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On older chip-sets it was not possible to have a dedicated graphics card in the PEG slot and have the Integrated Graphics Device (IGD) enabled at the same time.

After looking online I found this was because they "Shared the same bus lanes" so it was either one or the other and the BIOS would disable the IGD if it detected a card in the PEG slot.

I was just wondering why PCIe devices can't share the same lanes. It is my understanding that with the old PCI standard all devices shared the same lanes so why is this not possible with PCIe?

I know PCIe is serial and creates point-to-point connections between the Switch and 'End Devices' but I don't understand why if the IGD and Dedicated card share the same lanes why they can't work together?

What part of the PCIe design prevents this?


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I was just wondering why PCIe devices can't share the same lanes.

PCIe uses a point-to-point topology, so each lane expects one device on each end. If it wasn't, it'd be something like the original PCI - one issue with plain PCI is that when one device is talking on the bus, others have to wait.

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Thanks for the answer. I was sort of wondering why the lanes that are used for either the PEG card or the IGD could not be used in some sort of older PCI mechanism where they share same lanes even though they are PCIe devices. Is it just a design requirement with PCIe that each lane can only have one device at the end? – RJSmith92 Apr 19 '14 at 19:57
@RJSmith92 - I wouldn't call it a design requirement it simply was a design decision. – Ramhound Apr 19 '14 at 20:20
@Ramhound Ye, I could have phrased it better. Thanks for the help. – RJSmith92 Apr 19 '14 at 20:24
@RJSmith92 - I was going to make an example to your typical "highway" where a car can't "literally" share a lane with another car. There is always a car in front of you and behind you but two cars can't have equal weight. PCIex16 is 16 lanes but a single device can only use it, think that more of say a shipping "company" fleet all going to the same location. So each PCIex4, PCIex8, PCIex16 would be a single highway for each "shipping" company. – Ramhound Apr 19 '14 at 20:39
@Ramhound Thanks, my point was why when using PCIe you could not have 2 devices on the same lanes (like PCI did) and each device only responds when it's addressed and the devices share the bandwidth. Like you said it will be part of the PCIe design that this is not possible. – RJSmith92 Apr 19 '14 at 20:55

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