[AMDBulldozer]

Размеры буфера DMA резервируемого системой для видеокарты на сегодняшний день не зависят от объема её собственной памяти.
Давайте сравним суммарный размер адресных окон карты с 1.5ГБ памяти:

1.5GB VRAM

Код:

# lspci -v -s 3:0.0
03:00.0 VGA compatible controller: NVIDIA Corporation GF110 [GeForce GTX 580] (rev a1) (prog-if 00 [VGA controller])
        Subsystem: NVIDIA Corporation Device 086a
        Flags: bus master, fast devsel, latency 0, IRQ 26
        Memory at f8000000 (32-bit, non-prefetchable) [size=16M]
        Memory at c0000000 (64-bit, prefetchable) [size=128M]
        Memory at c8000000 (64-bit, prefetchable) [size=32M]
        I/O ports at d000 [size=128]
        [virtual] Expansion ROM at f9000000 [disabled] [size=512K]
        Capabilities: [60] Power Management version 3
        Capabilities: [68] MSI: Enable- Count=1/1 Maskable- 64bit+
        Capabilities: [78] Express Endpoint, MSI 00
        Capabilities: [b4] Vendor Specific Information: Len=14 <?>
        Capabilities: [100] Virtual Channel
        Capabilities: [128] Power Budgeting <?>
        Capabilities: [600] Vendor Specific Information: ID=0001 Rev=1 Len=024 <?>
        Kernel driver in use: nvidia

и карты, у которой видеопамяти в 4 раза больше:

6GB VRAM

Код:

]# lspci -v -s 1:0.0
01:00.0 VGA compatible controller: NVIDIA Corporation GK110 [GeForce GTX Titan] (rev a1) (prog-if 00 [VGA controller])
        Subsystem: ASUSTeK Computer Inc. GTXTITAN-6GD5
        Flags: bus master, fast devsel, latency 0, IRQ 32
        Memory at fa000000 (32-bit, non-prefetchable) [size=16M]
        Memory at d0000000 (64-bit, prefetchable) [size=128M]
        Memory at d8000000 (64-bit, prefetchable) [size=32M]
        I/O ports at e000 [size=128]
        [virtual] Expansion ROM at fb000000 [disabled] [size=512K]
        Capabilities: [60] Power Management version 3
        Capabilities: [68] MSI: Enable- Count=1/1 Maskable- 64bit+
        Capabilities: [78] Express Endpoint, MSI 00
        Capabilities: [100] Virtual Channel
        Capabilities: [128] Power Budgeting <?>
        Capabilities: [420] Advanced Error Reporting
        Capabilities: [600] Vendor Specific Information: ID=0001 Rev=1 Len=024 <?>
        Kernel driver in use: nvidia

Как легко видеть, объемы видеопамяти разные, а в адресном пространстве процессора они используют одинаковый по размеру диапазон адресов - 128+32+16=176МБ.