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I am using a Dell Vostro 1700 laptop which currently has 2GB of RAM. I was considering buying some more memory to upgrade it to 4GB. I am running 32 bit Windows Vista and I know that there can be issues that prevent it from making use of a full 4GB which I believe relate to the fact that memory mapped devices, e.g. graphics card, also need to be allocated addresses in the 4GB range addressable with 32 bits.

Consequently I was looking at device manager -> resources by connection -> memory to see what devices where allocated what memory addresses. I was surprised to see that there was an entry for [80000000 - F3FFFFFF] PCI bus. That is a 1.8GB range of addresses. When I expanded it the only thing in it was [E0000000 - EFFFFFFF] NVIDIA GeForce 8600M GT which is only 256MB.

So my question is does the PCI bus really occupy 1.8GB of address space and will it prevent my computer from making use of any more memory than it already has.

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2 Answers 2

What you are seeing is sections of the memory space mapped to devices on the PCI bus, colloquially known as the "PCI Memory Hole".

It's used to communicate with devices on the PCI bus. Basically, the contents of the memory in those addresses represent the status of your PCI devices, and by reading/writing them, the processor can sens/receive data from the PCI devices.

So, no, your PCI devices are not using 1.8GB of ram, as their address spaces are outide of the address range of your ram.

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I accept that the PCI bus as indicated above is outside of the address range of my ram ... at the moment. My question was really about what happens when I add more ram. –  Neil May 14 '10 at 11:04
    
Just to add some background, a friend of mine had a similar situation on his desktop and when he added more ram none of it showed up in the task manager. The total physical memory there was unchanged. I am trying to figure out if this is why and if the same thing will happen to me or whether his problem was related to something else. –  Neil May 14 '10 at 11:08
    
Huh, are you sure the ram was good? Also, the wikipedia article on PAE ( en.wikipedia.org/wiki/Physical_Address_Extension ) is an excellent resource on how this stuff works. Apparently "32" bit systems are actually 36+ bit, and the other sections are hidden by the OS, only exposing 32bit-~1000MB to each thread. This lets the OS map different sections of memory to different processes, which partly circumvents the 32 bit limitations. –  Fake Name May 15 '10 at 9:55
    
Yes, I am sure the ram was good because it is now running fine and seeing all the ram after upgrading to a 64 bit OS. –  Neil May 15 '10 at 22:58

Ummm I highly doubt it :) My Toshiba L300-1AS registers 3GB ram when it has 4GB remembering that 32bit OS only supports 4GB wasnt really a problem for me with inspection through start > run > dxdiag i discovered that my computer accesses 1341Mb of my RAM for graphics, but i just accept it, My computer definitely runs faster with the extra 2GB i know this doesnt quite address your error but i couldnt locate the PCI bus to work out if i was having the same results as you

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