A simple NMOS 6502 emulator.
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Yet another MOS 6502 emulator.

Designed to support no-std and not require an allocator nor any unsafe code, and be reasonably fast.

The API of this crate shies away from implementing interrupt handling,
instead having you step the emulator one opcode at a time and handle them yourself.

Note that this does not emulate cycle-by-cycle, and as such may not be 100% accurate.

Feature Flags

The following feature flags exist:

Name Description
bcd Enable binary-coded decimal arithmetic.
Enabled by default. Disable if you're writing a NES emulator.
Note that invalid BCD is left untested and will not function faithfully to the MOS 6502.
bytemuck Enables bytemuck support.
arbitrary Enables arbitrary support. This will pull in std.
serde Enables serde support.


extern crate std;

use std::eprintln;

use r6502::{Emulator, FunctionReadCallback, FunctionWriteCallback};

fn main() {
    let mut emu = Emulator::default()
        .with_read_callback(FunctionReadCallback(|state: &mut State, addr| {
            // Log reads  
            eprintln!("Read from #${addr:04x}");
            state.memory[addr as usize]
        .with_write_callback(FunctionWriteCallback(|state: &mut State, addr, byte|
            // Don't write to ROM 
            if addr < 0xFF00 {
                state.memory[addr as usize] = byte

    loop {
        let interrupt_requested = emu.step()
            .expect("found an invalid opcode (only MOS 6502 opcodes are supported)");
        if interrupt_requested { // Go to IRQ interrupt vector 
            let vector = u16::from_le_bytes([
            emu.state.program_counter = vector;


This may be licensed under either the MIT or Apache-2.0 license, at your option.