#cpu #simulator #catalyst


Cycle-accurate, exhaustively tested simulator of W65C02S microprocessor

3 releases

0.9.2 Sep 2, 2020
0.9.1 Aug 22, 2019
0.9.0 Jun 8, 2019

#62 in Simulation

Zlib license

1.5K SLoC

This crate is a cycle-accurate simulator for the WDC W65C02S, the most advanced direct descendent of that catalyst of the home computer revolution, the 6502.

This crate accurately simulates all bus signals of the W65C02S except RDY, SOB, and BE, which can all be simulated by outside code. It is written in such a way that the unused bus logic usually gets optimized out. Make sure LTO is enabled in your Cargo.toml for a tremendous speedup:

lto = true

This crate does not depend on any other libraries, including the standard library.

The W65C02S instruction set includes the original NMOS 6502 instructions, the additional instructions supported by all CMOS 6502s, the "Rockwell bit extensions" (BBRx/BBSx/RMBx/SMBx), and the WAI and STP instructions.

The accuracy of this simulation has been tested on the 65test test suite, which contains over 4500 tests. In every single test, the simulator's bus traffic is exactly the same as the real hardware—even down to the timing of IRQ and NMI signals. This means that this simulator is suitable for prototyping and simulation of real systems using the W65C02S processor, including systems based on the W65C134S MCU.

To use it, you will need an instance of W65C02S and an implementation of System. W65C02S simulates the CPU; System must simulate the hardware attached to the bus (memory, IO devices, et cetera).

use w65c02s::*;

pub fn main() {
    let mut system = HelloWorldSystem::new();
    let mut cpu = W65C02S::new();
    while cpu.get_state() != State::Stopped { cpu.step(&mut system); }

/// A simple system with 64K of RAM, along with an output-only "serial
/// port" mapped to $0000.
struct HelloWorldSystem {
    ram: [u8; 65536],

impl HelloWorldSystem {
    pub fn new() -> HelloWorldSystem {
        // initialize RAM with all 0xFFs
        let mut ram = [0xFF; 65536];
        // initialize the message
        ram[0x0001..0x000F].copy_from_slice(b"Hello World!\n\0");
        // initialize the program
            op::LDX_IMM, 0,     //   LDX #0
                                // loop:
            op::LDA_ZPX, 1,     //   LDA $01, X
            op::BNE, 1,         //   BNE +
            op::STP,            //   STP
            op::STA_ZP, 0,      // + STA $00
            op::INC_X,          //   INX
            op::BRA, 0xF6,      //   BRA loop
        // initialize the reset vector to point to $0200
        ram[0xFFFC..0xFFFE].copy_from_slice(&[0x00, 0x02]);
        HelloWorldSystem { ram }

impl System for HelloWorldSystem {
    fn read(&mut self, _cpu: &mut W65C02S, addr: u16) -> u8 {
        // all reads return RAM values directly
        self.ram[addr as usize]
    fn write(&mut self, _cpu: &mut W65C02S, addr: u16, value: u8) {
        if addr == 0 {
            // writing address $0000 outputs on an ASCII-only "serial port"
            print!("{}", String::from_utf8_lossy(&[value]));
        else {
            // all other writes write to RAM
            self.ram[addr as usize] = value

This simulator is based on the simulator in the original ARS Emulator.


w65c02s is distributed under the zlib license. The complete text is as follows:

Copyright (c) 2019, Solra Bizna

This software is provided "as-is", without any express or implied warranty. In no event will the author be held liable for any damages arising from the use of this software.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgement in the product documentation would be appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.

No runtime deps