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PPU Testing

Since the PPU is heavily dependent on the CPU for configuring the graphics memory, its logical to use the CPU to test the behavior of the PPU. Basically, a simple program was written that configure graphics with simple values and checking the output. The most difficult problem with testing the PPU is the time it takes to simulate. The refresh is designed only to happen every 20 ms which is quite a long time for a simulation considering the Nintendo CPU clock runs at 1 MHz. This means over 10000 cycles will be performed before the first pixel is processed. This can be fixed by tweaking the refresh clock to match when the test code is finished so that the PPU can begin earlier, however this still does not fix all of the problems. To truly see that the PPU is generating the correct image, its beneficial to view the actual pixels outputted. This means that PPU has to finish multiple scan lines before it creates anything recognizable.

Ian A. Buck
Wed May 20 12:50:42 EDT 1998