>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.
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This archive was generated by hypermail 2b29 : Fri Oct 20 2000 - 23:31:57 EDT