3D glasses and CPU cycle counting

Monday, 7th April 2008

I reintroduced joystick support to the emulator front-end over the weekend, using a much more sensible input manager. The control editor form uses the same interface design for keyboard and joysticks - you click the button you wish to edit, it appears pressed, you press the key (or joystick button) you wish to bind to it and it pops back out again. It'll also check to see if you've moved the joystick in a particular direction on any of its reported axes.

Another feature I added was better support for games that used the 3D glasses, using a simple red-cyan anaglyph output blender.

    

    

If the memory address range that the glasses respond to has been written to within the last three frames, it switches to the blending mode.

Irritatingly, with one glaring bug fix I managed to lower overall compatibility. Some instructions weren't being timed correctly (or at all) meaning that the emulator was executing too many instructions for the number of clock cycles it was asked to run. During the time each video scanline is run, 228 CPU cycles are executed. Upping this to 248 cycles (just for experimentation) fixes all known bugs - including the long-standing flickering pixels at the top of the screen in Cosmic Spacehead and display corruption in GP Rider. (However, digitised speech plays at a noticably higher pitch).

I'm not entirely sure why this is - it could be that some instructions are claiming to take too long, it could be yet another interrupt problem. To call this mildly frustrating is a bit of an understatement!