jNESb is a from-scratch Nintendo Entertainment System emulator written in Java. It faithfully re-creates the CPU (6502), PPU (including sprite pipeline and dynamic mirroring), APU (pulse/triangle voices) and I/O bus so commercial ROMs such as Donkey Kong or Adventures of Lolo can be executed without modification.
- CPU/Bus: Cycle-accurate 6502 core and NES memory map with mapper plug-ins (Mapper 0 & 1 supported).
- PPU: Background and sprite rendering, sprite-zero hits, mirroring updates originating from MMC1 writes, OAM DMA ($4014) support and palette management.
- Audio: Pulse + triangle channels mixed to a 44.1 kHz PCM stream played via Java Sound.
- Input: Standard NES gamepad emulation (keys:
Z=B,X=A,A=Select,S=Start, arrow keys for D-pad). - CLI & UI: Headless frame runner for automated checks plus a JavaFX desktop window to see and hear the emulator in real time.
- JDK 21 or newer (the project targets release level 25 via the Maven compiler plugin).
- Maven 3.9.11+.
mvn clean package
This produces target/jNESb-0.1.0-SNAPSHOT.jar containing the emulator classes.
java -cp target/jNESb-0.1.0-SNAPSHOT.jar org.jnesb.App <path-to-rom> [--headless] [--frames=N]
--headlesskeeps rendering disabled and exits after the requested number of frames (defaults to 1).--frames=Nlets you benchmark a ROM or confirm it boots without opening a window.
Launch the graphical emulator with audio and keyboard input:
mvn -q exec:java -Dexec.mainClass=org.jnesb.App -Dexec.args="<path-to-rom>"
Close the window (or press Ctrl+C in the terminal) to stop the emulation.
Run the automated CPU/PPU/APU/mappers test-suite:
mvn test
The suite includes unit tests, nestest trace validation, mapper smoke tests and APU frame counter checks.
- The emulator expects legal iNES ROM images. Mapper 1 titles (MMC1) rely on the dynamic mirroring callbacks implemented in the cartridge layer.
- Sprite DMA is triggered automatically when a ROM writes to
$4014; if you see misaligned sprites, ensure the ROM is authentic and not a hacked image.