Project 1: MMIX Simulator (Understand the Machine)
Build a working MMIX simulator to internalize machine-level execution and correctness.
Quick Reference
| Attribute | Value |
|---|---|
| Difficulty | Expert |
| Time Estimate | 3-4 weeks |
| Language | C |
| Prerequisites | C, bitwise ops, basic ISA concepts |
| Key Topics | instruction decoding, registers, memory model |
1. Learning Objectives
By completing this project, you will:
- Implement fetch-decode-execute for MMIX instructions.
- Model registers, memory, and condition codes accurately.
- Validate simulator output against reference test programs.
- Understand how high-level algorithms map to machine steps.
2. Theoretical Foundation
2.1 Core Concepts
- ISA semantics: Each opcode has precise behavior and side effects.
- State machine: CPU state changes deterministically per instruction.
- Memory model: Correct loads/stores and alignment handling.
2.2 Why This Matters
TAOCP depends on a concrete machine model. MMIX makes the cost and correctness of each instruction explicit.
2.3 Historical Context / Background
Knuth designed MMIX as a modern RISC model to replace MIX. It is a teaching machine with clean semantics.
2.4 Common Misconceptions
- “The PC always increments by 4”: Not on branches and traps.
- “All loads are aligned”: Unaligned access must be handled by spec.
3. Project Specification
3.1 What You Will Build
A command-line MMIX simulator that loads a binary image, executes instructions, and reports register/memory state.
3.2 Functional Requirements
- Parse MMIX instruction formats.
- Implement core arithmetic, branch, load/store, and trap instructions.
- Provide a step/run mode and register dump.
- Load program image into simulated memory.
3.3 Non-Functional Requirements
- Correctness: Matches MMIX spec behavior.
- Determinism: Identical inputs yield identical traces.
- Usability: Clear CLI interface.
3.4 Example Usage / Output
$ ./mmixsim hello.mmo
[pc=0x0000] LDA $1,0x100
[pc=0x0004] TRAP 0,0,0
HALT
3.5 Real World Outcome
You can step through a Knuth-provided MMIX program and see precise register effects.
4. Solution Architecture
4.1 High-Level Design
┌──────────────┐ ┌──────────────┐ ┌──────────────┐
│ program img │────▶│ decode + exec│────▶│ CPU state │
└──────────────┘ └──────────────┘ └──────────────┘
4.2 Key Components
| Component | Responsibility | Key Decisions |
|---|---|---|
| Decoder | Parse opcode/fields | Table-driven dispatch |
| CPU state | Registers, PC, flags | Struct with arrays |
| Memory | Byte-addressable RAM | Bounds checks |
4.3 Data Structures
typedef struct {
uint64_t g[256];
uint64_t pc;
uint8_t flags;
} MmixCpu;
4.4 Algorithm Overview
Key Algorithm: Fetch-decode-execute
- Fetch instruction at PC.
- Decode opcode and operands.
- Execute semantics and update PC.
Complexity Analysis:
- Time: O(1) per instruction.
- Space: O(M) for memory size.
5. Implementation Guide
5.1 Development Environment Setup
gcc --version
5.2 Project Structure
project-root/
├── src/
│ ├── main.c
│ ├── cpu.c
│ ├── decoder.c
│ └── memory.c
└── Makefile
5.3 The Core Question You’re Answering
“What exactly changes in machine state for every instruction?”
5.4 Concepts You Must Understand First
Stop and research these before coding:
- MMIX instruction formats
- PC update rules
- Endianness and memory layout
5.5 Questions to Guide Your Design
- How will you handle traps and halts?
- How will you validate unaligned loads?
- What is your minimal instruction subset for tests?
5.6 Thinking Exercise
Pick one instruction (e.g., ADD) and write its full state transition in pseudocode.
5.7 The Interview Questions They’ll Ask
- How does an instruction decoder work?
- Why is ISA modeling a state machine?
- How do you test a simulator for correctness?
5.8 Hints in Layers
Hint 1: Start small
- Implement LDA, ADD, TRAP.
Hint 2: Add memory
- Implement LD and ST.
Hint 3: Add branches
- Implement JMP and conditional branches.
5.9 Books That Will Help
| Topic | Book | Chapter |
|---|---|---|
| MMIX spec | TAOCP Vol 1 | MMIX sections |
| ISA basics | Patterson/Hennessy | ISA chapters |
5.10 Implementation Phases
Phase 1: Foundation (1 week)
Goals:
- Basic CPU state and decoder.
Tasks:
- Implement register file and PC.
- Decode a small instruction subset.
Checkpoint: Can run a trivial program.
Phase 2: Core Functionality (1-2 weeks)
Goals:
- Add memory and control flow.
Tasks:
- Implement load/store.
- Implement branches.
Checkpoint: Test programs execute correctly.
Phase 3: Polish & Edge Cases (1 week)
Goals:
- Correctness and diagnostics.
Tasks:
- Add step mode.
- Add register dumps and tracing.
Checkpoint: Matches reference trace for sample inputs.
5.11 Key Implementation Decisions
| Decision | Options | Recommendation | Rationale |
|---|---|---|---|
| Dispatch | switch vs table | table | Cleaner extension |
| Memory | fixed vs dynamic | fixed | Simplifies bounds |
6. Testing Strategy
6.1 Test Categories
| Category | Purpose | Examples |
|---|---|---|
| Instruction tests | Verify semantics | ADD, LD, ST |
| Control flow | Branch correctness | JMP, BEQ |
| Regression | Full program | MMIX sample |
6.2 Critical Test Cases
- ADD updates register and flags.
- LD/ST round-trip memory correctly.
- Branch changes PC as expected.
6.3 Test Data
Small MMIX programs with known traces
7. Common Pitfalls & Debugging
7.1 Frequent Mistakes
| Pitfall | Symptom | Solution |
|---|---|---|
| Wrong PC update | Infinite loops | Centralize PC logic |
| Endianness bugs | Wrong values | Explicit byte order |
| Missing bounds | Crashes | Check memory ranges |
7.2 Debugging Strategies
- Trace PC and opcode per step.
- Compare against known traces.
7.3 Performance Traps
Not critical; correctness is priority.
8. Extensions & Challenges
8.1 Beginner Extensions
- Add disassembler output.
8.2 Intermediate Extensions
- Add breakpoints.
8.3 Advanced Extensions
- Implement full MMIX opcode set.
9. Real-World Connections
9.1 Industry Applications
- CPU simulators and emulators follow this pattern.
9.2 Related Open Source Projects
- Official MMIX tools and simulators.
9.3 Interview Relevance
- Demonstrates low-level reasoning and correctness discipline.
10. Resources
10.1 Essential Reading
- TAOCP Vol 1 MMIX sections.
10.2 Video Resources
- Search: “MMIX simulator”.
10.3 Tools & Documentation
- MMIX spec and sample programs.
10.4 Related Projects in This Series
11. Self-Assessment Checklist
11.1 Understanding
- I can explain CPU state transitions.
- I can decode MMIX instruction formats.
11.2 Implementation
- Simulator executes sample programs correctly.
- I can trace instruction effects.
11.3 Growth
- I can map high-level code to machine steps.
12. Submission / Completion Criteria
Minimum Viable Completion:
- Run a small MMIX program with correct output.
Full Completion:
- Implement a meaningful subset of instructions and tests.
Excellence (Going Above & Beyond):
- Full instruction set and debugging features.
This guide was generated from LEARN_TAOCP_DEEP_DIVE.md. For the complete learning path, see the parent directory README.