ElectronAsh just posted his initial open-source PC Engine & TurboGrafx-CD code for adding CD support to MiSTer’s TG16 core. This is important not only for PC Engine CD, but also Sega CD, Jaguar CD, Playstation, and other disc-based systems, because portions of the code to stream CD data will be similar across platforms.
The work is at an early stage and Ash welcomes anyone to look at the code and help out. It could be weeks or months away from completion depending on the amount of support we can rally for it.
Ash’s TG16 fork for CD dev: https://github.com/ElectronAsh/TurboGrafx16_MiSTer