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Old 13th May, 2002, 07:47 PM
Aedan Aedan is offline
Chief Systems Administrator
Join Date: September 2001
Location: Europe
Posts: 13,075

I don't fully understand all of AGP - I don't profess to. However, simply setting the AGP aperture to the largest size might not be such a good idea for a number of reasons.

Firstly, the AGP aperture appears as a seperate set of addresses, EVEN THOUGH IT'S THE SAME PHYSICAL RAM! If you have 128Mb of RAM, and an aperture size of 128Mb, the system will allocate enough caching to handle 256Mb of RAM, increasing the usage of your RAM, despite the fact you haven't actually got any more RAM.

In Win9x machines, this causes problems, as the virtual address table for 512Mb RAM, coupled with the 128Mb aperture size means that VCache might run out of virtual addresses to use before finishing allocating virtual addresses to hardware. Obviously this is NOT a good state of affairs!

In NT based machines, the caching system can handle far more RAM, but still you'll lose RAM to the increased cache size.

Secondly, if your card has a large amount of RAM, you don't need such a large AGP aperture, as it's less likely to use any. The "rule of thumb" about setting it to half the RAM is invalid anyhow.

You can't have an AGP aperture less than 16Mb, as control structures that go with it are around 12Mb in size. This is why setting it less than 16Mb means you drop out of AGP mode.

Tuning the AGP aperture looks to be pretty badly documented. It looks like you'll have to run your favourite games, and bring the AGP aperture down until it starts to eat into your frame rate. The less memory you have on your card, the quicker this will happen!

Any views, thoughts and opinions are entirely my own. They don't necessarily represent those of my employer (BlackBerry).
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