Monthly Archives: November 2020

XGS on the Raspberry Pi 400

About a week and half ago I became the owner of Raspberry Pi 400, a small computer built around a Raspberry Pi 4. The kit I purchased came with a USB-C power adapter and a basic USB mouse; all I needed to supply was a monitor.

Naturally, the first thing I did after setting it up was get XGS running on it:

This was not quite as simple compiling XGS and running it, so let’s go over what changed, why it changed, and how to build XGS on the Pi after the changes.

Open GL Support on the Raspberry Pi

On older Pi models (1, 2, 3, and the Zero) the OpenGL support was provided through a proprietary firmware blob, and exposed through a set of custom GL libraries in /opt/vc/lib. Since 2017 XGS supported use of these libraries by specifying -DBCMHOST=true when running cmake.

As of the Pi 4 (and, by extension, the Pi 400) this proprietary firmware driver is no longer supported. Instead, there are now open source drivers supporting the Pi 3 and 4. They are fully integrated into Mesa now, so it is no longer necessary to link against proprietary libraries.

As a result of these changes I have decided to remove the BCMHOST option in XGS. Instead, XGS relies on SDL to pick the right driver; this will be KMSDRM when running from the console, or X11 when running from the desktop.

Note that this change has probably broken support for the Pi 1 or 2. Those models are sufficiently outdated now that I don’t feel the need to continue supporting them.

Building XGS

As of this writing Raspberry Pi OS does not ship a version of SDL with the KMSDRM driver enabled. So, before building XGS, you’ll need to build and install your own build of SDL. Fortunately this is very easy:

sudo apt-get remove -y --force-yes libsdl2-dev
sudo apt-get install libgbm-dev libdrm-dev
tar -xzvf SDL2-2.0.12.tar.gz
cd SDL2-2.0.12
./configure --host=arm-raspberry-linux-gnueabihf \
--disable-pulseaudio \
--disable-esd \
--disable-video-rpi \
--enable-video-kmsdrm
make -j4
sudo make install

You should also run raspi-config, navigate to Advanced Options -> GL Driver, and make sure you have the KMS driver selected. You’ll need to reboot after making this change.

With SDL built and the KMS driver enabled you can now build XGS:

cd xgs
mkdir build
cd build
cmake ..
make -j4

The resulting binary can be run from the desktop or direct from the console. It will open in a window when run from the desktop, and full-screen when run from the console.

The only problem I have noticed is that the video is a bit choppy when running from the console. It’s most noticeable when watching the sliding apple on the “Check startup device” screen. It does not happen when running from the desktop, so it is somehow related to the KMSDRM driver.

Raspberry Pi & the Future of XGS

XGS has certainly had a fragmented development history. I originally started it in 1996, but by 1997 development had already stalled. Five years later, in 2002, I briefly resumed work on it, but I also started a small business around that time and XGS ended up on a (virtual) shelf for nearly 15 years.

Fast forward to 2016, when I got the XGS itch again. I rewrote the code in C++ to make it easier to understand, removed non-SDL graphics support, and added a basic GUI.

By 2017 I had moved away from the project again despite making good progress. I briefly considered creating a retro emulated GS based around the Raspberry Pi (think of the C64 Mini) but that project never really got off of the ground, and by that point I was already spending the bulk of my time on other projects.

And so here we are in 2020. I’m currently between hardware projects (well sort of; more on that in another post) and I’m feeling the need to spend at least some of my free time on a software project. So, I am jumping back into XGS, with the intent of improving the Raspberry Pi support. My first task will be to create an ARM-optimized version of the M65816 emulation core. I’ve been wanting to learn ARM assembly, and this will be a great way to do that.

Stay tuned!