AGP and Graphics Memory

Don't I Need AGP2x Or 4x For The Future?

All AGP4x does is allow the graphics controller to assert more of its texture bandwidth demand on main memory. This is the source of the performance conflict, and 4x allows it to become a bigger problem. But there are two sides to examine: Texture demand and geometry transfer rates. We will do that in a minute.

When it is all said and done, the central issue is about graphics memory. The AGP vs. PCI issue gets dragged in because AGP is proposed as an alternative way to access more memory. Let me re-summarize my philosophy on this matter.

RULE #1 - Make sure that you have adequate graphics memory. If you have adequate graphics memory, you will not observe much of a performance difference between AGP and PCI. If you do not have adequate graphics memory, you are eventually going to run into texture related performance problems with PCI or with AGP. There are MANY variables that determine how big those problems are, and how they should best be solved.

Though I don't think AGP is bad, relying on AGP's execute mode texturing to recover from breaking Rule#1, is a less than ideal solution. It may potentially sacrifice MIPS, sacrifice accelerator performance, or both. If you already paid for AGP, and you inadvertently violated rule #1 in the process, AGP becomes a convenient, "no additional cost" way to deal with the problem. Convenient it is, but performance is another matter, and performance is not "free".

As I said, in order to be fully armed to make proper judgements, we will have to look at current and future requirements for texturing bandwidth and for the geometry stream. Let's start with texturing.