Simple Yet Powerful: Adding Stacks and Function Calls to an 8-bit CPU

Introduction

This project originated as a junior-level undergraduate course assignment. The initial requirement was to design a simple CPU with 4 registers and write a basic calculator application. I chose to push the boundaries of the project by implementing advanced features like stack operations and function calls. This was achieved through custom physical/virtual instructions such as push, pop, call, ret, and addpc. These additions enabled modular assembly programming, and I went further by developing:

• A standard library.
• An Integrated Development Environment (IDE) in C# with features like:
  • Syntax checking and highlighting
  • Code formatting
  • Assembler
  • Simulator
  • Debugger
  • FPGA programmer

The final product included a calculator application with full operator precedence and support for 16-bit multiplication and division. The IDE was open-sourced to help all 140 students in the class.

Features

1. Processor Design:

   • 8-bit, 4-register CPU implemented in Verilog.
   • Enhanced with PC-related operations LPCH, LPCL, SPC allowing more power virtual instructions:
     • stack operations (push, pop)
     • function call instructions (call, ret)
     • switch-case (addpc)

2. IDE (AssemblerGui):

   • Provides an assembler, simulator, and debugger.
   • Features syntax checking, automatic formatting, and support for various machine code outputs (e.g., Intel HEX, binary).
   • Open-sourced for collaborative development and use.

3. Calculator Application:

   • Supports full operator precedence.
   • Capable of performing 16-bit multiplication and division.

4. Educational Impact:

   • Open-sourced IDE was widely adopted by peers, benefiting 140 students in the course.

IDE Usage Guide

Editor

The editor allows you to create and modify MIPS files. Features include:

• Standard file operations: New, Open, Save, Save As.
• Automatic code formatting for consistent style (4-space indents, uppercase instructions, and registers).

Syntax errors are displayed with detailed information, and the cursor automatically moves to the error location for quick fixes.

Assembler

The assembler converts MIPS files into various machine code formats:

• Intel HEX: For ROM initialization in tools like Quartus.
• Binary/Hexadecimal Machine Code: Generates .txt files with binary or hexadecimal representation.
• Expanded Assembly: Converts extended instructions into raw MIPS instructions.
• ROM Download: Directly updates the FPGA ROM with assembled machine code.

Syntax errors during assembly provide detailed error reports, ensuring seamless debugging.

Debugger

The debugger simulates the execution of MIPS files. Key features:

• Start execution or step through instructions.
• Pause execution to inspect and modify registers or memory.
• Breakpoints for conditional pauses.
• Step through instructions, statements, or entire procedures.

Notes

• Be cautious when modifying the program counter (PC) during debugging to avoid unexpected behavior.
• Use advanced options (Enable Long Jumps and Enable Extended Instructions) only when necessary and with a clear understanding.

Contributions

This project showcases how extending a simple course requirement can lead to a robust, educational tool that benefits an entire class. By open-sourcing the IDE, I demonstrated the importance of collaborative development in education.

Repository

Find the source code and additional documentation on GitHub: Simple 8-bit CPU Project.