"TechNET"
The largest driver PAE compatibility issue involves direct memory access (DMA) transfers and map register allocation.
Many devices that support DMA, usually 32-bit adapters, are not capable of performing 64-bit physical addressing. When run in 32-bit mode, the device can address all physical address space. In PAE mode, it is possible that data would be present at a physical address greater than 4 GB. To allow devices with these constraints to function in this scenario, The Windows 2000 Server family and later provide double-buffering for the DMA transaction by providing a 32-bit address that is indicated by a map register. The device can perform the DMA transaction to the 32-bit address and the kernel copies the memory to the 64-bit address that is provided to the driver.
When the system runs with PAE disabled, drivers for 32-bit devices never require their map registers to be backed by real memory. This means that double-buffering is not necessary, since all devices and drivers are contained within the 32-bit address space. Based on testing of drivers for 32-bit devices on 64-bit processor–based computers, it is expected that most client-tested, DMA-capable drivers expect unlimited map registers.
To constrain compatibility issues, Windows XP Service Pack 2 includes hardware abstraction layer (HAL) changes that mimic the 32-bit HAL DMA behavior. The altered HAL grants unlimited map registers when the system is running in PAE mode. In addition, the kernel memory manager ignores any physical address above 4 GB. Any system RAM beyond the 4 GB barrier would be made unaddressable by Windows and be unusable in the system. By limiting the address space to 4 GB, devices with 32-bit DMA bus master capability will not see a transaction with an address above the 4 GB barrier. Because these changes remove the need to double-buffer the transactions, they avoid a class of bugs in some drivers related to proper implementation of double buffering support.
As a result of these changes to the HAL and memory manager, the impact to device driver compatibility is expected to be minimal on systems running Windows XP Service Pack 2 with data execution prevention enabled.