On 25 Jun 98 at 20:39, Bob Wright wrote:

> Why an accelerator card?  If you video card has 8mgs and
> you add an accelerator card with an additional 8 or 12mgs..
> well, you can see the boost in available graphic ram.

  Actually, this isn't generally true, because at any given moment
your system is only using one card or the other[*].  [Corollary:
Installing a 3D accelerator card will not speed up programs that only
do 2D.]
  [*] There are a few cards out there that *can* do 3D into a window
on an otherwise 2D screen.  They don't seem to get used this way very
much.

  The two cards use memory somewhat differently, though.

  A 2D card is going to dedicate memory first to the video frame
buffer (the actual pixels currently on screen), and may use the rest
for caching and the like.
  A 3D card, on the other hand, typically comes with a fixed
allocation of memory to various functions -- some for the frame
buffer, but some for texture maps and some for polygon lists and so
on.
  Few 3D cards will support a resolution beyond 800x600, while a 2D
card with 8MB might reasonably be expected to support 1600x1200 at
32bits of colour depth (or faster operation at lower resolutions).

  Why an accelerator card?  An accelerator card (of whatever sort)
has an on-board processor that can do some jobs while the system CPU
moves on to something else.  In 2D, the CPU can draw a line on the
screen by setting a bunch of pixels; or it can give the line
endpoints to an accelerator and go do something else while the
accelerator fills in the pixels to draw the rest of the line.  There
are analogous recurring operations in 3D video, where an accelerator
can handle the grunt work, often much faster than the CPU could
because of their special-purpose hardware.

David G