Pandowdy.Cpu 2.0.0

Pandowdy.CPU Emulator

A cycle-accurate 6502/65C02 CPU emulator written in C# for .NET 8.

Features

• Cycle-Accurate Emulation — Micro-op pipeline architecture provides true cycle-level timing
• Stateless Core — CPU execution engine is stateless; all state lives in CpuStateBuffer, enabling save states, multiple instances, and easy testing
• Multiple CPU Variants — NMOS 6502, 65C02, and Rockwell 65C02 with bit manipulation instructions
• Double-Buffered State — Clean instruction boundaries for debugging, single-stepping, and state comparison
• Interrupt Support — IRQ, NMI, and Reset with proper priority handling
• Extensible Bus Interface — Simple IPandowdyCpuBus interface for custom memory maps and I/O
• Pure C# — Single NuGet package with no F# dependencies

Validation

All CPU variants pass Klaus Dormann's 6502/65C02 Functional Tests, a standard test suite for 6502 emulator validation:

Cycle-Accurate Validation

All CPU variants are cycle-accurate and pass the Tom Harte SingleStepTests, which validate not only final register state but also cycle-by-cycle bus activity for every opcode:

See Pandowdy.Cpu.Harte-SST-Tests/README.md for instructions on running these tests.

Supported CPU Variants

Quick Start

using Pandowdy.Cpu;

// Create memory bus and CPU state
var bus = new RamBus();
var cpuBuffer = new CpuStateBuffer();

// Load program and set reset vector
byte[] program = { 0xA9, 0x42, 0x8D, 0x00, 0x02 }; // LDA #$42, STA $0200
bus.LoadProgram(0x0400, program);
bus.SetResetVector(0x0400);

// Reset and execute
Cpu.Reset(cpuBuffer, bus);
int cycles = Cpu.Step(CpuVariant.WDC65C02, cpuBuffer, bus);

Console.WriteLine($"A = ${cpuBuffer.Current.A:X2}"); // A = $42

Project Structure

Pandowdy.Cpu/              # Core library (CPU, MicroOps, Pipelines, CpuState, CpuStateBuffer)
Pandowdy.Cpu.Tests/        # xUnit test suite (2,149 tests)
Pandowdy.Cpu.Dormann-Tests/ # Klaus Dormann functional test runner
Pandowdy.Cpu.Harte-SST-Tests/ # Tom Harte SingleStepTests runner

Installation

dotnet add package Pandowdy.Cpu

Documentation

• CPU Usage Guide — Detailed usage instructions and examples
• API Reference — Complete API documentation for Cpu, CpuState, and CpuStateBuffer
• Building — Build instructions, running tests, and project structure

Quick Build

dotnet build
dotnet test

For detailed build instructions, external test suites, and troubleshooting, see BUILDING.md.

License

Licensed under the Apache License, Version 2.0. See LICENSE for details.

Author

Copyright 2026 Mark D. Long