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Reading on the AGP spec, the little bits I've found on sites like https://old.pinouts.ru/Slots/agp_pinout.shtml, say:

The Accelerated Graphics Port (also called Advanced Graphics Port) is a high-speed point-to-point channel for attaching a single device (generally a graphics card) to a computers motherboard, primarily to assist in the acceleration of 3D computer graphics. Many classify AGP as a type of computer bus, but this is something of a misnomer since buses generally allow multiple devices to be connected, while AGP does not. AGP originated from Intel, and it was first built into a chipset for the Pentium II microprocessor. AGP cards generally slightly exceed PCI cards in length and can be recognized by a typical hook at the inner end of the connector, which does not exist on PCI cards. Nowdays AGP is almost replaced by PCI-Express.

This tells me that the connector is, in fact, more generic and just a high-speed peripheral connect to the CPU. This has obvious advantages for graphics processing which needed the higher bandwidth of the time, but I can think of several other devices which would enjoy that connection speed (capture cards, disk controllers, cache devices, etc).

Were there reasons this wouldn't be done, and are there examples of hardware utilizing the AGP connector in this manner?

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    I am a little distressed that AGP is retro computing now! – trognanders Nov 13 '20 at 8:24
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    @trognanders: AGP can legally drink in the USA – Bryan Boettcher Nov 13 '20 at 18:52
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    OMG, AGP on the RC SE... I am ooold... – peterh - Reinstate Monica Nov 13 '20 at 20:58
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    Put another way, AGP was released closer to the PC’s creation than to this point in time. – Stephen Kitt Nov 13 '20 at 21:56
  • Doesn't really count as it's still a graphics card, but ATI's AGP-based 8500DV card was able to offer a FireWire port. techrepublic.com/article/… – Max Barraclough Nov 14 '20 at 12:57
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I researched this question online fairly thoroughly a while back. I could not find any reference to an AGP device that wasn't a graphics card. It wasn't exhaustive, and absence of proof is not proof of absence, but I strongly suspect no such cards were made. I also think there are technical and economic reasons that make it unlikely. It would have been a rather strange product, with no clear target market.

The main feature of AGP over PCI is the addition of an DMA channel to main memory. Importantly, this is an exclusive channel, not a proper bus. You can have only one AGP slot. So the only real market for such a card would be for computers that don't need graphics, such as servers.

But the server market already had high-bandwidth buses like PCI-X, and they were actual buses, so you could, for example, have both a high-speed SCSI and a Gigabit Ethernet card in the same machine. I suspect most customers who wanted a very high-end peripheral at the time, like a Gigabit Ethernet card, would not want to stick it in a low-end desktop motherboard with only one high-speed slot.

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    I'm fairly certain I've seen motherboards with two AGP slots. I know AGP was an exclusive channel so I assume it only allowed switching between one of the two cards during POST or had a jumper on the motherboard - or is that a false-memory of mine? – Dai Nov 12 '20 at 3:55
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    UPDATE: Ah, found it - the HP ES80. It had two AGP slots because it was technically two separate computer nodes on the same board. – Dai Nov 12 '20 at 3:56
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    The setup (dedicated DMA to main RAM) would be useful in the lab for data-capture or arbitrary waveform generation, but the transient recorders I've used have all been since PCIe (and GigE) were viable options. It seems like historically AGP wasn't used for capture, but machines built for it used AGP for graphics to free up their fastest general-purpose buses for the experimental hardware – Chris H Nov 12 '20 at 11:37
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    What do you mean by "DMA channel to main memory"? Actually every PCI device was able to do DMA to main memory, given the physical address of that memory for its own DMA engine. – lvd Nov 12 '20 at 17:25
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    @lvd Sure, PCI can do DMA too. But AGP can do it quickly enough to stream textures directly from system RAM. So, I meant an additional, dedicated DMA channel. – RETRAC Nov 12 '20 at 19:31
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Yes, because it did not provide the necessary connections for wider use

AGP had asymmetric bandwidths. It was very much faster in transferring data from the CPU to the graphics card. For transferring data back from the graphics card to the CPU though, it was no faster than a PCI slot.

For graphics cards, this is exactly what you need. You typically do not need to read much data back from the graphics card. Most other applications need symmetric bandwidths in both directions, and AGP simply was not designed for this.

This was a major obstacle to graphics cards being useable for more general processing, initially with more custom code and later with CUDA. Whilst input data could be transferred more quickly, retrieving output data was no faster than a regular PCI card. Since motherboards with more than one AGP card were very rare, there was no incentive for manufacturers to use AGP.

Lessons learnt from AGP are why PCIe was developed, because the industry realised the missed potential of needing a faster bus system which could be used by everyone.

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