In our ongoing series highlighting exceptional projects from the DigiSim community, we're thrilled to showcase Alex Thompson's remarkable achievement: a fully functional 16-bit RISC processor designed and simulated entirely within DigiSim.io.
Alex, a third-year Computer Science student at State University, spent four months developing this impressive system as part of an independent study project. The result is not just a working CPU, but a testament to the power of modern digital logic simulation tools in education.
Meet the Builder
Alex Thompson is a 21-year-old computer science student with a passion for low-level computing and processor architecture. When not buried in coursework, Alex enjoys tinkering with embedded systems and has a growing collection of vintage computers.
"I've always been fascinated by how computers work at the most fundamental level," Alex explains. "Building a CPU from scratch seemed like the ultimate way to understand what's really happening inside these machines we use every day."
Project Overview
The DigiSim-16 Processor
Alex's creation, dubbed the "DigiSim-16," is a 16-bit RISC (Reduced Instruction Set Computer) processor featuring:
Core Specifications:
- 16-bit data path with 16-bit address bus
- 8 general-purpose registers (R0-R7)
- Harvard architecture with separate instruction and data memory
- 32 instruction types covering arithmetic, logic, and control operations
- 4-stage pipeline for improved performance
- Interrupt handling system for I/O operations
Memory System:
- 64KB instruction memory (32K 16-bit words)
- 64KB data memory with byte-addressable storage
- Memory-mapped I/O for peripheral communication
- Stack pointer for subroutine calls and interrupts
The Design Journey
Phase 1: Architecture Planning (Week 1-2)
Alex began with extensive research into existing RISC architectures, drawing inspiration from the MIPS and ARM instruction sets while keeping the design simple enough to implement in a semester project.
"The hardest part was deciding what to include and what to leave out," Alex recalls. "I wanted it to be capable enough to run real programs, but simple enough that I could actually finish it."
Key Design Decisions:
- Load/Store architecture - only load and store instructions access memory
- Fixed 16-bit instruction format for simplicity
- Register-register operations for most arithmetic and logic
- Conditional branching based on flag register status
Phase 2: Component Implementation (Week 3-8)
The implementation phase involved building each processor component systematically:
Arithmetic Logic Unit (ALU)
The ALU supports 16 operations including:
- Arithmetic: ADD, SUB, MUL, DIV
- Logic: AND, OR, XOR, NOT
- Shift: SHL, SHR, ROL, ROR
- Comparison: CMP, TST
"Building the ALU was like solving a massive puzzle," Alex notes. "Each operation needed its own logic, but they all had to work together seamlessly."
Register File
The 8-register file includes:
- Dual-port read capability for source operands
- Single-port write for result storage
- Register R0 hardwired to zero (MIPS-style)
- Register R7 designated as link register for subroutines
Control Unit
The control unit implements:
- Instruction decoding with microcode-like control signals
- Pipeline control with hazard detection
- Interrupt handling with priority levels
- Branch prediction (simple static prediction)
Phase 3: Integration and Testing (Week 9-12)
The integration phase revealed the complexity of making all components work together:
Pipeline Implementation
Alex implemented a 4-stage pipeline:
- Fetch - Retrieve instruction from memory
- Decode - Parse instruction and read registers
- Execute - Perform ALU operation or memory access
- Writeback - Store result in destination register
Hazard Handling
The processor includes logic for:
- Data hazards - forwarding and stalling mechanisms
- Control hazards - branch prediction and flushing
- Structural hazards - resource conflict resolution
Technical Challenges
Challenge 1: Timing and Synchronization
Problem: Ensuring all components operate in perfect synchronization across the pipeline stages.
Solution: Alex implemented a comprehensive clocking strategy with:
- Master clock driving all sequential elements
- Clock enable signals for pipeline stalls
- Reset logic for initialization and error recovery
"Getting the timing right was crucial," Alex explains. "One misaligned clock edge could corrupt the entire processor state."
Challenge 2: Memory Interface Design
Problem: Implementing efficient memory access while maintaining pipeline flow.
Solution:
- Separate instruction and data caches to avoid conflicts
- Memory controller with burst access capabilities
- Write buffer to hide memory latency
Challenge 3: Debugging Complex Interactions
Problem: Identifying bugs in a system with thousands of interconnected components.
Solution: Alex developed a comprehensive testing strategy:
- Unit tests for individual components
- Integration tests for subsystem interactions
- Instruction-level simulation with detailed logging
- Waveform analysis using DigiSim's built-in tools
"DigiSim's debugging tools were invaluable," Alex notes. "Being able to probe any signal and see waveforms made finding bugs much easier than it would have been with physical hardware."
Software Development
Assembly Language and Toolchain
Alex didn't stop at hardware—a complete software toolchain was developed:
Custom Assembly Language
; Example: Fibonacci sequence calculator
MAIN:
LOAD R1, #1 ; First Fibonacci number
LOAD R2, #1 ; Second Fibonacci number
LOAD R3, #10 ; Counter for 10 iterations
LOOP:
ADD R4, R1, R2 ; Calculate next number
MOV R1, R2 ; Shift numbers
MOV R2, R4
SUB R3, R3, #1 ; Decrement counter
BNZ LOOP ; Branch if not zero
HALT ; Stop execution
Assembler and Simulator
- Two-pass assembler written in Python
- Instruction simulator for software testing
- Debugging interface with breakpoints and single-stepping
- Memory dump utilities for program analysis
Test Programs
Alex developed several test programs to validate the processor:
- Basic arithmetic - Addition, subtraction, multiplication
- Sorting algorithms - Bubble sort and quicksort implementations
- String processing - Character manipulation and searching
- Mathematical functions - Square root and trigonometric approximations
- Game logic - Simple Tic-Tac-Toe implementation
Performance Analysis
Benchmark Results
The DigiSim-16 achieved impressive performance metrics:
Clock Speed: 50 MHz (simulated) Pipeline Efficiency: 85% (accounting for hazards and stalls) Memory Bandwidth: 100 MB/s (theoretical maximum) Instruction Throughput: 42.5 MIPS (Million Instructions Per Second)
Comparison with Commercial Processors
While not competing with modern processors, the DigiSim-16 compares favorably with historical 16-bit processors:
| Processor | Year | Clock Speed | Performance |
|---|---|---|---|
| Intel 8086 | 1978 | 5-10 MHz | ~0.33 MIPS |
| Motorola 68000 | 1979 | 8-16 MHz | ~1.35 MIPS |
| DigiSim-16 | 2025 | 50 MHz* | ~42.5 MIPS* |
*Simulated performance
Lessons Learned
Technical Insights
Complexity Management: "Breaking the project into small, manageable pieces was essential. I couldn't have built this all at once."
Testing Strategy: "Writing tests before implementing features saved me countless hours of debugging later."
Documentation: "Keeping detailed documentation of my design decisions helped when I needed to modify things later."
Educational Value
Deep Understanding: "Building a CPU gave me insights into computer architecture that no textbook could provide."
Problem-Solving Skills: "Every day brought new challenges that required creative solutions."
Appreciation for Complexity: "I have so much more respect for the engineers who design real processors now."
Community Impact
Alex's project has inspired other students and educators:
Educational Adoption
- Course Integration: Three universities have incorporated Alex's design into their curriculum
- Student Projects: Over 50 students have built variations of the DigiSim-16
- Research Applications: Graduate students are using it as a baseline for architecture research
Open Source Contribution
Alex has made the entire project available under an open-source license:
- Complete source code with detailed comments
- Assembly language specification and examples
- Testing framework and benchmark suite
- Documentation including design rationale and tutorials
Future Enhancements
Alex isn't done yet. Planned improvements include:
Hardware Enhancements
- Floating-point unit for scientific calculations
- Cache hierarchy with L1 and L2 caches
- Superscalar execution with multiple ALUs
- Out-of-order execution for improved performance
Software Ecosystem
- C compiler targeting the DigiSim-16 architecture
- Operating system kernel with basic multitasking
- Standard library with common functions
- Development tools including debugger and profiler
Getting Started with Your Own CPU
Inspired by Alex's project? Here's how to begin your own processor design journey:
Prerequisites
- Digital logic fundamentals - Understanding of gates, flip-flops, and combinational logic
- Computer architecture basics - Familiarity with instruction sets and processor organization
- Programming experience - Helpful for writing test programs and tools
Recommended Approach
- Start simple - Begin with a basic 8-bit processor
- Study existing designs - Learn from proven architectures
- Build incrementally - Add features one at a time
- Test thoroughly - Validate each component before integration
- Document everything - Keep detailed records of your design decisions
Resources
- DigiSim.io tutorials - Step-by-step processor design guides
- Computer architecture textbooks - Hennessy & Patterson is the gold standard
- Online communities - Join forums and Discord servers for support
- Open source projects - Study existing processor implementations
Conclusion
Alex Thompson's DigiSim-16 processor represents the best of what's possible when passion meets powerful tools. This project demonstrates that with dedication, creativity, and the right simulation environment, students can achieve remarkable things.
"The most rewarding moment was when I ran my first program and saw it execute correctly," Alex reflects. "All those late nights debugging suddenly felt worth it."
The DigiSim-16 project showcases the educational potential of modern simulation tools. By providing a risk-free environment for experimentation, platforms like DigiSim.io enable students to tackle ambitious projects that would be impossible with traditional hardware-based approaches.
We're excited to see what Alex builds next and encourage other community members to share their own ambitious projects. The future of digital logic education is being written by students like Alex, one simulation at a time.
Want to showcase your own DigiSim project? Contact us at community@digisim.io with details about your creation. We love highlighting the amazing work our community produces!
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