A BASIC you can change while it runs, by hand or through an AI agent.
Claude Code pauses a running game over MCP, rewrites one SUB and lets the game continue with its score and enemies unchanged. Video, 73 seconds.
jdBasic is a BASIC interpreter with a bytecode VM and APL-style arrays.
Graphics, GUI, sound, web and local AI are built in. Programs can be compiled
to a native .exe, and an MCP server lets an AI agent inspect and change a
program without restarting it.
Try it in the browser · Download · Take the tour
Press F6 in the running game and the VM pauses. Ask Claude to turn the ship red
and set the lives to 100, and it changes two variables through jdb_eval. Ask
it to move the shield bar 20 pixels up, and it edits the source; jdb_recompile
then swaps the changed SUB into the paused program and restores the program
pointer. After resuming, the game continues where it stopped with the new
drawing code. Any MCP client works, for example Claude Code, Cursor, Cline or Zed.
MCP setup and tool reference · Blog post about it
N = 1000000
X = RND(IOTA(N)) * 2 - 1
Y = RND(IOTA(N)) * 2 - 1
PRINT 4 * SUM(X*X + Y*Y <= 1) / N ' about 3.14
Operators and functions work on whole arrays, as in APL: IOTA, SCAN,
OUTER, GRADE, matrices, SVD, FFT. See From loops to array pipelines.
DIM frm = FORM.CREATE("Hello", 320, 200, "MAIN")
DIM btn = FORM.BUTTON(frm, "btnGo", "&Go", 110, 80, 100, 28)
SUB BTNGO_CLICK(e)
MSGBOX("Hello from jdBasic.", 64, "Hello")
ENDSUB
FORM.RUN(frm)jdbasic -c hello.jdb compiles this to hello.exe. Events are bound by name.
The controls are Win32 controls, including menus, toolbars, list views, tabs
and MDI windows, and the VS Code extension has a visual form designer.
jdBasic OS is compiled with --target=kernel and boots on bare x86-64 without
libc or a host operating system. The screen driver, keyboard handling, editor,
RAM disk, an interpreter and a JIT for x86-64 are all written in jdBasic. Its
prompt runs a small C-like language: ? interprets a line, ?? compiles it to
machine code and runs it, and COMP compiles a whole program (Pong in the
screenshot) into 3006 bytes of x86. See Bare metal.
The jdBasic interpreter also runs on microcontrollers, on an RP2350 (PicoCalc) and on an ESP32-S3 with a touch display. See On a board.
- In the browser: jdbasic.org/live, no installation needed.
- On Windows: download a bundle from Releases, unzip it and run
jdBasic.exe. The VB6 pack covers most features and includes the demos; mcp-native is the bundle for working with an AI agent. - On Linux and macOS: build from source as described in doc/BUILD.md.
? PRINT "Hello, jdBasic!"
Hello, jdBasic!
? PRINT SUM(IOTA(100))
5050
? HELP "SORT"First run on Windows: the binaries are code-signed, but the certificate is new, so SmartScreen may show "Windows protected your PC". SmartScreen shows this for certificates without a download history; no malware was found. Click "More info", then "Run anyway", or tick Unblock in the properties of the
.zipbefore extracting it.
After that, the tour goes from the first PRINT to a compiled
.exe in about fifteen minutes.
| ... | Start here |
|---|---|
| learn the language | The tour, then the video lessons |
| look something up | Language reference (contents at the top, A-Z index at the end) or HELP "name" in the REPL |
| program together with an AI agent | Programming with an AI agent, MCP server reference |
| write games and graphics | Graphics and games, sample gallery |
| build desktop tools | Desktop applications: native Windows forms, ImGui or the terminal |
| turn a script into an .exe | Compiling to a native .exe |
| work with files, tables and dates | Working with data, array pipelines, vector and matrix cookbook |
| build a web app or an API | Web development |
| make music or process audio | Sequencer, Audio FX, FX how-to |
| run local LLMs and ML models | AI inside your program, AI and machine learning |
| script Godot 4 | The Godot embed |
| run it on a microcontroller | jdBasic on a microcontroller, ESP32 bring-up |
| reuse ready-made modules | Module library: 45 modules, from TESTKIT to XLSX |
| switch from Python | Idioms from Python |
| build jdBasic or contribute | Building from source, CONTRIBUTING.md |
All documentation, grouped by topic: doc/README.md.
An Apple II emulator, a Godot RPG with LLM characters, a spreadsheet, a music sequencer, Minesweeper in the terminal and the scoring app of a garden championship, all written in jdBasic. The sample gallery has more than 250 other programs.
Feature list
- Bytecode compiler and VM with inline caches, opcode fusion and a refcounted value type
- APL-style vectorization:
SIN,+,*, scatter/gather,IOTA,REDUCE,SCAN,FILTERandSELECTwork on whole arrays - Linear algebra and DSP based on Eigen:
SVD,QR,DET,EIG,FFT/IFFT - Native compiler via LLVM:
jdbasic -c program.jdbwrites a standalone.exe - SDL3 graphics with a batch plotter that draws 70 000 coloured pixels per frame at more than 30 FPS (
universe.jdb) - Dear ImGui for immediate-mode tools, native Win32 forms with a visual designer
- Music sequencer (
SOUND.*) and an effect chain for guitar and synth - llama.cpp for local LLMs, ONNX Runtime for ML models
- HTTP/HTTPS client and server, COM automation, serial I/O, SQLite
- Reactive variables (
->), hot reload, a persistent workspace (SAVEWS/LOADWS) - DAP debug adapter and a VS Code extension with lint, hover and the form designer
- MCP server (
jdbasic --mcp) for Claude Code, Cursor, Cline and other MCP clients - Godot 4 embed: a
.jdbfile is a Godot script, andGDX.*reaches nodes, physics, 3D and audio (experimental;godot/has four projects) - Microcontroller ports of the interpreter (RP2350, ESP32-S3), and a kernel target (
--target=kernel) that compiles jdBasic into a freestanding x86-64 operating system - A library of 45 modules written in jdBasic
The original v1 codebase is kept on the legacy-v1 branch.
The 14 video lessons
The Train jdBasic series goes from "Hello, World" to native compilation in
episodes of 5 to 10 minutes. The videos are produced by a jdBasic program that
handles voice, screen recording, FFmpeg and the upload, see jdb/tv/.
| # | Lesson | Topic |
|---|---|---|
| 01 | Hello jdBasic | PRINT, DIM, basic types |
| 02 | If and For | IF/ELSE, FOR/NEXT, FizzBuzz |
| 03 | Arrays | Vector ops, broadcasting, reductions |
| 04 | Strings | Slice, search, SPLIT |
| 05 | Functions and SUBs | FUNC, SUB, recursion |
| 06 | Maps | Key-value data |
| 07 | INPUT and DO Loops | User-driven programs |
| 08 | File I/O | TXTWRITER, TXTREADER$ |
| 09 | Graphics | SCREEN, shapes, colours |
| 10 | Modules | EXPORT, IMPORT, code reuse |
| 11 | REPL Workflow | PRETTY, LINT, SAVEWS |
| 12 | Higher-Order Functions | SELECT, FILTER, REDUCE, lambdas |
| 13 | HTTP and JSON | Talk to the web |
| 14 | Native Compilation | Compile to .exe with jdbasic -c |
Project layout
src/ interpreter, compiler, VM (vm.cpp + vm_builtins_*.cpp), runtime modules
lib/ the module library, written in jdBasic (lib/README.md)
jdb/ example programs, start at jdb/README.md
doc/ documentation (doc/README.md), screenshots in doc/img/
tests/ regression bank; tests/gate/ holds the pre-commit gate
embedded/ microcontroller ports (RP2350/PicoCalc, ESP32-S3) and bare metal
embed/ the Godot 4 GDExtension; godot/ has the Godot projects
selfhost/ jdbc, a jdBasic compiler written in jdBasic
wasm/ the browser build behind jdbasic.org/live
fluppi/ "Vallys Reise", a top-down RPG
tools/ helper scripts (doc index, doc examples, syntax highlighting, ...)
bridges/ optional native bridges; modules/ modules IMPORT finds by name
vscode_extension/ the VS Code extension (.vsix)
libs/ vendored third-party sources; SDL3, LLVM and the rest per doc/BUILD.md
Bug reports, ideas and code are welcome. CONTRIBUTING.md
describes the build, the conventions and the pre-commit gate: four suites that
must report 0 failed in the interpreter and, for compiler changes, also as
native programs. The test bank is described in tests/README.md.
MIT, see LICENSE.txt. Third-party components are listed in THIRD_PARTY_LICENSES.txt.