The Processor: Pipelining - Lecture Notes

Lecture Overview

  • Lecture titled: The Processor: Pipelining
  • Presented by: Dr. Marvin Andujar
  • Context: Herbert Wertheim College of Engineering, University of Florida
  • Focus: Mechanisms of pipelining, exceptions, and interruptions in CPU operations.

Pipelining Basics

  • Definitions and components of pipelining are essential for improving CPU performance by executing multiple instructions simultaneously.
  • Key components in a basic pipeline:
    • Instruction Fetch (IF)
    • Instruction Decode (ID)
    • Execute (EX)
    • Memory Access (MEM)
    • Write Back (WB)
  • Importance: Allows for overlapped execution of instructions leading to high throughput.

Exceptions and Interrupts

  • Unexpected events that disrupt normal flow of control in a program execution.
    • Different ISAs may use terms interchangeably or differently.
    • Exception
    • Defined as an unscheduled event that arises within the CPU, disrupting program flow, utilized to detect issues such as overflow events.
    • Interrupt
    • An exception stemming from external stimuli affecting the processor.
  • Challenges: Managing exceptions and interrupts while maintaining performance.

Types of Events and RISC-V Terminology

  • Categorization of events based on origin and type in RISC-V architecture:
    • External Events:
    • Interrupt: Requests from I/O devices.
    • Internal Events:
    • Exception: Includes events like:
      • Arithmetic overflow
      • Undefined instruction
      • Hardware malfunctions (could be categorized as either exceptions or interrupts).

Handling Exceptions

  • Actions taken during an exception:
    • Save the address of the offending instruction in the Supervisor Exception Program Counter (SEPC).
    • Transfer control to the operating system using a specified address.
  • SEPC: A 64-bit register that stores the address of the instruction that caused the exception.
  • SCause: A 64-bit register that logs the reason for the exception.

Vector Interrupts

  • Control transfer addresses are determined based on exception causes:
    • Separate addresses assigned for different exception types.
    • Example addresses for exceptions:
    • Undefined instruction: 00 0100 0000
    • System error (hardware malfunction): 01 1000 0000

Pipelining with Exceptions

  • Mechanism enhancements include:
    • New input in multiplexor providing new PC value for exceptions: 0000 0000 1C09 0000hex.
    • Introduction of SCause register and SEPC.
    • The input signal directs where instruction fetching begins when an exception occurs.

Exception Handling Example

  • Example sequence demonstrating exceptions in an instruction block:
    • Instruction sequences leading to exception:
    • add x1, x2, x1
    • Instruction pointers and handlers:
      • 1C090000hex (instruction handler)
      • Following addresses for post-exception processing: 1C090004hex.

Multiple Exceptions Management

  • In pipelined architectures, multiple exceptions can occur simultaneously.
    • Strategies for addressing:
    • Basic: Handle the earliest instruction exception first.
    • Complex pipelines: Cater to multiple instructions per cycle and out-of-order execution.
    • Precise exceptions require careful handling to maintain execution accuracy.

Precise vs. Imprecise Interrupts

  • Precise Interrupt:
    • Always correctly associated with the originating instruction.
  • Imprecise Interrupt:
    • May not relate precisely to the instruction that caused them, complicating processing in pipelined architectures.

Multiple Issue in Processor Design

  • Multiple Issue: Technique enabling simultaneous instruction dispatching within a single clock cycle:
    • Static Multiple Issue: Pre-execution decisions made by compiler to group instructions.
    • Instructions are packaged into issue slots avoiding hazards identified during compilation.
    • Dynamic Multiple Issue: Decisions made during execution for issuing instructions, allowing for more flexible hazard management.

Speculation in Instruction Processing

  • Speculation: The practice of predicting instruction outcomes to optimize execution.
    • Processes include:
      • Pre-emptive start of operations.
      • Verification of correctness post-operation, with roll-back strategy if predictions were incorrect.
  • Common applications include speculating on branch outcomes and loads.

Compiler and Hardware Speculation Techniques

  • Compilers can rearrange instruction sequences (e.g., loading before branching) to enhance operation efficiency.
  • Hardware capabilities include looking ahead to execute future instructions and buffering results until confirmed necessary, providing dynamic flexibility to handle false predictions.

Speculation and Exception Scenarios

  • Addressing speculation when exceptions occur during preemptive instruction execution:
    • Static Speculation: ISA could be enhanced to support deferred exceptions.
    • Dynamic Speculation: Possibility to buffer exceptions until the completion of the instruction.

Static Multiple Issue Configurations

  • Very Long Instruction Word (VLIW): A style of processor architecture where numerous independent operations are defined in long instructions.
  • Issue Packet: Group of instructions that can undergo execution in a single cycle, managed by the compiler or processor to avoid hazards.

Scheduling in Static Multiple Issue Systems

  • Compilers must optimize instruction placement within issue packets to eliminate interdependencies efficiently.
  • Sometimes requires padding with no-operation (nop) instructions to maintain execution flow and efficiency.

Static Two-Issue Pipeline Operations

  • Example shows simultaneous issue of ALU and data transfer instructions utilizing a five-stage pipeline architecture.
  • Maintaining register writing at the pipeline's end aids in simplifying exception management while preserving a precise exception model.

Dynamic Multiple Issue Strategy

  • Description and functionality of superscalar processors, which can execute multiple instructions each clock cycle by selecting from the instruction stream during execution.
  • Advantages include removal of compiler scheduling dependence, facilitated by CPU's capacity to ensure code semantics during execution.