MOS 6502 Simulator

MOS 6502 Simulator is like an emulator for the 6502 chip. It focuses mainly on the CPU, but it skips some details like BCD operations and doesn’t worry about instruction timings.

Why I Started This Project

I got inspired to create this simulator when I found my old BBC-B in the loft. It brought back memories of spending hours using it in low-resolution text mode. I even thought about making a NES or BBC emulator, but I didn’t get too far with that idea.

Some Challenges I Faced

• Programming Language: I wrote it in C++. Since the 6502 doesn’t have a dedicated IO bus, everything is done through memory. My idea was to have a virtual class that provided a general interface so devices could override parts of it or trap writes. But honestly, I kept thinking that using assembly language would’ve been better for manipulating flags and rotations instead of dealing with messy C++ code.


• Instruction Decoding: At first, I aimed to decode instructions correctly, but there were so many exceptions to consider (especially if I wanted to support the 65C02). This turned into a giant switch statement! I even thought about breaking it down into smaller files just to keep things manageable. Plus, since they’re not in numerical order or grouped by type, compiling them into a jump table was tricky.

The Image Format and Documentation

The image format is pretty straightforward. The file has to be at least 65536 bytes (64k), which represents the memory image for the system (because of its 16-bit address bus). There’s also some odd ASCII text display starting at address 0x200 that can show strings just fine. Keep in mind this project was thrown together in about six hours or so, and while it’s not very thoroughly documented, it's still something!

Want To Learn More?

If you're interested in checking out more details or downloading MOS 6502 Simulator for your own use, feel free to visit this link.