OC ability controlled by AGP aperture? WTF

[Frank_Fuster]

I have an eVGA 6800 Limited Edition. It came with the ultra 400/1100 BIOS and it was not stable AT ALL. Yet the guy that had sold it to me said it was completely stable at 400/1100. So I decided to switch to the ULTRA 350/1100 bios and reset my BIOS.

Lo and behold the card was completely stable. It was stable at 430/1150, so I was happy. I decided to switch back to the ULTRA 400/1100 BIOS to check if it was still unstable using that BIOS. Switched and surprisingly it was completely stable, and oc'ed to the same 430/1150 speed. I then realized it was either my CPU OC or some sort of BIOS setting.

I OC'ed my p4 back to 3.0 speeds, Primed/RTHDRIBL for a long time and it was stable like before. Then I realized that I had put the AGP aperture size to 256MB before the 6800 had arrived. So I switched it to 256MB booted, got into windows opened RTHDRIBL and it crashed to BIOS the second it started. Booted again tried DOOM 3 and crashed to BIOS the second the opening scene started.

I switched back to 128 MB aperture and everything was fine again. Basically one of the weirdest experiences of my OC'ing life. Any inputs on why the AGP aperture would cause this?

 

[arentol]

What the AGP aperture setting does is set a limit of how much System RAM your videocard can use if it needs more RAM than it has on board, and causes a GART table to be created to reserve that space in physical memory.

What this means is the more RAM you have on your videocard the lower you can afford to set your AGP aperture setting! (However going down to 32 is getting too low on even 256mb cards,.in part because developers sometimes specify a texture as "non-local" so it Always goes to the RAM set aside for the aperture. So 64 is the minimum really.)

Now, to get to the source of your problem (Sort of)...

Whenever you boot your PC with an aperture setting (which is always when using AGP) a table called GART, Graphics Address Remapping Table, is created. This table is a map of 4k pages of Physical System RAM that are then reserved for Videocard use (The RAM is still useable by the system until the videocard needs it though). The larger you set your AGP aperture the larger the table will be since it has to record enough 4k pages to total the amount of RAM in your aperture setting.

So all I can think of is that maybe your system is conflicting with the GART table somehow. Since the GART table is part of the Northbridge there may be a problem with your Northbridge, or there could be an issue with your GART driver (which you should be able to update at your motherboard manufacturers, or northbridge manufacturers websites).

 

[legcramp]

it doesnt effect overclokability..I set my aperture to 256..on stock speeds and it crashes all the time heh