Implemented H-Blanking support, which was causing Super Mario World to hang. In fact, SMW now runs into the game... and I'm sure it would be playable, if I had sprites implemented! There are some graphical glitches, related to me not supporting subscreens correctly yet. I'll optimise the BG layer code next, and simultaneously correct this.
Found a small bug related to NMI interrupts, although I don't think was affecting anything. SMW also showed up a bug with the LSR absolute opcode. I also corrected a bug related to scrolling tilemaps vertically, which only manifested itself on certain titles.
Currently, 80% of all opcodes are implemented. SMW, Super Off-Road, Super Mario All-Stars, Gradius III and Super R-Type are among a handful of titles that seem to run into the game (minus sprites). A few other titles do stuff, but require more of the PPU to be emulated correctly before progressing further.