> However, using Donkey Kong and 1943, I found that the time
> from the caps lock LED coming on to the first apparent on-screen action (first jump
> sequence frame, or first fired shot frame) was consistently 3 frames.
The important thing now is: How does it react when you use vsync in window and in fullscreen mode? Using it without any special options is just the base value. To check input lag, it's interesting to know how much longer it takes for vsync (and maybe triple buffer) in both, window and fullscreen.
> I also tried my desktop Linux system with an integrated AMD graphics card (on the
> motherboard) and SDLmame, can't remember what version, but a few versions old at
> least, and the delay was a consistent 7 frames.
That's much too long.
> I do wonder what the 'minimum' achievable delay is, I suspect it's not possible to do
> better than 3 frames
From a purely software point of view you can easily test it: Pause the game, then press Shift + P + jump button. (That means: While in pause mode, MAME advances one frame and on this frame, the jump button is pressed.) Then continue pressing Shift + P (further frame advance) until Mario jumps. This way you can see how the game is programmed.
In "Vs. Super Mario Bros.", it takes one frame to jump: If the moment where you press Shift + P + Jump counts as frame 0, then the jump occurs at frame 1.
At the character selection screen of "Street Fighter II" it doesn't even take this one screen. Press the button and the fighter changes immediately, at frame 0.
So, yes, the game itself on a software level reacts immediately.
However, this still doesn't explain how much it would take for an actual physical gamepad in real world time to send the signal. I'd be really curious if three frames is the actual delay or if the real arcade is faster.
One possible way to verify this: As far as I see it, the Vs. system is basically nothing more than an NES. Same with the PlayChoice-10. So, even if we don't have an actual arcade machine to alter, couldn't someone who knows a bit about that stuff connect an LED to an NES controller that lights when you press a certain button? Then the test could be done with the actual "Super Mario Bros." on a real NES. Then we can see if Mario needs three frames to jump or if the actual hardware is faster. The same results should apply for the arcade version.
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