Hi everyone,

I've ported my Game Boy (DMG) emulator from B4J to Objo, and I'd like to share the source with the community. It's been a great way to get to know Objo: an emulator touches almost every part of a language, from integer arithmetic and MemoryBlocks to timers, canvas drawing, menus, file I/O and interfaces.

Test results

Both standard test suites pass in the built Objo app:

Blargg's cpu_instrs: all 11 CPU tests pass, including the combined ROM.
dmg-acid2: the graphics test renders perfectly.

Features

A complete Sharp LR35902 CPU core, including interrupts, HALT behaviour and the EI delay.
A scanline PPU with background, window and sprites (10 per line, DMG priority rules, 8×16 sprites, flipping), and STAT/LYC interrupts.
MBC1, MBC2, MBC3 and MBC5 cartridges.
Battery saves: in-game saves are stored as .sav files next to the ROM and saved automatically.
Save states: one quick slot per game, with checks so a state from another game or a damaged file is never loaded.
Reset, pause, and a menu bar with keyboard shortcuts.
A scalable window that keeps the aspect ratio.
A headless command-line test harness that runs test ROMs and reports instructions per second.

Controls: arrow keys for the D-pad, X = A, Z = B, A = Select, S or Return = Start.

How it's built

The UI talks to the emulator only through a small EmulatorCore interface: load ROM, run a frame, get the frame, set buttons, save/load state. ObjoCore implements it in pure Objo. When the C# plugin SDK arrives, a native core can implement the same interface and the whole UI stays as it is.

A few techniques that may be useful for your own projects:

Frames: the PPU writes each frame straight into an 8-bit BMP in a MemoryBlock, so Picture.FromBytes can display it directly. The BMP's own palette handles the Game Boy colour palettes for free.
Drawing: background and window tiles are drawn with one MemoryBlock.CopyTo per tile row, instead of pixel by pixel.
Arithmetic: for byte and word values, Mod, __ and * give the same results as BitAnd/ShiftLeft, so I use them in the hot paths.

Performance

On my M3 Pro, a built app runs the CPU core at about 250,000 instructions per second headless. With graphics, games run at around 23 fps, roughly 40% of real Game Boy speed. Games are playable and fun to watch, and it makes a nice benchmark to follow as Objo evolves. I'm looking forward to trying the core as a C# plugin once the SDK is released, and to seeing how the pure-Objo version develops with future Objo releases.

Planned, if I have time

Tools windows ported from the B4J version: sprite viewer, cheats and memory debugger.
Audio, once there's a way to stream sound samples.

Feedback, bug reports and improvements are very welcome, especially from anyone who knows Objo's performance characteristics better than I do.

Thanks to Garry and team for a fun new language to explore!

gameboy.objo

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