exploring methods to resolve hazards including structural adaptations, data forwarding, and control prediction methods.

Future Topics

  • Continue with Control Hazards and Branch Prediction in the next class session.
  • Encourage questions and active participation to clarify upcoming modules.