On 28 Mar 99, at 15:36, Max Timchenko wrote:

> How exactly can 2 cards live in one system? they compete for same
> address space ($a000:0 and up), won't it cause conflicts?

  All VGA cards have to provide modes where the video RAM can be mapped
into the A000 segment (64K at a time) in order to be accessible from
DOS programs.  That is true.

  However, constantly mapping and re-mapping this address space is
ugly, time-sonsuming, and pretty boring.  Most modern video cards
*also* map their entire RAM to space at a high (non-swappable) virtual
address, where protected-mode software can read and write it directly;
the drivers for the card know where this mapping is, or how to find it.

  So there's only a conflict at A000 if both cards are trying to
provide real-mode (DOS-compatible) access to video RAM.  As long as at
least one of them has this mapping suppressed, there's no conflict.
[Actually, under NT and 9x, this mapping is generally done by the OS,
and used to connect to a "DOS window" on the screen, instead of
connecting directly to the video RAM.  So it's "extremely normal" for a
video card under Win32 not to map to A000 at all.

  [There is a secondary issue that two VGA/SVGA cards would normally
want to use the same range of I/O port numbers and IRQ (if any);
solving that part of the problem seems like a natural extension of PnP,
which would I think be why this feature is only supported by fairly
recent cards.]


David G

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