W65C02S Emulator

A complete W65C02S (CMOS 6502) processor emulator written in JavaScript with an interactive trainer/debugger interface.

🚀 Try it live!

🧪 Klaus Dormann 6502 Test Suite

• Full W65C02S instruction set - All 65C02 opcodes including CMOS-specific instructions
• Interactive trainer/debugger - Visual register inspector, memory viewer, single-step execution
• Browser-based - No installation required, runs entirely in the browser
• Validation tested - Passes Klaus Dormann's comprehensive 6502 functional test suite (~88M cycles)

Try the emulator immediately: https://inindev.github.io/65c02_emu/

1. Start the Web Server

sh http_server.sh

This starts a local HTTP server on port 8000.

2. Open the Trainer

Navigate to: http://localhost:8000/

├── index.html          # Main trainer/debugger interface
├── w65c02s.js          # W65C02S CPU emulator core
├── ram.js              # Memory implementation
├── display.js          # UI and debugger logic
├── kd_test/            # Klaus Dormann test harness
│   ├── README.md       # Test suite documentation
│   ├── index.html      # Test runner interface
│   ├── test_harness.js # Test execution engine
│   └── test_ui.js      # Test UI controller
└── test/               # Legacy test interface
    ├── index.html
    └── tests.js

The main interface (index.html) provides:

• View and edit all CPU registers (PC, A, X, Y, SP)
• Monitor processor flags (N, V, B, D, I, Z, C)
• Click any value to edit directly

• Hexadecimal memory dump
• Navigate to any address
• View disassembled instructions

• Step - Execute one instruction
• Run - Continuous execution
• Stop - Halt execution
• Reset - Reset CPU to initial state

• Enter 6502 assembly code or hex bytes
• Load programs into memory
• Execute from any address

This emulator has been validated against Klaus Dormann's comprehensive 6502 functional test suite, which exercises:

• All instruction opcodes and addressing modes
• Flag behavior (N, V, Z, C)
• Stack operations
• Interrupt handling

See kd_test/README.md for details on running the test suite.

Test Results:

• ✅ 6502 Functional Test: PASSED (87,947,229 cycles)
• ✅ 65C02 Extended Opcodes: PASSED

The emulator implements:

• All 17 addressing modes (absolute, zero page, indexed, indirect, etc.)
• Complete instruction set including 65C02 extensions
• Accurate flag behavior and timing
• Stack operations with proper push/pull semantics

Key classes:

• W65C02S - Main CPU emulator
• Register6502 - Register file (A, X, Y, PC, SP, flags)
• Flags6502 - Processor status register

• 64KB addressable memory space
• Read/write operations
• 16-bit word operations (little-endian)

• Real-time register updates
• Memory visualization
• Disassembly view
• Execution control

Works in all modern browsers with ES6 support:

• Chrome/Edge
• Firefox
• Safari

No build process required - pure JavaScript:

# Start local server
sh http_server.sh

# Or use Python's built-in server
python3 -m http.server 8000

• Architecture: 8-bit CMOS microprocessor
• Address Space: 64KB (16-bit addressing)
• Registers: A (accumulator), X, Y (index), PC (program counter), SP (stack pointer)
• Flags: N (negative), V (overflow), B (break), D (decimal), I (interrupt), Z (zero), C (carry)
• Stack: 256 bytes (0x0100-0x01FF), descending

• W65C02S Datasheet
• 6502 Flag Tutorial
• Klaus Dormann Test Suite

• ✅ PHP instruction - Now correctly sets B flag when pushing status to stack
• ✅ BRK instruction - Now properly pushes PC+2 and status, sets flags, jumps to IRQ vector

GNU General Public License v3.0

Copyright © 2018 John Clark

See LICENSE for full license text.

Bug reports and pull requests welcome! This emulator aims for cycle-accurate emulation of the W65C02S.

Always run the Klaus Dormann test suite after making changes:

# See kd_test/README.md for setup
cd kd_test
# Download test binaries (see README)
# Open http://localhost:8000/kd_test
# Load and run tests

• CPU Implementation: John Clark
• Test Suite: Klaus Dormann (6502_65C02_functional_tests)
• W65C02S Specifications: Western Design Center (Datasheet)