I asked this question about a week ago in the emulators group, but the
only thing I learned since then is that the latest version of AppleWin
supports the sensing of video refresh bytes, but the time of sensing is
unrelated to the actual emulator video refresh.
The reason I'm asking is that I have written some code that does
animation, and needs an absolute time reference (as opposed to cycle
counts on accelerated machines or emulators).
The objective is not to do something sophisticated, like avoid tearing
of the animation (though that would be nice), but simply to cause it to
run at its intended speed, regardless of the speed of the processor
(emulated or real).
Polling VBL works on an Apple //e, but *only* on a //e, since later
machines de-implemented VBL polling in favor of VBL interrupts.
Video byte sensing (the "vaporlock" or "Apple Split" approach) works
on all real Apple II's, and achieves the desired absolute 1/60th
second time reference.
Since so many people use emulators these days, I was hoping that the
same technique would work on at least one popular PC and one popular
Mac emulator.
I think AppleWin may work on the PC, but if it accelerates the "virtual
video" as it accelerates the processor, then it may not.
All the reasonable uses I can think of for video sensing and VBL sensing
are related to an absolute time base, and should not be subject to
acceleration (just as it is not in a real Apple II), but I'm not sure
that emulator writers agree...
What I'd like to know is which emulators *do* keep the emulated video
period at 1/60th second (or 1/50th for PAL ;-).