Emulating consoles on modern PCs is a complicated business. There are significant low-level differences between the architecture of a console and that of a PC. While some of these issues can be brute-forced by raw computational horsepower, others heavily resist this kind of solution. Dolphin, the popular emulator for GameCube and Wii titles, recently released a major update to its own source code that finally enables emulation on the last title standing: Star Wars: The Clone Wars.
The team published a blog post detailing how it accomplished this task. It’s an interesting example of how the original developer’s decision to take advantage of some rarely-used GameCube capabilities prevented their game from being emulated for over a decade. In this case, the problem boiled down to the GC’s memory management unit, or MMU.
The MMU inside the GameCube provides games with virtual memory addresses that it maps internally to physical memory locations. The MMU is capable of providing either a Block Address Translation (BAT), which remaps a large chunk of memory, or with finer-grained page tables for small amounts of memory. The GameCube used an MMU in the first place because it allowed the CPU to cache memory accesses, and because there’s only 24MB of RAM in the system, despite having a 4GB maximum addressable space. The diagram below shows how MMUs work, in general terms.
Dolphin was already capable of emulating the GameCube’s MMU in several ways. Most games just use the default BAT mappings provided by Nintendo. Support for this mode is hard-coded into Dolphin. The emulated console doesn’t “know” that its data requests are being filled from a very different memory subsystem than the one the GameCube shipped with. In this mode, the MMU isn’t actually doing any address translation — it knows where the GameCube expects to find memory based on the address passed to it, and it retrieves the corresponding block of information from the PC’s RAM.
The second form of emulation that Dolphin supports is predictable invalid memory requests. The GameCube included 16MB of RAM that was mapped to the audio DSP, but many games use this pool for additional memory. Provided they do so in predictable fashion, Dolphin can grab those addresses (even though they’re invalid) and hand the game the data its requesting — provided it always requests information from the same set of invalid memory addresses.
But what happens when the addresses are both invalid and unpredictable? In the past, this killed game performance. Actually emulating the MMU takes thousands of CPU cycles, and the pace of improvement in CPU clock speed has all but stopped. The GameCube CPU might have only run at 485MHz, but our modern chips are just 10x faster in terms of raw clock speed — and there are functions that simply can’t be emulated effectively when doing so costs thousands of CPU cycles.
Memchecks are the core of what Enable MMU does, and it’s the key reason why MMU Enabled titles have been so slow in Dolphin. There are cases that require falling back to the interpreter, it doesn’t work with fastmem, and they’re even slower than normal memory accesses on console!
Recent code contributions by Fiora boosted performance in one MMU title (Star Wars: Rogue Squadron) by 10x and in most others by an average of 100%, but that wasn’t enough to solve the problem of Star Wars: The Clone Wars. This particular title actually defined its own BAT — the only game known to exist that did so. Because Dolphin fundamentally relied on the idea of a hardcoded BAT, there was no way to address this edge case without rewriting the entire memory emulator subsystem, and possibly not without ruining performance in a great many other games in the process.
One of the Dolphin contributors, magumagu, evidently came up with a way to rewrite Dolphin to support The Clone Wars’ custom BAT mapping without compromising performance in any other title. According to the Dolphin team, previous attempts to support the game had the side effect of reducing performance in other games by up to 30%. While the team has taken an 8-15% hit in other MMU titles, it believes it can recover this loss through further tweaks and improvements. Other games should actually see improved performance and fewer bugs as a result of the rewrite, so the gains should be well worth it in the longer term.
Console emulation has actually moved a bit more mainstream these days, thanks to high profile efforts from Microsoft to emulate the Xbox 360 on the Xbox One. Even Sony has dipped its toe in the waters with PS2 emulation on the PlayStation 4. Nintendo’s upcoming console, the NX, reportedly won’t be backwards-compatible — which could spur interest in emulating that platform as well.