Need clarification on `eadk_display_wait_for_vblank()`

[Obscure2020]

Hello, Numworks team. I have some questions regarding the usage of eadk_display_wait_for_vblank() which have not been sufficiently answered either in your own documentation, or elsewhere online. (At least not that I could find.) Could you take the time to answer the following for me?

1. The full signature for the function in question, as reported in eadk.h, is bool eadk_display_wait_for_vblank();. The fact that this function apparently returns a bool suggests that its purpose is not purely waiting or sleeping, as the main sleep functions eadk_timing_usleep() and eadk_timing_msleep() return void. How should the bool return value from this function be interpreted? For what reasons might it return true or false?

   • I tried to answer the above question for myself by tracing back its implementation in your code, but the farthest I could get was the handoff to SVC_DISPLAY_WAIT_FOR_V_BLANK. I couldn't find the actual implementation of that written anywhere, which is why I'm turning to you guys for some assistance.

2. Does the eadk_display_wait_for_vblank() function actually do anything on a physical, hardware Numworks calculator? Or is it only actually implemented to function in simulators?

[yannis300307]

Hello, I'm not from the Numworks team but I can answer your second question. The calculator's screen has a constant frame rate of 40 FPS. Epsilon doesn't have a double buffering for the screen. So in order to help the application to avoid being drawn between 2 frames, you can call eadk_display_wait_for_vblank(). 40 FPS is 25ms/frame. So if you start the rendering of the frame after the last one and that your frame takes 10ms to render, calling eadk_display_wait_for_vblank() will wait for 15ms. But if your frame takes more than 25 ms to render, let's say 30ms, calling the function will cause the calculator to wait until the next frame. So your app will be running at 50 ms / frame or 20 FPS.

[yannis300307]

And the function seems to always return true on the simulator (Probably due to the fact that the simulator updates the screen only when the keyboard events are pooled). See this line. The implementation for the actual calculator is in the closed source kernel.