Graphics Cards

AGP Cards
There are 2 types of graphics cards, the AGP and PCI types.
AGP stands for Accelerated Graphics Port. AGP cards have several advantages over the older PCI cards. (PCI stands for Peripheral Component Interconnect) It offers a minor data transfer rate advantage when it comes to moving the geometry stream from the CPU to the graphics card. AGP's GART table allows the OS(Operating System) to manage textures in off screen memory as well as in system memory. It also allows the graphics card to access these textures directly in either location.

AGP has a more flexible memory management and tha data transfer rate is faster due to its faster clock speed and bus pipelining. AGP cards have a higher bandwidth than PCI cards and even without using any of the extra features of AGP, the AGP bus runs at 66 MHz compared to the PCI bus which runs only at 33MHz. The theoretical bandwidths of AGP is 533 MB/sec while PCI's is 133 MB/sec.

AGP offers 2 ways to deal with textures.
One is called DMA(Direct Memory Access) mode :
The Card places the most frequently used textures in off screen memory, then lock down a few additional megabytes of system memory for the remainder of the texture database. If the graphics chip needs a tecture that is not in the graphics memory, then the accelerator uses AGP mode to copy the needed texture on demand into texture swap space in graphics memory.

The other is called the Execute Mode :
Allows the graphics chip to access the texture information in main memory without first copying it to graphics memory. The effective bus throughput of Execute mode and DMA mode are the same.

AGP's DMA mode offers excellent concurrency because the game can detect if it must swap textures before the texture is actually needed. This way, the card can begin fetching the tecture before it is needed to paint pixels on the screen.

The performance and speed of a card depends very much on how the card manages its textures.

AGP texturing allows the card to take control of the main system memory bus in order to access the texture data. When this happens, the CPU is locked out of main memory. If the CPU is engaged in a computationally challenging task, the CPU may stall, waiting for its turn to access the main memory. This problem can probably be overcome if the card takes only the required texels of the texture map. In DMA mode, the entire texture map has to be copied to the local memory.

The only way texturing does not impact CPU performance is if all the texture an application needs can fit into the local video memory on the graphics board, which is not very possible due to the large number and size of the textures required. Therefore, make sure you have adequate graphics memory. Otherwise, you are eventually going to run into texture related performance problems with PCI or AGP. Depending on AGP's execute mode texturing to recover from insufficient grpahics memory may potentially sacrifice MIPS and/or accelerator performance.

AGP 2x, 4x and the Pro-Slot
This is one thing that many people are unsure of. That is the difference between AGP 1x, 2x and 4x. There is no difference in speed between the 3. The only difference is the bandwidth of data transfer.

What about the Pro-Slot?
The suffix "Pro" only indicates the extension for the AGP slot. Its stands for an additional power supply for AGP graphics cards and has nothing to do with the bandwidth of the bus. This extension became necessary because the number of transistors on high-end graphics chips has increased.