Overview

The goal of this project is to understand how CPUs execute instructions by implementing a 6502 emulator from scratch.

Unlike many simple emulators, this one aims to emulate individual instructions accurately while keeping the implementation clean and easy to understand.

Features

• 151 official instructions
• Multiple addressing modes
• Configurable memory bus
• Disassembler
• Debug logging
• Unit tests

Motivation

I wanted to better understand how processors execute instructions and how machine code interacts with memory.

This project also serves as preparation for building a small operating system and experimenting with computer architecture.

Architecture

           +----------------+
           |      CPU       |
           +-------+--------+
                   |
        +----------+----------+
        |                     |
+-------v------+      +-------v------+
|   Memory     |      |   Registers  |
+--------------+      +--------------+

Instruction Execution

Each instruction follows roughly the same flow:

1. Fetch opcode.
2. Decode instruction.
3. Read operands.
4. Execute.
5. Update flags.
6. Increment program counter.

Example

fn lda(&mut self, value: u8) {
    self.a = value;
    self.update_zero_and_negative_flags(self.a);
}

Current Progress

Problems Encountered

The hardest part wasn't implementing instructions—it was realizing that several instructions behave differently depending on the addressing mode.

Another issue was understanding the processor status flags.

Lessons Learned

• Small abstractions make emulator code easier to extend.
• Writing tests early saves hours of debugging.
• Reading datasheets carefully is worth the effort.

Future Improvements

• Cycle accuracy
• Interrupt support
• Decimal mode
• Interactive debugger
• WebAssembly build

Screenshot

Related Lab Entries

• Understanding Zero Page Addressing
• Debugging ADC
• Implementing Status Flags

References

• MOS Technology 6502 Datasheet
• Programming the 65816
• NESDev Wiki

Source Code

GitHub:

https://github.com/Muhammed-Rajab

Demo:

https://example.com