I can see the attraction!

"Next (and last) unimplemented mode (as defined by the MD bits) is the 2-plane 8-bit (pattern name) mode which is used by the games themselves. That shouldn't be too difficult to implement (or more correctly, fix) but it does take a while for the simulation to get that far."

Famous last words! It has taken a few re-writes of a good chunk of the logic in the pattern generator, but I'm finally seeing at least Xevious and Mappy running in attract mode! Not much to see of Galaga except a few words on the screen, but that could be palette issues.

The aforementioned mode also requires both the page table and base address registers to be implemented and included in the pattern generator calculations. Not overly complex but a number of different combinations to account for in shifting values around relative to one another. It does pay to read - and re-read - the documentation as much of the logic that I ended up re-writing was summarised quite nicely in a few tables in the YGV608 Application Manual.

Anyway, here's some eye candy from the attract mode:

Xevious original and Arrangement versions (simulation)

Mappy original and Arrangement versions (simulation)

Not every mode is fully implemented yet either; only 8x8 & 16x16 tiles, and it assumes only Page 0 is in use on each plane. I will have to look at scrolling soon too!

One issue I have is that I currently have only a fraction of the graphics ROM loaded into the real MiSTer project, as it is residing in on-chip RAM. So I can only test that the correct tiles are being displayed under simulation. And under simulation, it takes 81 mins to get to the original Xevious attract mode, and a tad over 2hrs to get to the original Mappy attract mode!

Now that I've gotten this far, it's time to get the graphics ROM loaded into SDRAM. I have the option of using the 2nd SDRAM module on my development MiSTer, but haven't decided if that's necessary just yet...