> I'd guess it can be reduced by recording changes to the relevant hardware registers along with timestamps in the emulation so that the timing of your scanlines' construction becomes less of an issue.
This would be possible in most cases, but the SNES throws another problem at you: the video renderer can set flags that can affect the operation of the CPU. Range/tile over sprite flags, H/Vblank signals, etc.
In my model, I chose to forgo timestamps because they are very tricky to get right with subtle details. Instead, I render one pixel at a time, but I use a cooperative threading model. Whenever the CPU reads something from the PPU, it checks to see if the CPU is currently not caught up with the PPU. If so, it will switch and run the PPU. The PPU does the same with respect to the CPU.
Even with that, all the extra overhead of being -able- to process one pixel at a time knocks the framerate from ~240fps to ~100fps. And it fixes maybe a half-dozen minor issues in games for all that overhead.
This is because scanline-based renderers are notoriously good at working around these issues. There are lots of games that do a mid-scanline write in error, but only a few that do more than one on the same scanline. So all you have to do is make sure you render your line on the correct 'side' of the write. We actually took every game we could find with this issue, and averaged out the best possible position within a line to run the highest number of games correctly. Other emulators take that further and can make changes to that timing on a per-game basis to fix even more issues.
Air Strike Patrol's shadow is actually the only known effect where two writes occur on the same line, and there is no one point that would render the line correctly.
This would be possible in most cases, but the SNES throws another problem at you: the video renderer can set flags that can affect the operation of the CPU. Range/tile over sprite flags, H/Vblank signals, etc.
In my model, I chose to forgo timestamps because they are very tricky to get right with subtle details. Instead, I render one pixel at a time, but I use a cooperative threading model. Whenever the CPU reads something from the PPU, it checks to see if the CPU is currently not caught up with the PPU. If so, it will switch and run the PPU. The PPU does the same with respect to the CPU.
Even with that, all the extra overhead of being -able- to process one pixel at a time knocks the framerate from ~240fps to ~100fps. And it fixes maybe a half-dozen minor issues in games for all that overhead.
This is because scanline-based renderers are notoriously good at working around these issues. There are lots of games that do a mid-scanline write in error, but only a few that do more than one on the same scanline. So all you have to do is make sure you render your line on the correct 'side' of the write. We actually took every game we could find with this issue, and averaged out the best possible position within a line to run the highest number of games correctly. Other emulators take that further and can make changes to that timing on a per-game basis to fix even more issues.
Air Strike Patrol's shadow is actually the only known effect where two writes occur on the same line, and there is no one point that would render the line correctly.