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System Board Issues: DAC Capabilities for Buses
Various chipsets are capable of supporting more than 4 GB of physical memory. By using PAE, the Windows Datacenter and Advanced Server operating systems can use this memory.
On a 64-bit platform, for optimal performance, all PCI adapters (including 32-bit PCI adapters) must be able to address the full physical address space. For 32-bit PCI adapters, this means that they must be able to support the Dual Address Cycle (DAC) command to permit them to transfer 64-bit addresses to the adapter or device (that is, addresses above the 4 GB address space). Adapters that cannot provide this support cannot directly access the full address space on a 64-bit platform.
Unfortunately, Microsoft is finding that not all PCI buses on a system board support DAC, which is required for a 32-bit PCI adapter to address more than 4 GB of memory. Furthermore, there is no way for a DAC-capable PCI device (or its associated driver) to know that it is running on a non-DAC-capable bus.