Let's say gc spoils an experiment every once in a while.  You can
detect that and discard the test.  Use explicit System.gc() at
convenient times.

There is the RDTSC instruction for high res timing.

This is the sort of thing that would have been ever so much easier
back in the DOS days where you could poke at the hardware yourself any
time you wanted and there was documentation around on how to do it.

Leo:

   As Roedy said, you can use System.gc() to try to encourage a garbage
collection in Java (The method is a hint, not a command IIRC). The safer
solution is do the usual real-time stuff like pre-allocating the objects you
will need to avoid GC's. Also unless you are doing many allocations and
abandoning them, the GC most likely will not be doing anything.

   As far as reporting when it was written to the display memory, that can
be done, but what probably matters is when it is streamed to display. The
only signal possibly visible to the PC is the vertical retrace of the video
and then the time of display is relative to that as the display raster is
written. e.g. a 640x480 screen non-interlaced at 60Hz draws a line of 640
pixels every ~34usec or a pixel every ~50nsec. It would take ~32x34usec = 1
msec to draw a 32 row high icon and the offset from the vertical retrace is
approximately 1 msec for every 32 rows down the screen. These numbers can be
adjusted for other resolutions/refresh rates. What you need to do is access
the refresh signal and either write immediately for display lower down the
screen, or delay from the refresh until you know the display position has
been scanned out to the monitor and then write the data for display on the
next pass. No hardware I know will tell you when it is rendered to the
display, but refresh signals are not uncommon.

   I think it would be useful for you to describe what you are trying to
achieve, beacuse if you are trying to display something for n exact
duration, then you have the same problem with turning off the image and the
time quantum will be 1 screen redraw.

   FYI, I believe that the fastest refresh rates for low res. (640x480) are
approaching 160-180Hz on a top end video card/monitor combo and 100-120Hz
for higher resolutions.

Mark...