85cocoa wrote:
Just to make sure everyone is aware:
http://www.bannister.org/ubb/ultimatebb ... p=1#000005There appears to be a request to get more information on what writes to $2007 do during rendering, specifically in the context of the game "Saint Seiya - Ougon Densetsu Kanketsu Hen."
I'm not sure about that game, but writing to 2007 during rendering is very bad news. There appears to be two "control units" so to speak on the PPU that can operate the memory interface.
The main "rendering" control interface, and the 2007 read/write control interface. The two are simply OR'd together on the control lines. It is possible to do both a read AND write cycle at the same time because of this. I used my "3 in 1 tester" to monitor the PPU's address, data, and control lines during rendering to see what happened when reads and writes were performed during rendering.
The result is very unpredictable as you can imagine, since the timing of the CPU read/write and the timing of the PPU data fetch both come into play. The net result of writing during rendering could be corruption of at least 2 memory locations and possible corruption of the address counter/pointer. Reading isn't so bad but it too could corrupt the address counter during rendering and if nothing else, cause glitchy video due to improper data being fetched for display.