scott <sco### [at] scottcom> wrote:
>  > Even if the GPU has its own dedicated RAM,
> > you can't put the entire scene in it, especially with humongous open sandbox
> > games. You have to be selective about what you send the GPU to render. The
> > CPU makes the decision on what to render and with what LOD level. And it
> > obviously makes this decision based on what it as on its own RAM.
> >
> >    The more RAM you have, the more and higher-definition scene data you can
> > have at a time to tell the GPU to render.

> You would be correct if data could be transferred between CPU RAM and 
> GPU RAM much faster, but this isn't the case.  During a game it would 
> take several seconds to replace the GPU RAM completely, and in that time 
> you can probably load the data from the HD rather than having to keep it 
> in CPU RAM.  In typical games you don't need to replace the entire GPU 
> RAM anywhere near as often as that (in fact it would be a really bad 
> game engine design to do so), so it's even less of an issue.  That's why 
> it's possible for such games to run on a PS3 with a "puny" 256MB of CPU RAM.

  The majority of the data on the GPU's RAM is textures and other bitmaps
which do not change. You don't "replace the GPU RAM completely". You replace
the scene geometry. The geometry of the scene is being constantly transferred
from the CPU RAM to the GPU (if they don't share RAMs), especially on an open
sandbox game. There may be some geometry that is kept constantly on the GPU
RAM (such as stock objects and character models, if they can be animated
from inside the GPU itself), but the scenery itself has to be updated as
the player moves.

  The reason is rather obvious: The entire world cannot fit into either RAM
at once, but must be dynamically loaded and updated as the player moves.
The CPU makes the decision on what to load and what to display, and obviously
constantly updates the geometry from its own RAM to the GPU.

  You still make it sound like you don't need CPU RAM *at all* to play an
open sandbox game. That's obviously ridiculous.

  And you still haven't answered my question: Why is 512 MB better than
for example 128 MB of CPU RAM for large games? Or 32 MB. Or 1 MB.

  If you think about that question, then think why eg. 2 GB would be better
than 512 MB. Or is 512 MB some kind of magic limit after which any additional
RAM is obsolete? Can you explain why this would be so?

-- 
                                                          - Warp

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