CSCI 343 April 8: Midterm 2 Review (Datapaths, Pipelining, and Hazards in MIPS Architecture)

Tuesday April 8, 2025

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Drawing Individual Datapaths

• You may be required to draw one or two individual datapaths for specific instructions.
• No control lines are needed for these, except for the branch instruction, which includes a single multiplexer controlled by a zero-check.
• All wires and paths must be labeled with bit widths (e.g., 32-bit, 16-bit, 5-bit, etc.) and meaningful annotations (e.g., operand, offset, address).
• For branch, store, load, jump instructions, be able to explain how the target address is calculated.

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Multiplexers in the Full Datapath

Key Multiplexers:

1. Register Destination (RegDst)
2. Memory to Register (MemtoReg)
3. Jump/Branch Control
4. ALU Source (ALUSrc)
5. PC Source Multiplexers (e.g., for branch/jump)

What to Know for Each Mux:

• Which instructions use the mux.
• What are the inputs, output, bit widths, and control signals.
• Diagram of each mux showing:
  • All labeled inputs.
  • Labeled output.
  • Control line and setting.

Additional Note:

• Only one mux (in branch datapath) connects to the zero-check signal.

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Control Signal Logic and Target Address Calculation

• Know how PC + 4, branch target address, and jump address are computed.
• The control lines involved include branch, jump, and zero-check.
• Know what values feed into the PC via muxes depending on the instruction.
• Understand sign-extend vs. shift-left-2 order in target address computation.

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Execution Time: Single-Cycle vs. Pipeline

Single-Cycle Design:

• All instructions take one cycle (fixed-length clock cycle).
• Clock cycle length must accommodate the slowest instruction.
• Example: Compute time required for a load by summing durations of all involved components.
  • Give brief explanations for each component’s time.
  • Multiply by instruction count for total execution time.

Pipelining:

• Five pipeline stages (not four):
  1. Instruction Fetch (IF)
  2. Instruction Decode/Register Fetch (ID)
  3. Execution (EX)
  4. Memory Access (MEM)
  5. Write Back (WB)
• Must draw pipelined execution diagrams.
• Cycle duration is determined by longest stage.
• Compare total execution time between single-cycle and pipelined architectures.

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Pipeline Hazards and Solutions

Structural Hazards:

• Not an issue in MIPS, due to consistent stage use across instructions.

Data Hazards:

• Occur when instructions depend on results of previous ones.

• Types:

  • Read-after-write (RAW) is most common.

• Examples:

  load $s1, 24($s0)
  sub $t0, $s1, $s2

  • sub needs $s1 before it is updated by the load.

• Solutions:

  • Stalling with bubbles
    • Introduce NOPs to delay dependent instruction.
    • Implemented by the Data Hazard Unit (DHU).
  • Forwarding
    • Uses pipeline registers to forward data.
    • Not always effective, e.g., load followed by dependent instruction.
    • Works when result is available at EX stage and needed at ALU input.

Control Hazards:

• Caused by branches and jumps.
• Uncertainty in next PC value leads to stalls.
• MIPS uses Delayed Branch strategy:
  • Always execute next sequential instruction.
  • Ignores branch prediction logic.
  • Simple, but may execute incorrect instructions, requiring pipeline flush.

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Registers Between Pipeline Stages

• Needed to transfer data and control signals between pipeline stages.
• Important for data forwarding and ensuring instructions in different stages don’t interfere.
• Example: Load result must be passed from MEM to WB via a register.

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Instruction Phase Behavior

• Not all instructions are active in all pipeline phases.
• Load is active in all 5 stages.
• Understand instruction behavior across each phase (especially for Load, Store, R-type).

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Hazard Units

• Forwarding Unit:
  • Complex and costly.
  • Implements data forwarding to minimize stalls.
• Hazard Detection Unit (DHU):
  • Detects RAW hazards.
  • Inserts bubbles as needed.

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Compiler Techniques for Optimization

• Compilers may reorder instructions to reduce hazards.
• You are not expected to reorganize instructions, but should be able to:
  • Identify dependencies.
  • Understand why reordering could help.

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Exam Topics

• Expect problems related to:
  • Datapath diagrams
  • Mux control logic
  • Pipelining vs. single-cycle comparisons
  • Hazard detection and resolution
  • Instruction dependencies

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Exam Preparation Notes

• Practice drawing and annotating datapaths and pipeline stages.
• Understand how each instruction flows through the pipeline.
• Memorization of control settings isn’t required—reason through what control signals are needed.
• Use pipeline diagrams to reason about stalls, bubbles, and forwarding.
• Be able to provide examples of data and control hazards, and how they are resolved.
• Review PowerPoint slides mentioned in class for concrete diagrams and forwarding examples.