Could someone help me read this information better, I am suspicious of this line: Memory at d0000000 (64-bit, prefetchable) [size=256M]

My video card is supposed to have 1GB of memory, however I'm not sure if this output reflects that or if I'm looking at the wrong thing?

01:00.0 VGA compatible controller: ATI Technologies Inc Cypress [Radeon HD 5800 Series] (prog-if 00 [VGA controller])
    Subsystem: ATI Technologies Inc Device 0b00
    Flags: bus master, fast devsel, latency 0, IRQ 44
    Memory at d0000000 (64-bit, prefetchable) [size=256M]
    Memory at fbee0000 (64-bit, non-prefetchable) [size=128K]
    I/O ports at d000 [size=256]
    Expansion ROM at fbec0000 [disabled] [size=128K]
    Capabilities: [50] Power Management version 3
    Capabilities: [58] Express Legacy Endpoint, MSI 00
    Capabilities: [a0] MSI: Enable+ Count=1/1 Maskable- 64bit+
    Capabilities: [100] Vendor Specific Information: ID=0001 Rev=1 Len=010 <?>
    Capabilities: [150] Advanced Error Reporting
    Kernel driver in use: fglrx_pci
    Kernel modules: fglrx, radeon

I wouldn't worry about this in that it happens to the best of us. My card has 1536 MB of onboard video RAM yet this is what lspci shows:

03:00.0 VGA compatible controller: nVidia Corporation GF110 [GeForce GTX 580] (rev a1) (prog-if 00 [VGA controller])
        Flags: bus master, fast devsel, latency 0, IRQ 16
        Memory at f9000000 (32-bit, non-prefetchable) [size=16M]
        Memory at e0000000 (64-bit, prefetchable) [size=128M]
        Memory at ee000000 (64-bit, prefetchable) [size=32M]
        I/O ports at 7f00 [size=128]
        [virtual] Expansion ROM at e8000000 [disabled] [size=512K]
        Capabilities: <access denied>
        Kernel driver in use: nvidia
        Kernel modules: nvidia_current, nvidia_current_updates, nouveau, nvidiafb

There are filed bugs and a billion and five forum threads (all around the internet, not just Ubuntu) about this.

I'm personally of the opinion that this is a lspci bug (or feature omission) and that it doesn't actaully effect anything negatively. For nvidia users, I trust nvidia-settings (or Catalyst for the AMD/ATI users) when it reports the value correctly:

enter image description here

X also knows:

oli@bert:~$ grep -i memory /var/log/Xorg.0.log
[    26.504] (--) NVIDIA(0): Memory: 1572864 kBytes
[    26.585] (II) NVIDIA: Using 3072.00 MB of virtual memory for indirect memory
[    26.777] (==) NVIDIA(0): Disabling shared memory pixmaps
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Me the same, I think 256M means mega but not megabytes, I think it means we have 256 mega(1024x1024) blocks of 64 bit addresable memory words thats 8 bytes so:

256M * 8b = 2048Mb (2Gb)

that's double as expected now, so it must mean that of those 64 bits is a trick and only 32 bits are really in physical memory

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