[Tuxnes-devel] Mid-scanline updates?

[Jim U. <ji...@3e...>]

Do PPU updates made during a scanline take effect immediately?  My
understanding is no---they are either disallowed or delayed until hblank or
vblank.  I ask because if this is the case, I can optimize the rendering
code in a way that is a huge win for the dreamcast.  

I also ask because my reading of the emulator code indicates that it
accepts mid-scanline updates, and calls drawimage() immediately, but in all
cases that I can see it delays any visible effect until hblank.  For
example, drawimage keeps a local static copy of hscrollreg in curhscroll
and only synchronizes them during hblank.

I am experimenting with writing eight pixels to the screen per loop
iteration, rather than one, and also combining two 16-bit pixels into one
32-bit write.  As I mentioned this provides a big speedup on the dc.
However the code becomes more complicated if you don't start on an
eight-pixel boundary, or write fewer than eight pixels.  To solve this I
need to make additional changes to the code, which will result in most
mid-scanline updates being delayed (although mmc2_4_latch will still work).
But I don't want to make these changes if it will break a large number of
games and I can't find a workaround.

Jim

-- 
"You can eat if you want to; that's your prerogative...  I used to play
that game... I know what it's like...  But I'm just not interested in it
any more... I'm not interested in going out, and working for the money to
buy the food, then shopping for it, then going home and preparing it, then
eating it, then washing the dishes, then just letting it all out in the
toilet... and in the end, you end up in a box, in the ground, on the
outside of town."  -- breatharian.com
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