On 27 Mar 99, at 1:31, Kenneth Alan Boyd Ramsay wrote:

> The answer is a definite YES!
>
> My system is a COMPAQ 486 DX-66 using DOS/WINDOWS 3.11.
> I have a MDA (Monochrome Display Adapter) Display with a Hercules Graphics
> Card and a built-in VGA (Video Graphics Adapter) on the motherboard with a
> monochrome VGA monitor.
>
> Using the DOS MODE command at the "C:\>" prompt as follows:
>
> C:\>mode mono
>
> switches the video to the MDA monitor and:
>
> C:\>mode co80
>
> switches to the VGA - one at a time.
>
> I read that there are a few programs (I think BORLAND C++ is one) that can
> actually produce separate output (e.g. text on MDA and an image on the
> VGA) on both at the same time (or almost the same time).

  This is a special case.

  Because the MDA (monochrome) and CGA (colour) adapters used different
parts of the B000 segment and different control register I/O addresses,
there was no difficulty in having both in a single machine.  One or the
other (but not both!) could be replaced by a VGA card set to
appropriate (mono/colour) configuration.
  [We used to call such a machine a "two-headed monster".  There were a
few games that would display graphics on the colour monitor and
status/statistics on the mono, but the main use was for debugging
Windows software, with the debugger running the mono screen and the
Windows desktop and applications on a (colour) VGA.

  I believe the question is about a recent feature which allows a single
Windows desktop to spread across multiple VGA monitors; this is the first
(and, so far, only) way to have multiple VGA adapters in a single machine.

  [I recall a card from 5-6 years ago that implemented some kind of "dual
VGA".  As I recall, it was very expensive and driver support was limited.]

> I have also heard of high-end 3-D/Virtual Reality systems with two
> AGP (Advanced Graphics Processors) - one for the left eye image - and
> one for the right.  This would use two monitors with crossed Polaroid
> filters and a half-silvered mirror (the reflected image is reversed
> on the monitor so that they line up); with matching Polaroid glasses.

  There are 3D cards which can be installed in pairs -- one renders the
even-numbered lines on the screen, and the other the odd-numbered ones
("SLI").  These are not, however, VGA cards; they rely on a separate VGA
card for regular 2D output and the 3D output is routed through that
connector, *to a single monitor*.

>   I have also read of rapidly switching Kerr cells to flash the
> appropriate eye's image to the correct eye, using a single monitor.
> Another possibility is, of course, a VR headset.

  I think simulating 3D by rendering separate images for left and right
eye is a whole other kettle of fish.


David G

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