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Why is the following memory address range on the PCI bus?

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The image is from my Netbook that uses shared video memory with no dedicated video memory so I am unsure as to why the VGA memory range would be on the PCI bus. Wouldn't this range need to go to system memory which then the PCI device reads from?

This isn't an issue but rather just curiosity as to why it is.

Any help would be appreciated.


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Because that particular GPU is connected to the PCI bus. –  Ramhound Mar 2 '14 at 0:26
@Ramhound Thanks, I understand that but the GPU has no dedicated VRAM so I don't know why that address would be on the PCI bus. –  RJSmith92 Mar 2 '14 at 0:31
Note it's 128k and is likely PCI device configuration registers. –  Brian Mar 2 '14 at 2:07
Makes perfect sense to me –  Ramhound Mar 2 '14 at 3:14
Brian and Ramhound, the questioner actually stated in the question and again in a comment what the memory range was. It's not device configuration registers. Xe knows what the memory is. Xe wants to know why Device Manager shows Windows to have associated it with the PCI bus. –  JdeBP Mar 2 '14 at 11:33

1 Answer 1

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Your graphics adapter doesn't have literally no dedicated memory at all. If it did, how would it know what it was in the middle of doing? If it was, for example, operating from a particular area of main memory, how would it know which area of main memory that was or what it was using it for? This is a very small mapping for the exchange of control information between the GPU and CPU. It is implemented as a very small amount of special function memory on the GPU, made visible to the CPU through a PCI memory mapping.

This is basically the same way it's done if the GPU does have dedicated memory, except the mapping would typically be larger. Though, generally, it doesn't include all of the video memory -- there's no need for that. Small windows into the video memory are sufficient.

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Thanks for the answer. You're right, I should have said in my question that the controller has no dedicated VRAM. I understand it has a small amount of memory for configuration but I still don't understand why this VGA memory range is on the PCI bus? –  RJSmith92 Mar 4 '14 at 10:41
Just because it's easier to do it that way than to invent a new interface for it. –  David Schwartz Mar 4 '14 at 10:49
Thanks again, The initial confusion came because I know 0xA000 to 0xBFFFF is in system memory and yet Windows shows this to be on the PCI bus. So even when Windows shows this address range to be on the PCI bus, when anything writes to this address the chipset will write it to system memory from which the PCI card will then read from? ...and the only reason Windows shows the address on the PCI bus is because it is a PCI device? –  RJSmith92 Mar 11 '14 at 12:05
Is my above post correct? thanks. –  RJSmith92 Mar 17 '14 at 21:30

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