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Performing DMA in 64-Bit Windows

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Adding 64-bit addressing support to your driver can significantly improve overall system performance. This is particularly important for device drivers that perform direct memory access (DMA). In 64-bit Microsoft Windows, device drivers that perform DMA but do not support 64-bit addressing are double-buffered, which results in lower relative performance.

Although double-buffering usually has a relatively small impact (single percentage points) on 8 GB systems, this is enough to impact I/O-intensive tasks, such as database activity. As the amount of physical memory increases, this negative performance impact increases as well.

To support 64-bit DMA, drivers should observe the following guidelines:

  1. Use PHYSICAL_ADDRESS structures for physical address calculations.

  2. Treat the entire 64-bit address as a valid physical address. For example, drivers should not call MmGetPhysicalAddress on a locked buffer, discard the high 32 bits, and pass the truncated address to a 32-bit component adapter. This results in corrupted memory, lost I/O, and system failure.

  3. Use the high-performance scatter/gather routines (GetScatterGatherList and PutScatterGatherList) that were added in Windows 2000.

  4. Check the value of the Mm64BitPhysicalAddress global system variable. If it is TRUE, the system supports 64-bit physical addressing.

  5. Set the Dma64BitAddresses member of the DEVICE_DESCRIPTION structure to TRUE to indicate that your driver supports 64-bit DMA addresses.

The DMA routines in 32-bit Windows are 64-bit-ready. If your device driver uses these routines correctly, your DMA code should work without modification on 64-bit Windows.