A custom virtual machine for the Stars! 4X game

The signed releases on GitHub embed a compressed copy of the original 16-bit game, so that single EXE is ready to play out-of-the-box with no further setup or downloads. I’m distributing 32-bit builds (but requires SSE2) because there’s no advantage to 64-bit here, and these builds work (almost) everywhere except 16-bit Windows. 32-bit Windows could run the original 16-bit game, but the VM-encapsulated version is better behaved. It doesn’t dump a Stars.ini under C:\WINDOWS, it interacts properly with the task bar, and copy protection is neutralized via the OS bridge.

If you ever been curious about Stars!, now’s the time to try it. The game has a thorough, built-in tutorial, but also check out the wiki, the official strategy guide, and AutoHost (play-by-email service). The game predates the modern search engine concept, otherwise they might have chosen a better name. I suggest using “stars 4x” in your searches.

If you want to build from source and hack on the VM yourself, the best tool for the job is w64devkit, of course, because it comes with everything you’ll need. Plus the game itself: stars.exe from stars27jrc3.zip.

Implementation details

The emulator itself requires x86 or x86-64 because it does not implement x87 (80-bit floating point) in software, but instead runs these operation directly on the host’s x87 hardware. This is simple, fast, and precise. The project validates the emulation as a whole with a differential fuzzer against the host. The fuzzer randomly generates a 16-bit instruction, emulates it, then runs it with JIT on the host and compares the results.

Handles on Windows are pointer-sized, and so the Win16-to-Win32 bridge maps 16-bit handles to host handles. It marshals between different struct layouts when translating these calls, services the DOS interrupts the game requires, an copies data in and out of guest memory. It’s rather like running a Wasm instance, which of course makes sense in retrospect.

The Win32 bridge is also monitorable and manipulatable using the Model Context Protocol (MCP). AI agents can “see” the UI “DOM” as it’s built, and can drive it by injecting synthetic events into the event pump, all without going through the usual desktop control. The MCP can also read and write guest memory. Opus 5 played a complete game through MCP — which is quite fun to watch — requesting my assistance at just two points when it got stuck in the UI. A foundation for a new Stars!Bench?

Targeting old 16-bit computers, the authors couldn’t afford to build a sloppy, wasteful UI, and so by modern standards the game UI is remarkably fast and responsive. They don’t make ‘em like they used to. Computing the next turn, or “turn generation,” is the computational bottleneck, and so that’s where I focused my optimization efforts. The emulator can trace executed instructions, so I gathered traces of turn generation, then had Fable 5.1 identify and reverse engineer the hottest common routines (e.g. the game’s L’Ecuyer MCG PRNG) and basic blocks. Each was re-written in C and mapped into the instruction decoder as new 80286 instructions. On load the emulator identifies these routines and patches them with the new instruction. This resulted in a nearly ~2x speedup of turn generation. By exploiting local conditions, emulating a particular known program, I get JIT performance without JIT complexity.

The original game doesn’t use buffered I/O, and instead issues many small reads and writes. Passing these small reads/writes straight to Win32 made I/O take ~5% turn generation time, probably worse today than it was back then. Plus it’s just rude. The emulator buffers the game’s I/O calls, further speeding up turn generation.

The game has some sound effects in the “battle VCR” and the final version of the game shipped with Microsoft’s WaveMix.dll. Rather than load and link this DLL, the emulator implements the DLL’s interfaces natively, and these routines are dynamically linked into the 16-bit process. You will not need this DLL with the emulator, nor is it embedded in releases.

The game also has art assets embedded uncompressed in the original EXE, forming the bulk of its ~3MB. Stars!VM uses a custom LZ-based compression algorithm tailored to compressing the original game. It’s compressed when embedded in releases. So the 32-bit version of the game is half the size of the original, at ~1.5MB.

The original game requires a serial code, serving as its copy protection. A code is 8 alpha-numeric characters that must pass two checks. Failing the first is loud, but failing the second will sabotage your game with penalties. You’ll know because it will announce that your people suspect you are a usurper. Most codes you’ll find online are such “usurper” codes. Play-by-email (PBEM) saves embed a hardware signature derived from C and D drive configuration. People playing on different machines using the same serial code recieve the usurper penalty.

I bought a serial code back in the day, but it hasn’t been possible to purchase one a for at least decade now. So the emulator injects a fixed serial code on first run (disable with --prompt-serial), and you won’t need to worry about it. The VM also produces a fixed hardware signature (same as any other emulator), so it looks like everyone running Stars!VM is sharing the a machine, meaning no penalty for key reuse. I cracked the serial code checks anyway, allowing me discover interesting ones. My favorites: CLONEMUM, CROSSNUT, EGGSWAIN, GHOSTKIN, GONKSHOW, GRIPMIME, SEEKCAPS, SIFTBOLD, SIRBUOYS, SLIMFAZE, SLOTMOPS, SPAWNELK, SUNBLOND, WANTNEAR, and WRONGPOX. These look like some of my passwords.

Endless possibilities

I’m quite pleased and excited with the results, especially for a weekend project. It’s breathed life back into the game for me, not only having a better experience running it, but also that I can trivially bend the game to my will in the ways I dreamed about. A few hooks in the right places should open the game to easy modding, but I’m more engineer than modder.