Well, it’s been a lot of hours, and I’ve only just hit the point where I can test out all this bloody PPU code. The background layer & palettes appear to be working correctly now, all my problems were down to pretty much autopiloting my way through the tile rendering and reading code, missing off simple things like having “iY” instead of “iY * NESPPU_TILE_W” when indexing into the tiles. Bleh, it’s done, and I have a progress shot, as well as a sanity check shot:

An expression of emotion in relation to some of this pointer twiddly code

Simple test tiles and background layer generated, and also loaded in my font tile sheet.