Sergiy "srg320" Dvodnenko, creator of the MiSTer's amazing Sega Saturn core, has admitted that his in-development 3DO core for the platform may never achieve 100% accuracy.
Speaking to Read Only Memo, Dvodnenko explains that while the core itself is "not complex and does not consume a lot of FPGA resources," the big headaches come from VRAM emulation:
"This is likely what prevented other developers from working on the 3DO core. Accurate VRAM emulation requires a certain amount of internal FPGA memory, which it doesn't have. Therefore, unfortunately, the core cannot be accurate on the MiSTer."
Later in the discussion, the FPGA developer explains the complexities of adapting the console's most complex component: its sprite engine:
"The Sprite Engine renders most of 2D and 3D graphics. Only FMV does not use it, but instead copies data directly into VRAM (framebuffer memory). Some parts of the Sprite Engine are described in the documentation. But that’s not enough to figure everything out. The sprite engine is closely tied to DMA. So, I connected my homemade logic analyzer to the original 3DO board.
I wrote a few test programs and captured a lot of data from the buses. This allowed me to understand how DMA works and implement it in the core. By understanding the sequence and timing of the DMA components, I figured out the sequence of the Sprite Engine modules. Unfortunately, some details can’t be clarified, such as the capacity of the various FIFOs that are part of the Sprite Engine. So, absolute accuracy won’t be achievable. Perhaps someday the MADAM and CLIO chips will be reverse-engineered, and we’ll learn more."
The MiSTer platform is a little long in the tooth these days, so it's hardly surprising that developers are reaching the limit of its potential.
It remains to be seen if experts like Dvodnenko can eke out a little more performance from the system, or if we'll need to wait for superior FPGA hardware to arrive before we can achieve complete accuracy with 3DO games.
But then again, does a core need to be 100% faithful for you to enjoy playing games on it? Dvodnenko himself says his superb Saturn core isn't totally accurate, so perhaps it's not as big a deal as you might imagine.