总线主控pci设备的DMA

对于总线主控pci设备的DMA

PDMA_ADAPTER
  IoGetDmaAdapter(
    IN PDEVICE_OBJECT  PhysicalDeviceObject,
    IN PDEVICE_DESCRIPTION  DeviceDescription,
    IN OUT PULONG  NumberOfMapRegisters
    );

最后一个输出参数NumberOfMapRegisters，啥意思？

分配内存缓冲区是用
PVOID
  AllocateCommonBuffer(
    IN PDMA_ADAPTER  DmaAdapter,
    IN ULONG  Length,
    OUT PPHYSICAL_ADDRESS  LogicalAddress,
    IN BOOLEAN  CacheEnabled
    );
函数吧？

高手帮忙解答一下，谢谢！

the maximum number of map registers that the driver can allocate for any DMA transfer operation.