New Core: PX68k (Android/iOS/Windows/Linux/Mac)

hizzk

Disclaimer: This article was written by Tatsuya79, who has also contributed many improvements to the X-68K core. Developer r-type is the one who made the port

The Sharp X68000 was a home computer released exclusively in Japan in 1987. It was a powerful machine for its time and saw a great number of arcade ports, exclusive titles and doujin (indie) games developed for it, even years after the last model was launched in 1993.

Until now the only way to run Sharp X68000 games in RetroArch was with MAME. Its driver isn’t really the most advanced one and it is quite demanding, excluding many platforms such as smartphones.

Outside Retroarch, PX68k was aimed to be fast enough for that usage. Based on Winx68k, targeting the PSP and ported to iOS and Android by its Japanese developer Hissorii, it was possibly the only X68000 emulator on those platforms. As its development stopped some years ago, compatibility issues due to OS upgrades made its usage rather complicated.

Developer R-Type decided to port it to RetroArch, replacing its old 32 bits based CPU emulation by a 64 bits one from Yabause core. There is also a back end for the cyclone cpu on arm/android but surprisingly it didn’t give any speed enhancement and had more problems than the previously mentioned c68k.

After a common effort to fix various issues resulting from this change (thanks Retro-Wertz), it should now be at the same level of compatibility as the original emulator.

Running some tests on an old Samsung Galaxy S3, where we could barely emulate a 16MHz CPU before with PX68k stand-alone, we now achieve smooth results with a 66MHz setting. This makes it 4 to 5 times faster than before, and the libretro port is probably now the best performing Sharp X68000 emulator you can get for various cheap or old devices.

Testing on an i5-3570K@4GHz with “Akazukin Cha Cha Cha” achieved upwards of 1000fps on the default 10MHz emulated CPU. The same test gives 136fps in RetroArch using the Mame core.

The PX68k-libretro core still keeps the same main limitation of the original: no MIDI emulation. We also need to bring a virtual keyboard back, you can only use real ones at the moment. However, we did make some improvements:

1.) You don’t need to load a particular utility to define the amount of RAM the machine uses any more, there’s now a core option for that.

2.) You can change the CPU speed in real time.

If, like some old DOS games behaved, you encounter one that runs too fast (ex. Arkanoid), you can directly slow down your CPU from a fast 25MHz to the 10MHz clock speed it was programmed for.

We also added some overclock steps as high as 200MHz. High frequencies have the side effect of speeding up the floppy loading time, which is a much welcomed accident on this machine. (100MHz is already a lot faster for that.)

-We made some 8 buttons gamepad profiles which weren’t used that much on the system, but are great for the various Street Fighters II iterations.

You’ll need the bios files, which have been made publicly available by Sharp. Place them in your system/BIOS directory, in a subdirectory named “keropi”. The iplrom.dat and cgrom.dat are necessary, but you do not need the sram.dat. See the core information for a complete list.

L2 button or F12 key brings up the original px68k menu where you can change the inserted disks. They have to be unzipped to be accessible from this menu but can be zipped/archived when launching directly from RetroArch.

After the first boot a “config” file will be generated in the “keropi” folder. You can enter your rom folder into the “StartDir” line to make it accessible from the PX68k-libretro core’s in-game menu.

SDL Libretro Proof of Concept

Libretro Team

The larger design goal behind Libretro is to make programs modular. To us, modularity means that a program should run from within the confines of a dynamic library, and that it should be possible for this program to then run inside any of the libretro-compatible players that exist out there.

In order to make a libretro core as it stands right now, you need to be familiar with how the API works. There is some obligatory documentation available for this purpose but we understand that API familiarity is still not where it should be, and that to some developers out there looking to get started with libretro, it might be intimidating to get started.

To that end, we are searching for ways to ease the difficulty and learning curve that comes with getting to grips with Libretro. We know that SDL for instance is already heavily used out there by game developers and emulator creators alike.

SDL and libretro cannot reasonably be compared. The entire purpose behind Libretro is to make a cohesive, consistent ecosystem of modular programs that, like a plugin, can be inserted into any frontend player that supports our API. Something like SDL is more generic in that it doesn’t really care what your program is going to be; it just acts as convenient middleware for your program so that you don’t have to write against a myriad of programming APIs across all the various platforms. And while libretro allows for something of that nature too, it does so with distinct design goals in mind that are more or less forced on you for the purpose of a better play experience.

SDL Libretro

SDL Libretro is a project that was started out by me half a year ago. Back then it was more or less in an unusable state. To date, I had ported a couple of SDL programs already to libretro (like NXEngine), but previously I always did so by manually baking in parts of SDL and then shoehorning the runloop such that it would fit inside libretro. A libretro core’s runloop consists of a ‘lifebeat’ that lasts for exactly one frame, which can pose a problem for many SDL programs, because how the programmer implements the runloop there is entirely up to the programmer, whereas libretro forces this runloop model on you. It does this for good reasons, so that the frontend can easily do advanced operations like fast forwarding, rewinding, etc. But nevertheless, if you have an existing program, it might take time to whip it into shap such that it fits the confines of a libretro program.

Developer r-type has done an awesome job of making SDL Libretro finally a viable project. Right now it exists as a Proof of Concept that works on both Linux and Windows, and to illustrate that it works, r-type has made available three Proof of Concepts to show off SDL Libretro:

  • An OpenTyrian SDL port
  • A Mandelbrot game port (using SDL)
  • A Tetris game port (using SDL)

Right now, this SDL port is obviously in its infancy, and this might be an area where we could make use of further contributions.

To go over some of these:

  • Because an SDL program can be implemented any number of ways, right now we have to rely on libco in order to implement the main runloop.
  • Right now we are targeting SDL 1.2.15. There are currently no plans for SDL 2 support, although if we do, it’s likely it would be a separate project.
  • Lastly, be cognizant of the fact that when we say ‘Proof of concept’, we really mean it. Things are not perfect yet and it will take some time to iron out all of the bugs.

To use libretro SDL in conjunction with your game program, right now you would first build the SDL libretro part. You run the Makefile and once successful, it will create a statically linked ‘archive’ (such as ‘libSDL_unix.a’ and/or ‘libSDL_win.a’). From there, you would manually link this archive into your libretro core. That way, your libretro program can interface against SDL.

If you want to see some test examples of how this is done in practice, go to the directory ‘tests‘. ‘opentyrian’, ‘sdl-mandelbrot’ and ‘sdl-tetris’ are three current proof of concepts.

What does this mean for endusers?

It means that developers familiar with SDL have an easier time getting themselves acquainted with libretro. It also will mean that we can get SDL ports up and running quicker instead of having to reimplement and rewrite everything from scratch.

Right now, my current plan is to take the quick and dirty OpenTyrian port, and divorce it from most of its SDL idiosynchrasies and turn it into a nice, native, fleshed out libretro core. However, at the same time, I also want to help improve, build and foster further work going into libretro SDL. So if anything, we need to strive for even more well fleshed-out tests at the same time.

Credit to r-type

We want to thank r-type a lot for coming up with this wonderful Proof-of-Concept. Without him, this project would have barely stumbled out of the gates and it would have taken many more months for it to end up running anything. Hopefully we can once return the favor for all the hard work and effort guys like this have provided to our project. It’s the passion and the commitment of most of the followers surrounding this project that keeps us going.