MapTransfer routine

The MapTransfer routine sets up map registers for an adapter object to map a DMA transfer from a locked-down buffer.


  _In_    PDMA_ADAPTER DmaAdapter,
  _In_    PMDL         Mdl,
  _In_    PVOID        MapRegisterBase,
  _In_    PVOID        CurrentVa,
  _Inout_ PULONG       Length,
  _In_    BOOLEAN      WriteToDevice


DmaAdapter [in]

Pointer to the DMA adapter object returned by IoGetDmaAdapter and previously passed to AllocateAdapterChannel for the current IRP's transfer request.

Mdl [in]

Pointer to one of the following: the MDL that describes the buffer at MdlAddress in the current IRP or the MDL that describes the common buffer set up by the driver of a subordinate device (auto-initialize mode).

MapRegisterBase [in]

Specifies the map registers allocated for the DMA operation. The system passes this value to the driver's AdapterControl routine.

CurrentVa [in]

Pointer to the current virtual address of the data to be transferred for a DMA transfer operation.

Length [in, out]

Specifies the length, in bytes, to be mapped. If the driver indicated that its device was a bus master with scatter/gather support when it called IoGetDmaAdapter, the value of Length on return from MapTransfer indicates how many bytes were mapped. Otherwise, the input and output values of Length are identical.

WriteToDevice [in]

Indicates the direction of the transfer operation: TRUE for a transfer from the locked-down buffer to the device.

Return value

MapTransfer returns the logical address of the region mapped, which the driver of a bus-master adapter can use. Drivers of devices that use a system DMA controller cannot use this value and should ignore it.


MapTransfer is not a system routine that can be called directly by name. This routine is callable only by pointer from the address returned in a DMA_OPERATIONS structure. Drivers obtain the address of this routine by calling IoGetDmaAdapter.

The DmaAdapter must have already been allocated as a result of the driver's preceding call to AllocateAdapterChannel.

The number of map registers that can be set up cannot exceed the maximum returned when the driver called IoGetDmaAdapter.

A driver can get the initial CurrentVa for the start of a packet-based DMA transfer by calling MmGetMdlVirtualAddress. However, the value returned is an index into the Mdl, rather than a valid virtual address. If the driver must split a large transfer request into more than one DMA operation, it must update CurrentVa and Length for each DMA operation.

The driver of a bus-master device with scatter/gather support can use the returned logical address and updated Length value to build a scatter/gather list, calling MapTransfer repeatedly until it has used all available map registers for the transfer operation. However, such a driver could more simply use the GetScatterGatherList routine.


Target platform


Available starting with Windows 2000.


Wdm.h (include Wdm.h, Ntddk.h, or Ntifs.h)



DDI compliance rules


See also




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