Messages in this thread | | | Date | Fri, 20 Oct 2000 20:35:53 -0700 | From | "Timothy A. Seufert" <> | Subject | Re: Any dual AGP slot motherboards? |
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>So they started with the PCI spec, but they changed the logical meaning >of a lot of the bus signals, they added a lot of bus signals, they run it >about 8 times faster, they changed the voltage and a bunch of other stuff. >I say it is a different animal now.
Sorry to post yet another correction, but you've misinterpreted the AGP spec a bit. To really understand what's going on, you must read farther into the spec.
AGP adds "sideband" signals to 100% standard 66 MHz 3.3V PCI. The sideband signals are for setup, control, and clocking of AGP mode transactions. AGP transactions re-use the PCI data pins for data transfer, but at no time is the PCI logical or electrical protocol violated. During AGP transfers, the PCI state machines on both sides are simply told the bus is busy.
At least as of AGP 2X, AGP was such a compatible superset of PCI that it was possible to connect an AGP 2X device to a PCI bus and have it work. You can't do that if you want to use AGP transfers, since they require a point-to-point bus, but if the AGP pins are left unconnected the device act as a normal PCI device.
As a matter of fact, most if not all current PCI video cards have the same chips as the respective AGP versions. I personally own an ATI PCI video card which has "AGP 2X" silkscreened on top of its Rage Pro chip. This was likely a key element of Intel's plan for marketing AGP to the industry; AGP would have been a much harder sell if it had forced companies to develop different chips for the AGP and PCI versions of their cards.
It does sound like they've finally changed signalling levels for AGP 4X. If PCI-mode transfers have to use the new electrical spec, it's the first time the PCI part of AGP has departed in any way from standard PCI. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org Please read the FAQ at http://www.tux.org/lkml/
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