MIPS Single-Cycle Processor Emulator Tutorial for CS312

Introduction to the MIPS Single-Cycle Processor Emulator Project

If you're taking CS312, you've likely encountered the MIPS single-cycle processor emulator project. This assignment challenges you to build a program that reads a binary file containing MIPS instructions, disassembles them into human-readable assembly, then simulates execution cycle by cycle. It's a foundational exercise in computer architecture that gives you hands-on experience with instruction encoding, register files, and memory management.

Think of it like building a retro game console emulator—but instead of playing Super Mario, you're executing MIPS assembly instructions like ADD, LW, and BEQ. The skills you gain here translate directly to understanding how modern processors work, from the CPU in your laptop to the AI accelerators in data centers.

Understanding the Project Requirements

Your MIPS simulator must accept command-line arguments (mipssim –i INPUTFILENAME –o OUTPUTFILENAME) and produce two output files: a disassembly file (_dis.txt) and a simulation file (_sim.txt). The disassembly file lists each instruction word in binary, its memory address, the opcode, and the arguments. The simulation file shows the state of all 32 registers and memory after each instruction executes.

One key twist: the opcode is only 5 bits, preceded by a valid bit. If the valid bit is 0, the instruction is a NOP. This is a simplified version of real MIPS, but it teaches you to handle bit manipulation—a skill essential for low-level programming and embedded systems development.

Step 1: Building the Disassembler

Start by reading the binary file and parsing each 32-bit word. For each word, extract the valid bit (bit 31) and opcode (bits 30–26). Then, based on the instruction type (R-type, I-type, J-type), decode the remaining fields: registers, immediate values, shift amounts, or jump targets.

For example, an ADD instruction has opcode 0 and function code 0x20. You'll need a lookup table or a series of if-else statements. A beginner-friendly approach is to first print whether each instruction is valid or not—this helps you debug the binary parsing before tackling full disassembly.

Use a modular design: separate the disassembler logic from the simulation logic. This makes your code reusable and easier to debug. In fact, many students find that writing the disassembler first and testing it thoroughly saves hours of frustration later.

Step 2: Implementing the Simulation Engine

Once disassembly works, you can simulate execution. Start with the program counter at 96 (the default start address). For each cycle, fetch the instruction, decode it, execute it, then update the register file and memory. After each instruction, output the state.

You'll need to implement instructions like ADD, SUB, LW, SW, BEQ, J, JR, and others. Pay special attention to branches and jumps—they change the program counter and can cause infinite loops if not handled correctly.

A helpful analogy: imagine you're debugging a video game AI that decides enemy movements. Each instruction is a decision: move left (LW), check health (BEQ), or attack (ADD). The register file is like the AI's memory of player positions. By simulating step by step, you can trace exactly what the AI is doing.

Step 3: Formatting the Output

The output format is strict. The disassembly file must have four tab-separated columns: binary instruction, address, opcode, and arguments. The simulation file must start with 20 equal signs, then cycle info, then register values in groups of 8, then data memory in rows of 8 words.

Create a dedicated output function to handle this formatting. It's a small investment that prevents headaches. For example, in C++, you could write a printState() function that takes the cycle number, PC, instruction string, register array, and memory array, then prints everything in the required order.

Remember: the diff command with -w ignores whitespace differences, but you should still aim for exact matches. Use tabs consistently, and ensure that negative numbers are printed in decimal with a minus sign.

Common Pitfalls and How to Avoid Them

Endianness: The binary file is likely big-endian. Make sure you read bytes in the correct order.
BREAK instruction: The last instruction is always BREAK. After that, the remaining data is program data, not instructions. Your disassembler should stop disassembling at BREAK.
Valid bit: If the valid bit is 0, treat the instruction as a NOP. Don't skip it entirely—it still occupies a cycle and should appear in the simulation output.
Register zero: Register $0 is hardwired to 0. Any write to it should be ignored.

Testing and Debugging

Use the provided sample inputs and outputs to verify your program. But also create your own small test programs—like a simple loop that adds numbers. This helps you catch edge cases. Run your program on the Linux home server to ensure compatibility.

If you're stuck, think of this as similar to debugging a popular mobile app like Instagram's photo filter pipeline. Each filter is an instruction, and the image data is memory. By stepping through the pipeline, you can find where the filter goes wrong.

Conclusion

Building a MIPS single-cycle processor emulator is a rewarding project that deepens your understanding of computer architecture. By breaking it into disassembler and simulator components, you can tackle it step by step. Use the tips in this tutorial to avoid common mistakes, and remember: the skills you gain—bit manipulation, state management, and precise output formatting—are valuable for any software engineering career, especially in embedded systems, game development, and AI hardware.

Good luck with your CS312 project! With careful planning and testing, you'll have your emulator running in no time.