Wed, 09 Aug 2000 daniel sheltraw wrote:
> Hello RTers
>
> This is a bit off the RTL and RTAI topic but I have a quetion similar to
> Patricks regarding RT display. I am willing to accept the course
> grained timing given by the reciprocol of the refresh rate but I would
> like to be able to draw the image to screen with hard RT scheduling. There
> exists a register in most cards that "resets" the sequencer.
Standard VGA register? (Long time since I programmed VGA on the hardware
level...)
> I am not sure what reset means and can find no documentation to tell so. I
> am wondering if anyone knows whether this resets the sequencer so that I can
> start at a well defined spot in the sequencer cycle and therefore achieve
> hard RT scheduling. Of course there may be latencies for the reset which
> would have to be taken into account.
Any video card registers (except for the one with the blanking bits, in case
you have no VBL IRQ...) would generally be of interest only when dealing with
hardware accelerated rendering. (If you're using mode-X or bitplane [16 color]
modes, you're in trouble, though - these modes require register access all the
time, which means you cannot share the card at the hardware level.) AFAIK, the
only effect they could have on any normal card is that ongoing hardware
accellerated operations could steal some VRAM bandwidth, which of course could
be a problem if high bandwidth is required for the RT operations.
As to standard VGA registers, I only know about the usual bitmap access "helper"
logic and that stuff, and it is indeed slow on some modern cards, but I doubt
it gets much worse than fiddling with ports in general. "Only" the usual
hundreds of cycles wasted on waitstates...
If you really need h/w acceleration for the RT stuff, the driver has to run in
RT context, and on nearly all non-pro cards you also need to dedicate the card
to the RT task, since consumer video cards in general cannot switch context in
the middle of an operation. (Pro cards for CAD/CAM, real time 3D animation etc
can do this kind of task switching, and it seems like the new generation of game
cards are getting there too, as a result of game and pro cards beginning to use
the same chipsets. Lower costs and higher performance for both classes.)
David Olofson
Programmer
Reologica Instruments AB
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