Video Cards

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Today's software is increasing in graphics intensity. Even "mundane" business software uses icons, charts, animations, etc. When you add 3D games and educational software to the equation, one can see that there is a crunch in bandwidth for graphical information. With newer software and games getting much more graphics intensive, the PCI bus is maxed out. In fact, the PCI bus, once considered very fast, can now be considered a bottleneck.
Intel knew this. In response, they designed the Accelerated Graphics Port, or AGP. Intel defines AGP as a "high performance, component level interconnect targeted at 3D graphical display applications and is based on a set of performance extensions or enhancements to PCI." In short, AGP uses the main PC memory to hold 3D images. In effect, this gives the AGP video card an unlimited amount of video memory. To speed up the data transfer, Intel designed the port as a direct path to the PC's main memory.

AGP sounds groundbreaking, and it is, no doubt, the latest craze in the need for graphical speed. One reason it is faster than PCI is that, while PCI runs at 33MHz, the AGP bus runs much faster. A 4X AGP bus runs at 4 times 33MHz, or 133MHz! Also, a normally clocked PCI bus can achieve a throughput of 132MB/s. Yes, this is fast, but when compared to the throughputs of 3D games, one finds that it is not enough. AGP, running in 2x mode (2 x 33 = 66MHz), can achieve a throughput of 528MB/s! AGP pulls this off by constantly transferring data on both the rises and falls of the 66MHz clock cycle. Also, AGP makes use of sideband transfers and pipelining so it can constantly transfer data without depending on other components in the PC.

The pipelining ability of the AGP bus is a key point that explains why it provides a performance advantage. Since AGP pipelines operations it can process quicker and more efficiently than PCI bus can. AGP uses a special organization process for all pending and processing requests. In effect, the bus can process one instruction while still recieving the next instructions. This allows much more to be accomplished in a shorter amount of time.

For a diagram of how the AGP bus is structured, see this diagram provided by Intel Corporation.

One can easily see why the need for a new graphical interface is needed. While PCI served us well, and still continues to do so, it is bogged down by the demand of full screen 3D graphics. It works great for 2D business software and most games, but intense 3D challenges the bandwidth limitations. For true 3D, there is much information that must be transferred for a single image.

AGP, as stated above, uses the main PC memory to store all 3D information, including textures and the Z-buffer. This rids us of a prime problem of PCI video. Textures add reality to what we see on screen. The Z-buffer creates an illusion of depth. Both of these take up loads of memory, and they use the same chunk of memory. Therefore, manufacturers were forced to choose between textures or the z-buffer. Often, they had to design software that was weak in both areas in order to deal with the PCI bus. With AGP, this restriction is gone.

To create lifelike 3D images, the CPU must perform intensive 3D calculations. The graphics controller processes the texture data and bitmaps. In many cases, the controller must read elements from 7 or 8 different textures and average them into a single pixel on the screen. When this calculation is performed, the pixel must be stored in the memory buffer. Because these textures are so large, they cannot be stored on the video card's buffer. With AGP, they instead are stored in the main system memory. Because of this, it is recommended that you have a large amount of system memory in your machine. This should be no problem due to the low prices of RAM. Intel, no doubt, took this into account when they decided to use your RAM for graphics.

To access the texture data from the main memory, AGP uses a technique called Direct Memory Execute, or DIME. In short, this connects the memory directly to the AGP/PCI chipset. This lets the graphics card access the textures in the main memory, which is limited only by the amount of memory you have in your system.

Like PCI, AGP uses a 32-bit connector. But, there is a difference. The AGP connector has 64 contacts, just like the old MCA adapter. AGP uses a 64-bit wide data path. This extra contact provides new roadways for the pipelining and queuing of data requests. Another difference is that AGP uses an extra eight sideband address lines that allows the controller to issue simultaneous commands while also accessing all 32 of the main data pathways. This is called Sideband Addressing, or SBA. All this comes together to give AGP a faster throughput then PCI.

The Requirements

AGP is still in the preliminary stages. Although it works, the support for it is not adequate for most users.

In order to use AGP with the P2, you must have a motherboard with Intel's 440LX or BX chipset. All such boards offer SDRAM support, an absolute must have for AGP. If you want to use AGP with a Socket 7 processor, you'll find yourself using chipsets like the Via Apollo VP3 and the ALI Alladin V.

AGP also requires software support, including both the OS and graphics drivers. Windows 95 and NT4 can be modified to support AGP, but Windows 98 has built-in support. NT5 will have built-in support for AGP. Windows 95 users can get the Windows 95 OEM Service Release 2.1 or a patch program called USBSUPP.EXE. Your current Win95 PCI device driver will support AGP, but you will need to get DirectX5, which is the only version of DirectX to support DIME. You must make sure your video drivers include VGARTD.VXD as well. This is a virtual device driver that turns on the DIME feature.

Most mainstream graphics card vendors have produced AGP versions of their PCI based video cards. Among these are ATI, Diamond, Matrox, NVidia, STB, and Number Nine. These AGP cards are not always all they are cracked up to be. Each vendor implements a different set of 3D instructions and effects. Some vendors implement these effects through software, a practice that negatively effects performance.

Often, these AGP cards come with a large amount of video memory. 4 MB is minimum, with 8 MB being more standard. Some offer support up to 16 MB of RAM. This video memory gives a large amount of space for texture storage.

The Reality

At this point in the development of AGP, I would not recommend going out of your way to upgrade your system to AGP. The benefits over PCI video are definitely staggering, but the hardware required to support it in the first place can be rather staggering to the typical PC user still using a low-end Pentium PC.

Reviews indicate that many AGP video boards do not perform significantly better than their PCI counterparts. With some boards, AGP makes no difference at all. But, these reviews sometimes do not make mention of the fact that, like MMX, AGP requires software to actually take advantage of it. Without proper software support or software that would ever benefit, AGP will provide little improvement.