Sarasota PC Monitor
Tech Talk (5/05)
A Graphics Card Primer
by Brian K. Lewis, Ph.D.*
Member of the Sarasota Personal Computer Users Group, Inc.It used to be very simple when you wanted to upgrade your graphics or video card. All you had to consider was whether you had an IDE slot or a PCI slot for your expansion cards. Then Intel came along and decided that something faster was needed and that became the AGP or Accelerated Graphics Port. That was too bad, it meant you had to have an expansion slot on your motherboard that was different from all the others in appearance. This slot would only accept an AGP graphics card. But of course, it didn't end there. The next thing you know there was an AGP 2x, then an AGP 4x, next an AGP 8x.
The worst of all this is that the cards were not interchangeable and, under some circumstances not even backward compatible. In fact, the wrong card in the AGP slot drawing the wrong voltage could not only destroy the graphics card, it could destroy your motherboard. So after causing all this confusion, guess what Intel and the other manufacturers have come up with? It's called PCI Express or PCIe and it's not compatible with any of the older expansion cards. In fact, the plans are to convert all the expansion slots on new motherboards to PCIe and then you'll have to upgrade all your expansion cards or buy a complete new computer system. Guess who's laughing all the way to the bank?
So now you need to educate yourself on the various forms of graphics cards so you won't waste your money anytime you want to upgrade. It may also be useful when you start looking for a new computer. After all, the transition to this new setup will take some time. It will probably take until 2007 for all the manufacturers to get on board. By then there will be something newer on the drawing boards.
Let's start by looking at the AGP card. To understand the functioning of these cards we need to clarify some of the terminology. Every PCI expansion card communicates with the central processor (CPU) via a "bus". This bus is the electrical pathway that transmits data packets from any device connected to the bus to the CPU. Thus every PCI slot is connected to the CPU via this bus. Multiple packets from different sources (slots) can be transmitted at the same time. When they arrive at the CPU each data packet is processed one at a time. The standard PCI bus is 32 bits wide, operates at 33 MHz and provides a maximum bandwidth of 132 MB/s. This bandwidth is shared by all devices connected to the PCI bus. So the maximum amount of data being transmitted from all the devices is 132 MB/s. The very first AGP card also had a 32 bit bus width, but it operated at 66 MHz. Since no other devices are connected to the AGP port, it has a dedicated path connected directly to the CPU and to system memory. This immediately doubled the bandwidth for graphical data to 266 MB/s.
The AGP card provided additional advantages. The AGP card can receive and reply to multiple packets of data in a single request. With PCI bus the graphics card could only receive single packets at a time and it would have to wait until all the information was received before it could be sent out. A third advantage is called "sideband addressing". With the PCI bus the complete data packet must be examined to determine where the data should go. With AGP port, the addressing is included in eight additional lines of information that are essentially outside the data packet. This speeds up the reading of the address to which the data packet is to be transmitted.
If any of the graphics images you view on your video monitor are 3-D images, the AGP cards improves this also. The 3-D images are built with a "texture map". With PCI graphics the texture map had to be stored twice. With AGP, the map is stored only once in a "Graphics Address Remapping Table" or GART. The GART communicates directly with the CPU and replaces the dual storage required by the PCI system. With all these changes, there is no doubt that the AGP cards provide a considerable improvement over the PCI graphics system.
So why were the other versions of AGP developed? Speed, naturally! As demand grew for faster rendering of graphics images manufacturers came up with faster AGP cards. The demand wasn't just for people playing games on their computers. The increase in the number and complexity of images on web pages was probably just as important in pushing the speed envelope for graphics cards. User problems with the newer cards did not relate to the increases in speed, but to the differences in voltages required by the newer generations of AGP cards. The original AGP card used a 3.3v connection. AGP 2x cards used a 1.5v connection but could also work with 3.3v. This dual voltage setup is covered by an AGP specification called AGP 2.0.
Then came AGP 4x that works only with a 1.5v connection. That was followed by AGP 8x that uses 1.5v but only requires 0.8v for signaling. Now there is a 4x/8x combination, covered by AGP specification 3.0. This newer version of the 4x/8x slot will work with either 4x or 8x graphics cards. Don't confuse the card designations (using an x) with the overall specifications using the decimal point. So you have three different AGP specifications. It is essential that you know exactly which specification is supported by your motherboard before you start shopping for a new graphics card. If your owner's manual doesn't have the information then you will have to check with the computer or motherboard manufacturer.
So now that we have some background on the AGP cards, what have these changes done to the speed of data transmission? This table shows why you want a 4x or 8x AGP card. However your motherboard must support the 4x/8x voltages or your cards won't work.
AGP 1x mode = 266MB/s (megabytes per second data transfer rate)
AGP 2x mode = 533MB/s
AGP 4x mode = 1.07GB/s (gigabytes per second. 1GB/s = 1024MB/s)
AGP 8x mode = 2.14GB/s
As usual the manufacturers weren't satisfied with their success. Now, there is a new standard on the way. This is the PCI express bus that I mentioned earlier. This new bus uses a two-way serial connection to optimize transmission between the device and the CPU. More new terms: a link is the connection between the PCIe device and any other device such as the CPU or system memory. Each link can be composed of 1 to 32 lanes. As many lanes as needed can be assigned to any device. So a graphics card needing extra speed and bandwidth compared to slower devices, such as a network adapter, could be assigned 16 lanes. That would enable a usable bandwidth of 4 GB/s!
Other devices that might be able to use the extra bandwidth of PCIe are gigabit Ethernet controllers or RAID controllers. Consequently, it appears all the expansion slots on future motherboards will be PCIe.
So you can see that CPU's are not the only things changing in the computer world. How many of you have upgraded your computer's hard drive to an SATA drive? If you buy a computer anytime after this year, you'll be getting SATA drives and AGP 8x or possibly PCIe expansion slots. If you are really into graphics you'll probably want a motherboard which doesn't have an integrated graphics card. Oh yes, remember when graphics cards had 8 or 16 MB of RAM. Now you should be shopping for 128 to 256 MB of DDR RAM. Just another little change for you to consider.
Happy upgrading!
*Dr. Lewis is a former university & medical school professor. He has been working with personal computers for more than thirty years. He can be reached via e-mail at bwsail@yahoo.com.
Copyright 2005. This article is from the May 2005 issue of the Sarasota PC Monitor, the official monthly publication of the Sarasota Personal Computer Users Group, Inc., P.O. Box 15889, Sarasota, FL 34277-1889. Permission to reprint is granted only to other non-profit computer user groups, provided proper credit is given to the author and our publication. We would appreciate receiving a copy of the publication the reprint appears in, please send to above address, Attn: Editor. For further information about our group, email: admin@spcug.org/ Web: http://www.spcug.org/The Sarasota Personal Computer Users Group, Inc. has 1,100+ members and was established in 1982. We are members of the Assoc. of PC User Groups (APCUG), the Florida Assoc. of PC Users Groups, Inc., and we are members of the America Online Ambassador Program.
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