During Computex we saw photos of several Intel 925x boards equipped with both PCI Express and AGP slots.  Since Intel?s new chipset doesn?t offer simultaneous support for PCI-E and AGP, there was a serious question as to how Gigabyte and ECS (the two manufacturers with boards on display) were accomplishing it. 

At the time, I speculated to Tuan that one or both of the companies might have used an AGP connector on a PCI slot, as the only other solution would seem to be an AGP-to-PCI-Express bridge chip that left only one of the slots useable at a time, the latter option being only a proposed theory, as no such design exists to our knowledge).

As Anand revealed here, both manufacturers have gone the PCI route, and hung an AGP port off the PCI bus.  This is perfectly feasible from a manufacturing standpoint, but the performance penalty should be huge.  Amazingly, SiS claimed only a 5-20% performance penalty from using this solution.

The PCI bus provides 133 MB/s of bandwidth shared between all the PCI slots.  AGP 8x provides a dedicated 2.1 GB/s.  For ECS to claim that a 93.7% drop in available bandwidth only equated to a 20% drop in performance indicated one of two things:  Either the usefulness and necessity of AGP had been fundamentally misrepresented through the years, or ECS was ludicrously optimistic when estimating the performance penalty of an AGP-compatible PCI solution. 

My curiosity piqued, I decided to investigate just how much AGP bandwidth affected modern game and application performance, and, by extension, how much PCI-Express is likely to improve it.  While we won?t be able to directly predict PCI-E performance based on AGP scaling, we can use one to establish a rough trend for the other.

Before we delve into a discussion of AGP performance lets discuss briefly what it is.

The History of AGP:

AGP was developed in the mid-to-late 1990s to address the twin problems of limited PCI bandwidth and the very high cost of video RAM.  At the time, 8 meg of VRAM was the practical limit that could be built into the PCB.  If a card used a 4 meg frame buffer, that left only 4 meg for any sort of processing work.  This was sufficient for basic 2D, but not for higher-level 3D applications. 

AGP addressed both issues simultaneously by creating a dedicated port between video card and processor, a port whose bandwidth would be reserved for only the video card.  In order to facilitate the usage of system memory for video performance, AGP was designed to use DMA.  Although we tend to view ?integrated? video systems with a jaundiced eye today, the original AGP standard was explicitly designed with them in mind.  Given the primacy of 2D applications and the limited amount of available VRAM when AGP was designed, it was believed that frame buffer memory would remain on the video card, while AGP facilitated the use of system memory for 3D performance. 

The original AGP specification was based on the PCI 2.1 spec and ran on a 66 MHz bus.  This explains why its possible for an AGP slot to be fitted on a standard PCI system without much effort?fundamentally, the bus architecture is identical.

The original AGP 1x specification ran at 66 MHz and provided 266 MB/s of bandwidth.  AGP 2x offered 533 MB/s of bandwidth and first appeared in August 1997 with the release of Intel?s 440LX chipset.  AGP 4x first appeared in late 1999 but the high price of RDRAM and unattractiveness of i820 combined with the low performance of VIA?s AGP solutions probably slowed adoption of the standard.  By the summer of 2000 and the launch of Intel?s i815, however, a wide variety of mature AGP 4x chipsets were available.

In August of 2002 VIA launched their KT400 with AGP 8x support.  In five years, available AGP bandwidth had quadrupled from 533 MB/s on the 440LX to 2.1 GB/s using VIA?s KT400.  In a few weeks, Intel will launch the first PCI-Express motherboards whose x16 PCI-E links will offer 4.1 GB/s of memory bandwidth.  In seven years we?ve nearly octupled total video card bandwidth?but how necessary is the gain?  Is PCI-Express needed in the video card industry? 

next >

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• System COnfiguration--Slowing Down to AGP 1x. (1 of 6)
• System Configuration and Details, Cont. (2 of 6)
• 3DMark 2001, 2003, AquaMark, Quake 3. (3 of 6)
• Comanche4, GunMetal, Final Fantasy, SPEC. (4 of 6)
• Unreal Tournament 2003 & 2004, Far Cry, Painkiller. (5 of 6)
• Analysis, Conclusions. (6 of 6)