8086 Pipelining Concepts and Performance Enhancement
Understand how the 8086 microprocessor achieves performance improvements through pipelining, instruction queuing, and parallel execution techniques.
Introduction to Pipelining in 8086
Pipelining is a technique where multiple instruction phases are overlapped to improve processor throughput. The 8086 implements a simple two-stage pipeline that separates instruction fetching from instruction execution, allowing the Bus Interface Unit (BIU) and Execution Unit (EU) to work simultaneously.
Foundation Concepts
- Temporal Parallelism: Different phases of instruction processing occur simultaneously
- Instruction Overlap: While one instruction executes, the next instruction is being fetched
- Queue-Based Architecture: Instructions are prefetched and stored in a buffer
- Performance Improvement: Reduced idle time for processor components
8086 Pipeline Architecture Overview
Pipeline Stages in Detail
| Stage | Unit | Operations | Duration | Parallel Activity |
|---|
| Fetch | BIU | • Memory access
• Instruction retrieval
• Queue storage | 4 clock cycles | EU can execute previous instruction |
| Execute | EU | • Instruction decode
• ALU operations
• Register updates | 1-4+ clock cycles | BIU can fetch next instruction |
Instruction Queue - The Heart of 8086 Pipelining
The 6-byte instruction queue is the central component enabling pipelining in the 8086. It acts as a buffer between the fetch and execute stages, allowing continuous operation of both units.
Queue Characteristics and Specifications
Instruction Queue Specifications:
- Size: 6 bytes (can hold multiple instructions)
- Organization: First-In-First-Out (FIFO) structure
- Access Method: EU reads from front, BIU writes to rear
- Pre-fetch Strategy: Automatic when queue is not full
- Variable Instruction Length: 1 to 6 bytes per instruction
Queue Operation States
Instruction Queue Management
The queue management follows specific protocols to maintain optimal performance:
Queue Filling Protocol:
- Automatic Fetch: BIU fetches when queue has space
- Priority to EU: Execution takes precedence over fetching
- Bus Availability: Fetching only when bus is free
- Alignment Consideration: Word alignment affects fetch efficiency
Queue Emptying Scenarios:
Normal Sequential Execution:
Queue maintains steady state with continuous refilling
Branch Instructions:
Queue is flushed, causing pipeline stall and performance penalty
Interrupt Handling:
Current queue state is preserved during interrupt processing
Pipeline Performance Analysis
Understanding pipeline performance requires analyzing instruction execution patterns, queue efficiency, and timing considerations.
Performance Metrics
Pipeline Timing Analysis
Sequential Instruction Execution:
Performance Calculation Examples
Problem 1: Pipeline Efficiency Calculation
Given: A sequence of 10 instructions, each taking 2 clock cycles to execute. Queue is initially full.
Calculate: Total execution time with and without pipelining.
Without Pipelining (Sequential):
- Fetch time per instruction: 4 cycles
- Execute time per instruction: 2 cycles
- Total per instruction: 6 cycles
- Total time: 10 × 6 = 60 cycles
With Pipelining (Overlapped):
- First instruction: 4 (fetch) + 2 (execute) = 6 cycles
- Remaining 9 instructions: 9 × 2 = 18 cycles
- Total time: 6 + 18 = 24 cycles
Performance Improvement:
Speedup = 60/24 = 2.5×
Efficiency = (60-24)/60 × 100% = 60% improvement
Pipeline Hazards and Stalls
Pipeline performance can be degraded by various hazards that cause stalls or require queue flushing.
Types of Pipeline Hazards
1. Control Hazards (Branch-Related)
Cause: Branch instructions that change program flow
Effect: Instruction queue must be flushed
Performance Impact: 4-6 clock cycles penalty
Branch Types and Impact:
- Unconditional Jumps (JMP): Always cause queue flush
- Conditional Branches (JZ, JNZ): Flush if branch is taken
- Procedure Calls (CALL): Queue flush plus stack operations
- Returns (RET): Queue flush plus stack operations
2. Data Hazards
Cause: Instructions that require immediate data from memory
Effect: EU must wait for BIU to complete data fetch
Performance Impact: Variable delay based on memory access time
3. Resource Hazards
Cause: Competition between BIU and EU for bus access
Effect: Temporary pipeline stall
Performance Impact: 1-2 clock cycles delay
Pipeline Stall Analysis
Optimization Strategies
Software Optimization:
- Minimize Branches: Use conditional instructions where possible
- Loop Unrolling: Reduce branch frequency in loops
- Instruction Scheduling: Arrange instructions to minimize conflicts
- Data Locality: Keep frequently used data in registers
Hardware Features:
- Larger Queue: More buffering capacity (limited to 6 bytes in 8086)
- Branch Prediction: Not available in 8086, introduced in later processors
- Multiple Pipelines: Not available in 8086, superscalar in later designs
Practical Pipeline Examples and Exercises
Example 1: Pipeline Timing Diagram
Assembly Code Sequence:
MOV AX, 1234H ; 3 bytes ADD AX, BX ; 2 bytes MOV CX, AX ; 2 bytes JMP LABEL ; 3 bytes (causes queue flush) LABEL: INC AX ; 1 byte
Execution Timeline:
| Clock Cycle | BIU Activity | EU Activity | Queue Status | Notes |
|---|
| 1-4 | Fetch MOV AX,1234H + ADD AX,BX | Idle | 5 bytes | Initial fetch |
| 5-6 | Fetch MOV CX,AX | Execute MOV AX,1234H | 4 bytes | Pipeline active |
| 7-8 | Fetch JMP LABEL | Execute ADD AX,BX | 5 bytes | Queue full |
| 9-10 | Idle (queue full) | Execute MOV CX,AX | 3 bytes | BIU waiting |
| 11-12 | Queue flush | Execute JMP | 0 bytes | Branch taken |
| 13-16 | Fetch from LABEL | Idle | Refilling | Pipeline restart |
Performance Analysis Problems
Problem 1: Branch Penalty Calculation
Scenario: A program has 20% branch instructions, and each branch flushes the queue.
Calculate: The average performance degradation due to branch penalties.
Solution Approach:
- Normal instruction: 2 cycles (with pipeline)
- Branch instruction: 2 + 4 = 6 cycles (including flush penalty)
- Average cycles = 0.8 × 2 + 0.2 × 6 = 2.8 cycles/instruction
- Performance degradation = (2.8 - 2)/2 × 100% = 40%
Problem 2: Queue Efficiency Analysis
Scenario: Analyze queue utilization for different instruction lengths.
Given: Instruction mix: 40% (1-byte), 35% (2-byte), 20% (3-byte), 5% (4-byte)
Analysis:
- Average instruction length = 0.4×1 + 0.35×2 + 0.2×3 + 0.05×4 = 1.9 bytes
- Queue capacity = 6 bytes
- Average instructions in queue = 6/1.9 ≈ 3.16 instructions
- Queue utilization efficiency = (3.16 × 1.9)/6 × 100% ≈ 100%
Advanced Pipeline Concepts
Pipeline vs. Later Processors
While the 8086 implements a simple two-stage pipeline, understanding its concepts is crucial for grasping advanced pipeline architectures in modern processors.
| Processor | Pipeline Stages | Key Features | Performance Gain |
|---|
| 8086 | 2 (Fetch, Execute) | Basic instruction queue | 1.5-2.5× over non-pipelined |
| 80286 | 3-4 | Improved branch handling | 2-3× |
| 80486 | 5 | On-chip cache, FPU pipeline | 3-4× |
| Pentium | 5 (dual) | Superscalar, dual pipelines | 4-6× |
| Modern CPUs | 12-20+ | Out-of-order, prediction | 10-100× |
Learning Outcomes
After studying 8086 pipelining, you should understand:
- Pipeline Fundamentals: How instruction overlap improves performance
- Queue Management: FIFO operation and optimization strategies
- Performance Analysis: Calculating speedup and efficiency metrics
- Hazard Recognition: Identifying and mitigating pipeline stalls
- Design Trade-offs: Benefits and limitations of simple pipelining
- Optimization Techniques: Software and hardware approaches
