AGP - PCI interesting read.

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pie

Banned
http://members.hyperlink.net.au/~chart/agpinfo.htm

The 'traditional' PCI bus that we are all very familiar with is rated to a maximum speed of 33 MHz. (without overclocking). The PCI bus standard, however, allowed for an optional extension of the design (with a longer connector) which operates at 66 MHz. This optional extension of the standard was very rarely ever implemented by anyone, even though it offered a theoretical doubling of PCI performance. (I saw it once on a Gigabyte board, I think, but I've never seen a 66 MHz card, ever).

When the AGP standard was designed, very little was altered from the PCI standard (which AGP is based on). The main changes that AGP implemented were to finally recommend the utilization the optional 66 MHz extension offered by the PCI standard, and for some reason the connector was also changed physically (electrical signal-wise it's virtually identical). AGP also introduced a new host bridge standard that removed some of the bottlenecks between the AGP bus and system memory, which in itself accounts for some speed improvements on 3D cards, but really doesn't help things any for text or 2D graphics. (It was mainly put in place to allow fast updating of textures on 3D accelerators).

The downfall of this simplistic 're-hashing' of PCI to create AGP was that the standard was actually too flexible. Rather than making 66 MHz mode a requirement, it was left optional (just like in the PCI standard)!! What all this means that a hardware designer can take an existing PCI design and re-hash it to work on AGP in a very short space of time, with virtually no alteration of the design at all. Think about that for a moment. It may physically be an AGP card, but it doesn't have to support 66 MHz. If it isn't running at 66 MHz, it must be running at 33 MHz; the exact same speed as a PCI card!!! In other words, there is ZERO percent improvement over a PCI based adapter!!!

Granted, the AGP bridge may offer some improvement by way of bottleneck reduction, but this improvement is probably negated by the fact that the same software drivers, unaltered, are undoubtedly also in use. If the drivers aren't optimized for AGP, then there is a huge wasted potential since all the 'extra' features of AGP can't be called on: i.e. AGP 2x mode, sidebanding, fastwrite transfers, etc etc.
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that reviewer is a dumb ass. 66mhz PCI cards are very common in the server space. 66mhz PCI slots are also backwards compatible with 33mhz cards and have been shown to offer significantly better performance. there was a site awhile back that stuck a promise 33mhz RAID card in a 66mhz slot and benched it versus the same card in a slower PCI slot. The results were around 60% faster if i remember correctly. It was theorized that the increase in performance was due to the DPU being able to talk to the card faster and more frequently than when on a 33mhz slot.

The problem though is that things like sound cards and add in IDE cards etc.. just dont max out the 33mhz bandwidth. Also with things like hypertransport even the once saturated PCI bus (saturated from multiple IDE cards sound cards and the advent of USB was beginning to clog the once hampered PCI bus) there arent many situations a consumer would require a 66mhz slot.
 
pie said:
http://members.hyperlink.net.au/~chart/agpinfo.htm
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Uh... what the hell? 64-bit 66MHz PCI cards are everywhere. So are 64-bit 33MHz PCI cards. And PCI-X 133 cards.

This guy doesn't have a clue.

Examples:

64/66 PCI: Adaptec 29160
64/33: 3ware Escalade 7500-4
PCI-X 133: Adaptec 29320

Not to mention countless other RAID/SCSI/network/fibre channel cards.

I honestly didn't read any farther beyond the part where he said that it was rarely implimented, that he only saw it once (on a Gigabyte board :lol: ,) and that he had never seen a 64-bit card. Kinda makes me think he doesn't know what he's talking about.
 
is there a date on that webpage? i could have been true at the birth of agp.. but not anymore.
 
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