Tech Files
AGP – From A to … P!
Unveiled by Intel in 1997, the Accelerated Graphics Port (AGP) was rapidly adopted by motherboard manufacturers as a standard component for virtually all PC systems. AGP found quick acceptance among the PC community because of its unique mix of features—very high bus speed and the ability to use system RAM in lieu of dedicated video memory (and of course, who could forget Intel’s extremely clever marketing—a “Let’s-give-it-away-for-free!” tactic to encourage adoption by motherboard manufacturers and consumers).
AGP is not a flash-in-the-pan idea, however; instead, the AGP
port you find on a modern motherboard is governed by a set of highly technical
and precise standards. As is the case with any bit of computer technology,
AGP has undergone several updates and improvements to remedy certain issues
that were discovered in the early versions of the AGP standard.
Before we dive in, I want to clarify where this article is going. Needless
to say, there’s a lot of information available concerning AGP and how
it works. However, this article is going to cover only the aspects of AGP that
come up as issues to technicians. I’m going to completely ignore some
of the more “neat-o!” technologies, such as AGP’s sidebanding
function—not because it’s unimportant but because there’s
really nothing for you as a technician to do about it.
You can consider this more of a “How to Deal With AGP!” article as opposed to “What does AGP do for me?” If you want to learn more about AGP and the bits that make it tick, then check out Intel’s great webpage:
http://www.intel.com/technology/agp
AGP 1.0
Intel released the first version of AGP, appropriately titled “AGP specification 1.0,” in 1997. AGP 1.0 defined the basic components of AGP still used today—a 66 MHz, 32-bit, video-only data bus. The 1.0 specification also defined the connector—a 124-pin slot that provided 3.3 volts to the video card.
If you take a close look at the slot in the figure below, be sure to note the position of the orientation notch on the brown AGP slot. It’s closer to the back of the motherboard. A notch is only seen in that position on 3.3-volt, AGP 1.0 expansion slots.
Figure 1: AGP 1.0 slot (click for larger image)
While the first AGP 1.0 video cards ran at 66 MHz x 32 bits (4 bytes), for a total throughput of 266 MB/s, the 1.0 standard included a 2X spec for AGP to “double pump” and move twice as much data on each clock cycle. This would bring the throughput speed up to 533 MB/s.
The original 266 MB/s speed is called 1X. To get the improved 2X speed, you needed both an AGP 2X capable video card and a 2X capable AGP slot. You can’t tell a slot or a video card’s speed by looking at them. This information is included in the documentation of both the motherboard and the video card. If you see a 3.3-volt AGP slot or video card, however, you know they are either 1X or 2X.
Figure 2: 3.3-volt AGP card (click for larger image)
AGP 2.0
In 1998, Intel released the AGP 2.0 specification. This new spec included a number of improvements, but the two that are most interesting were a change in voltage and a new 4X speed.
The AGP 2.0 specification defined a 1.5-volt power requirement. To prevent users from installing a 3.3-volt card in a 1.5-volt slot (or vice versa), the AGP 2.0 spec defined a new connector. This connector moved the orientation notch towards the front of the motherboard—compare Figure 3 to the Figure 1. The difference is subtle but critical–putting the wrong card in the wrong type of AGP slot could be disastrous for both the card and the slot (and the motherboard that happens to be attached to the slot as well!) The hassle involved with getting the right card for your motherboard became quite a bother for most techs.
Figure 3: 1.5-volt AGP slot (click for larger image)
As you might imagine, this new electrical configuration caused quite an uproar with users and video card and motherboard makers! As a workaround solution, the AGP 2.0 specification also defined a Universal AGP slot that would accept both 3.3- and 1.5-volt cards. You should note that the Universal AGP slot does not any type of orientation mechanism. Universal AGP slots were very common for a a time to provide complete compatibility for AGP cards.
Figure 4: Universal AGP slot (click for larger image)
AGP 2.0 also defined a universal video card connector (not to be confused with a Universal AGP slot!) that would fit both 1.5-volt and 3.3-volt AGP slots. They are distinct with their double orientation notches. This universal video card connecter is very common, like the Universal AGP slot, because it ensures compatibility for video cards across a wide variety of motherboards.
Figure 5: Universal AGP video card (click for larger image)
Finally, AGP 2.0 defined a quad-pumped 4X speed that pushed the throughput up to a whopping 1,066 GB/s.
Fortunately, the AGP 2.0 specification is backwardly compatible—so for example if you put a AGP 4X card into an AGP 1X slot, it will work fine, albeit at the much slower AGP 1X speed. You should always try to match the speeds of your AGP slot and AGP video card to achieve the best possible results.
AGP Pro
One problem with AGP at this time was that the standard AGP slot could only provide about 50 watts of power to any device. While this was (and still is) sufficient for most video cards, higher-end video card makers clamored for more power. To address this need, Intel released an addendum to the AGP 2.0 spec called AGP Pro. The AGP Pro addendum provided for an extension to the AGP slot that could provide either 75 or 110 watts of power to the AGP slot.
AGP Pro slots are visually distinct as they have an extension to both ends of an otherwise standard AGP slot. The slots are usually covered with yellow warning tags to prevent users from trying to install non-AGP Pro cards into the wrong position.
Figure 6: AGP Pro slot (click for larger image)
AGP Pro slots also come in 3.3-volt (rare), 1.5-volt, and universal variation. The AGP Pro slot in the figure is universal – no orientation notch. Also notice that the AGP Pro slot in the figure above has had the front warning tag removed. This is a very common practice. AGP Pro slots accept regular AGP cards (assuming the voltage is correct) and most regular AGP cards come with a registration tab on the back of the video card’s AGP connector that fits into the front part of the AGP Pro slot. This registration tab has no pins –it simply helps support the card. Just in case you are wondering, AGP Pro cards will not work in a regular AGP slot!
Figure 7: AGP Pro video card (click for larger image)
Note the strange double-wide card! The AGP Pro standard defines that 75 watt cards need to be two cards wide and the 110 Watt cards be THREE cards wide. They only use the one AGP slot but they need the extra space to give these cards plenty of room for cooling fans, heatsinks, etc. Think how hot a 75 or 100 watts light bulb can get and you’ll understand why they made that requirement!
AGP Pro slots are not uncommon but they are in the minority. AGP Pro cards are quite rare, as video card makers have greatly reduced the power needs of high-end video cards.
AGP 3.0
In 2002, Intel released the greatest, and probably the last, AGP specification: AGP 3.0. AGP 3.0’s major improvement is an8X speed, boosting the throughput of the AGP bus to slightly over 2 GB/s. Of course, the AGP 3.0 specification includes many other critical improvements, including support for multiple AGP ports, but these extras are transparent from a tech’s standpoint.
AGP 3.0 also defines an 0.8-volt signaling voltage. Luckily, the AGP 3.0 specification allows for AGP 3.0 video cards to use standard 1.5-volt AGP slots. Of course, you’ll still need a motherboard with AGP 3.0 compliant AGP slots to take advantage of the extra power of this new specification.
Given that AGP 3.0 video cards use standard connectors, there’s nothing to show you since AGP 3.0 cards look exactly like any other 1.5-volt AGP card.
Where do we go from here?
Even though AGP 3.0 is still rather new (at this writing in late 2003), a replacement is already on the horizon in the form of the upcoming PCI Express standard. PCI Express is a serial expansion bus with throughput of up to 8 GB/s as well as host of “Oooo Aaah” features like hot-swapability. With new technology being developed that aims to replace AGP, it appears that AGP technology is living on borrowed time.