Several plans surfaced since for the normal tiled modes combined with SPI RAM, while I also started developing an own Uzebox-style console based on non-overclocked ATmega1284's (2x ATMega1284 + SPI RAM).
I ended up with the decision of the following system, simplified from the original Mode 74 (the current Mode 748's solution is also a rather simplified variant of the original M74):
- Two VRAMs, one in SPI RAM, one in normal RAM. Each VRAM is one byte / tile.
- The SPI RAM VRAM is a large scrollable surface of ROM tiles, capable to access 256 tiles.
- The normal VRAM has a switch on Bit7 selecting between an overlaid RAM tile and the bg. The low bits select the RAM tile.
Of the SPI RAM bitmap modes, the 2x1284 could display the 8 color mode (pictures using the 16 color mode don't fit in a 1284's RAM), and the 2 color 384 pixels wide mode without attributes. Of course M748 will keep having those, I am rather just noting these for compatibility, progressing towards the possibility of graphics-rich RPG and adventure (which I would like to have on the other system too if I eventually complete that).
For the design I mostly keep an RPG game's use cases in mind, so in the end I will also very likely add a 384 pixels wide tiled mode as well (for 48 characters wide text which is very useful for both RPG and adventure), this is about the same which I use in the Mode 72 prototype (although I will likely change that too, matching it with the format I introduced with Mode 40/41/42 to make it more straightforward to generate charsets, or you could just use those in Mode 40).