rustycoat

Rustycoat is a fun little project to create a 6502 emulator in Rust.

The 6502 was a revolutionary 8-bit microprocessor - its simple and inexpensive design made it practical to build low-cost personal computers, and together with the Z80, the 6502 really kickstarted the home computer movement. The 6502, or variants of it, were used in Apple, Atari, Commodore and Nintendo machines.

The core CPU emulator (found in c6502.rs) is pretty much complete, and timing-accurate with the 6502 chip. Tests of the instruction set are found in c6502_tests.rs.

Along with the CPU, there are software equivalents of a few components: a clock with adjustable frequency, a memory implementation with customizable banks (compatible with the 6502's bank-switching memory architecture), a set of gates, and even some "UI", in the form of LEDs built on top of the iui crate.

The programming model is designed to make it easy to hook up and run the CPU with a clock, memory, and some set of components - much like one would with a breadboard. Here's a trivial example that wires up a clock directly to an LED:

    // Create an LED
    let mut led = Led::new(Color::new(1.0, 0.0, 0.0), Color::new(0.4, 0.4, 0.4));

    // Create a 5Hz clock and wire it up to the LED.
    let mut clock = Clock::new(5);
    clock.output().connect_to(led.input());

    // Create the computer, add components, and start it up.
    let mut c = Computer::new();
    c.add_async(clock);
    c.add_ui(led);

    c.run();

An actual CPU example can be found in the examples directory.

This was mostly just a fun project to go down memory lane and learn a bit of Rust while doing it. At some point, maybe it could be grown into an actual computer emulator.

References

While working on Rustycoat, I came across a lot of great materials on the 6502.

• A good summary of what the 6502 does on each cycle of an instruction.

• The best cycle-by-cycle guide of a 6502's instruction set.

• 6502 instruction set with length and timings of each instruction.

• Another instruction set guide, with better formatting.

• A great description of the 6502's overflow flag.

• A description of how interrupts work on a 6502.

• The original 6502 datasheet, helpful to understand the hardware and pin layouts.

• A fun list of bugs on the 6502 and its variants.

• Another list of 6502 bugs and implementation quirks.

• An explanation of when the status register on a 6502 is updated.

• A great guide on how comparison instructions on a 6502 work.

• A guide to the C64 memory layout.

• A guide to how the C64 keyboard works.