>>353
ttp://wiki.nesdev.com/w/index.php/MMC3#Hardware
Regarding PPU A12 rises:
If the BG uses $0000, and the sprites use $1000, then the IRQ will occur after PPU cycle 260 (as in, a little after the visible part of the target scanline has ended).
If the BG uses $1000, and the sprites use $0000, then the IRQ will occur after PPU cycle 324 of the previous scanline (as in, right before the target scanline is about to be drawn).
When using 8x16 sprites: When there are less than 8 sprites on a scanline, the PPU makes a dummy fetch to tile $FF for each leftover sprite. In 8x16 sprite mode, tile $FF corresponds to the right pattern table ($1000).


PPUアドレスバスA12がスキャンラインカウンタのクロックとなっている。
The counter is clocked on each rising edge of PPU A12, no matter what caused it, so it is possible to (intentionally or not) clock the counter by writing to $2006
BGとSPRをどっちに配置するかで、立ち上がり、立下りが変わるので、
IRQが起きる位置とライン数が変わるようだ。