6502 Emulator in C++

An emulator library for the 6502 CPU built in C++

Inspired by: Dave Poo and Ben Eater on YouTube.

Based on: This Documentation

How to add the library to your project

Method 1: CMake FetchContent

The easiest way to include the library in your C++ project is by using CMake's FetchContent.
Add the following to your CMakeLists.txt

FetchContent_Declare(
  6502Library
  GIT_REPOSITORY https://github.com/Harry-Skerritt/6502-Emulator.git
  GIT_TAG main
)

FetchContent_MakeAvailable(6502Library)

target_link_libraries(MyProject PRIVATE 6502_Library)

Method 2: Manually Adding the Library

Download the .zip source code from the releases this can then be dropped into a /external or /include folder within your project

You should then add these lines to your CMakeLists.txt, replacing external with your external libraries folder

add_subdirectory(external/6502Library)
target_link_libraries(MyProject PRIVATE 6502_Library)

How to setup the emulator

An instance of the CPU and Memory are required! \

The first step is loading memory

To load memory you can use a few different functions

Hard coding memory

memory.initMemory(); // This will set the entire 65535 bytes of memory to 0x00

memory.setMemory(0xEA); // You can put any value as the argument and the entire 65535 bytes of memory will be set to that value

If using either of the above memory inits, the values in memory will need to be specified in a way similar to below

    memory.data[0x8000] = 0xA9;
    memory.data[0x8001] = 0x42;
    memory.data[0x8002] = 0x8D;
    memory.data[0x8003] = 0x00;
    memory.data[0x8004] = 0x60;

Loading from a file

memory.loadMemory("/path/to/file"); // You can specify the path to a .bin file which will be loaded into memory

memory.promptMemoryLoad(); // This will give you instructions to aid in memory setting within the console

You only need ONE of these lines

The second step is to reset the CPU

The cpu needs to go through its reset sequence and move to the sector of memory which stores the program. This address is specified by the reset vector (0xFFFC and 0xFFFD) and the start address of your program should be stored at these two values using little endian.

To reset the cpu you can use:

cpu.reset(memory);

The final step is executing code

To execute code the function

cpu.execute(x, memory);

can be used. The amount of clock cycles you wish to be performed should be passed in as x.

This number can be less than the total for the program, but cannot be more unless the memory is initialised to 0xEA

An Example

The code below shows a basic program for setting up the emulator.
The hardcoded program here takes the value 42, stores it in the A register. Then stores the A register at address 0x6000

// main.cpp
#include <emulator_6502.h>
using namespace emulator_6502;

int main() {
    CPU cpu;
    Memory memory;

    memory.setMemory(0xEA); // Set the memory to NOP

    // Set the reset vector values to an address
    memory.data[0xFFFC] = 0x00;
    memory.data[0xFFFD] = 0x80;

    cpu.reset(memory); // <- Resets the CPU and starts the program

    // Set other memory addresses here
    memory.data[0x8000] = 0xA9;
    memory.data[0x8001] = 0x42;
    memory.data[0x8002] = 0x8D;
    memory.data[0x8003] = 0x00;
    memory.data[0x8004] = 0x60;

    cpu.execute(6, memory); // <- Pass number of required clock cycles (6 here)

    return 0;
}