An MCP server for developing BASIC and Assembly Language programs for the Commodore 64

• Optimal C64 development stack
  • Emulator: VICE
  • Assembler: 64tass
  • Tokenizer: petcat
• REPL - A minimal command line client for interacting directly with the MCP server.
• MCP Server working with the ability to:
  • Assemble 6502 assembly language programs to .prg format
  • Tokenize BASIC programs to .prg format
  • Launch and run .prg format programs in the emulator

REQUIREMENTS

Install

• Node + npm - Javascript runtime environment

  • Install the latest stable Node release (comes with npm).

• VICE - The Versatile Commodore Emulator

  • Read the Manual
  • Download binary for your platform
  • Includes emulator and tokenizer/detokenizer (petcat)

• 64Tass - The 6502 assembler for C64

  • Read the Manual
  • Install
    • MacOSX - brew install tass64
    • Windows - Download the latest binary

Configure

Create .env file

• Create file .env in the root of your project.
  • If you have downloaded the project source, you can copy .env-example
• Add / modify vars
  • ASSEMBLER - pointing to the 64tass assembler executable (full path if not in PATH)
  • TOKENIZER - pointing to the petcat tokenizer executable (full path if not in PATH)
  • EMULATOR - pointing to the x64sc emulator executable (full path if not in PATH)
  • ASM_PATH - pointing to your assembly source folder
  • BASIC_PATH- pointing to your basic source folder

Example .env file

ASSEMBLER=64tass
TOKENIZER=/Users/zaphod/vice/bin/petcat
EMULATOR=/Users/zaphod/vice/x64sc.app/Contents/MacOS/x64sc

ASM_PATH=./asm/hello
BASIC_PATH=./basic/token

Example mcp.json file for Cursor

{
  "mcpServers": {
    "mcp-c64": {
      "command": "npx",
      "args": ["-y", "mcp-c64"],
      "env": {
        "ASSEMBLER": "64tass",
        "TOKENIZER": "/Users/zaphod/vice/bin/petcat",
        "EMULATOR": "/Users/zaphod/vice/x64sc.app/Contents/MacOS/x64sc",
        "ASM_PATH": "/Users/zaphod/Projects/retro-game/asm/",
        "BASIC_PATH": "/Users/zaphod/Projects/retro-game/basic/"
      }
    }
  }
}

Example mcp.json file for VSCode

{
  "mcp": {
	"servers": {
      "mcp-c64": {
        "type": "stdio",
		"command": "npx",
		"args": [
		  "-y",
		  "mcp-c64"
		],
		"env": {
		  "ASSEMBLER": "64tass",
		  "TOKENIZER": "/Users/zaphod/vice/bin/petcat",
		  "EMULATOR": "/Users/zaphod/vice/x64sc.app/Contents/MacOS/x64sc",
		  "ASM_PATH": "/Users/zaphod/Projects/retro-game/asm/",
		  "BASIC_PATH": "/Users/zaphod/Projects/retro-game/basic/"
		}
	  }
	}
  }
}

Example configuration for Github Copilot

{
    "servers": {
         "mcp-c64": {
             "type": "stdio",
             "command": "npx",
             "args": ["-y", "mcp-c64"],
             "env": {
                 "ASSEMBLER": "64tass",
                 "TOKENIZER": "/Users/zaphod/vice/bin/petcat",
                 "EMULATOR": "/Users/zaphod/vice/x64sc.app/Contents/MacOS/x64sc",
                 "ASM_PATH": "/Users/zaphod/Projects/retro-game/asm/",
                 "BASIC_PATH": "/Users/zaphod/Projects/retro-game/basic/"
             }
         }
    }
}

The REPL

A minimal MCP Client which allows you to interact with the mcp-c64 server. It doesn't invoke LLMs but rather allows you to invoke the tools as an LLM would.

Running the REPL

• Invoke with npm run repl
• You will see a menu of available commands.

MCP Interactive Client
=====================
Connected to mcp-c64 server via STDIO transport

Available commands:
  help                       - Show this help
  list-tools                 - List available tools
  call-tool <name> [args]    - Call a tool with optional JSON arguments
  quit                       - Exit the program

> 

The list-tools command

• Lists the available tools in the MCP server.
• These are the tools that the LLM could call, and the descriptions it would see

> list-tools

Available tools:
  - assemble_program: Assemble an assembly language program.
        Only the .asm source filename is required in file parameter, output .prg and .map files will be generated.
        Any additional args such as -a, -cbm-prg, etc. should be supplied in an array in the args parameter.
  - tokenize_program: Tokenize a BASIC program.
        Only the .bas source filename is required in file parameter, output .prg file will be generated.
        Any additional args such as -w2, etc. should be supplied in an array in the args parameter.
  - run_program: Run an assembled or tokenized program.
        The .prg filename is required in file parameter, and the program's path is required in path parameter.
        Any additional args such as -console, etc. should be supplied in an array in the args parameter.
        
>

The call-tool command

• Allows you to call a tool on the MCP server just as the LLM would
• You pass the tool name and its arguments as a JSON string

assemble_program tool

• Uses 64tass to assemble a 6502 assembly language program
• To target the Commodore 64 using Unicode or standard ASCII input, include the -a and -cbm-prg arguments
• Refer to the 64tass command line options reference for more info on configuring the assembler behaviors

> call-tool assemble_program {"file":"hello_world.asm", "args": ["-a", "--cbm-prg"]}
Calling tool 'assemble_program' with args: { file: 'hello_world.asm', args: [ '-a', '--cbm-prg' ] }
Tool result:
{
  output: '64tass Turbo Assembler Macro V1.60.3243\n' +
    '64TASS comes with ABSOLUTELY NO WARRANTY; This is free software, and you\n' +
    'are welcome to redistribute it under certain conditions; See LICENSE!\n' +
    '\n' +
    'Assembling file:   asm/hello/hello_world.asm\n' +
    'Output file:       asm/hello/hello_world.prg\n' +
    'Passes:            2\n',
  error: '',
  status: 0
}

>

tokenize_program tool

• Uses the petcat command included with the VICE emulator
• To tokenize a Unicode or standard ASCII file for the Commodore 64, include the -w2 argument
• See the petcat command line options reference for more info on configuring the tokenizer input and output behavior
• See the C64-Wiki for PETSCII control characters that can be used in BASIC strings for such things as cursor movement and character color

> call-tool tokenize_program {"file":"token_test.bas", "args": ["-w2","-v"]}}
Calling tool 'tokenize_program' with args: { file: 'token_test.bas', args: [ '-w2', '-v' ] }
Tool result:
{
  output: '/Users/zaphod/Projects/retro-game\n',
  error: '\nLoad address 0801\nControl code set: enabled\n\n',
  status: 0
}

> 

run_program tool

• Uses the x64sc executable included with the VICE emulator package
• To take a screenshot on exit, include the -exitscreenshot argument followed by a file name argument
• See the x64sc command line options reference for more info on configuring the emulator behavior

> call-tool run_program {"file":"token_test.prg", "path":"/Users/zaphod/Projects/retro-game/basic/token", "args": ["-exitscreenshot","token_test.png"]}
Calling tool 'run_program' with args: {
  file: 'token_test.prg',
  path: '/Users/zaphod/Projects/retro-game/basic/token',
  args: [ '-exitscreenshot', 'token_test.png' ]
}
Tool result:
{ output: '', error: '', status: 0 }

>