Home Page Forums MAME MAME The SNES WIP topic

Oh, you meant double buffered in terms of tiles. I was thinking scanlines, since we were previously talking about line buffering sprite generators. Your explanation makes perfect sense (whereas buffering a scanline ahead makes no sense at all)

I figured that the reason the SNES PPU has the modes it does is because of memory bandwidth as soon as I noticed that the total number of 2-bit planes (doubled in the hires modes) plus the number of layers (counting the OPT table as a layer) always adds up to no more than 8:

Mode 0: 4 layers + 4 planes (1 + 1 + 1 + 1) = 8
Mode 1: 3 layers + 5 planes (2 + 2 + 1) = 8
Mode 2: 3 layers + 4 planes (2 + 2) = 7
Mode 3: 2 layers + 6 planes (4 + 2) = 8
Mode 4: 3 layers + 5 planes (4 + 1) = 8
Mode 5: 2 layers + 6 planes (2*2 + 1*2) = 8
Mode 6: 2 layers + 4 planes (2*2) = 6

ETA: I just noticed that this also explains the limitations of Mode 4 compared to the other OPT modes (i.e. each tile can only have a horizontal or vertical offset applied, not both)