card needs memory to hold object data such as object shape, size and mass. if an object has been moved then its location and orientation needs to be stored somewhere, now storing it in main system ram is going to be slow since all these maths calculations need very low latency memory so that there is no hitching or jerking when lots of objects are moving around. object collision would also take up some memory. objects are not stored as whole units, just their wire frame status is required. the rest the cpu handles.
it could be something like this:
Look at an explosion scene in cellfactor when that level starts all the world objects need to be loaded into the ppu's ram and not just the objects you can see on screen. if an explosion takes place and a box files out of the players view and hits another object causing explosions or movment of other objects etc then its status in the world needs to be updated. and if the ppu is only calculating things in the players view then its not going to be of any use at all.
now imagine a land mine has been placed on the road outside a building and then the player runs inside the building the ppu needs to know about the mina and all the other things in the world otherwise explosions that take place out of the players main view will not be processed by the PPU if the ppu is configured to only process stuff in the players direct view due to memory limitations.
now the ppu needs onboard memory to hold all this data. most of this if not all of this is vector based data that the card holds on the entire world terrain and objects.
e.g. a table like this could be used by the ppu:
ObjectName, ObjectSize, ObjectDimensions, ObjectWeight, ObjectMaterial
box1 , small , 12x9x11 , 22kg , cardboard
something like the above needs to be held, along with lots of other information such as if the object is in motion then its rate of acceleration needs to be calculated and its friction level on the terrain. ObjectMaterial e.g cardboard will link to another table showing how to calculate friction e.g cardboard vs concrete or cardboard vs grass. lots of other factors to take into consideration such as mavity, air density, box strength etc.
all this table data is what takes up space in the ppu's memory. now the above is assuming that the ppu uses the most advanced form of physics which i really doubt the ageia implementation uses.
if you all sit back and look at your pc tables for a minute just imagine how much physics calculations would be needed to move an object over the desk alone.
the weight of you monitor etc could show stress points up on certain areas of your table. real physics is next to impossible to simulate since there is jut too many factors that need to be calculated for a single object. i really doubt the ageia implementation has this power to begin with. but then again im assuming that the physics will mimick that of the real world. the physics in halflife2 were very good best i have come across in a game but most of the time it was just scripted events that programmers already knew about.
e.g in real physics i could just blow up a wall or blow up a gate that i need to get through to get to the other side. but then this opens up another question, physics in a game will have to be limited to some extent otherwise its going to be too easy to clock a game.
there is so much to physics that ageia have not addressed and i believe its to do with the lack of power on their ppu.
what they are trying to do is make the movement of objects more realistic but thats not what they should be aiming for. the aim of this generation should be to get deformable terrains and breakable materials into play. then improve on that over the later years.
