Article ID: 253233 - Last Review: March 1, 2007 - Revision: 3.2

INFO: NdisQueryMapRegisterCount Is Obsolete on Windows 2000

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This article was previously published under Q253233

SUMMARY

The NdisQueryMapRegisterCount function does not provide useful information on a Windows 2000-based computer.

On a Microsoft Windows NT 4.0-based computer, busmaster NIC miniports call the NdisQueryMapRegisterCount function and obtain information about the total number of available map registers on the system. This information can assist a driver to make decisions about how many map registers to attempt to allocate with the NdisMAllocateMapRegisters function. Regardless of this information, driver writers have had to work within the restriction that they must operate with less than the desired number of map registers.

Under Windows 2000, the NdisQueryMapRegisterCount function does not always return the total number of map registers available in the system; the function may return an invalid value. Driver writers should not use this function.

Here is an example of how driver writers can allocate map registers. Driver writers can call the NdisMAllocateMapRegisters function in a loop, reducing the number of map registers requested from the ultimate desired number to the minimum number acceptable in each iteration until the function succeeds or the loop terminates. If the minimum is not available, the driver should write an event log entry and return an appropriate error code from the MiniportInitialize handler.

Driver writers should also refer to the NdisMInitializeScatterGatherDma function for an alternative DMA approach, where the miniport driver does not have to manage map registers:

Adapter->MapRegisters = MAX_MAP_REGISTERS_NEEDED;

      while(Adapter->MapRegisters > (MIN_MAP_REGISTERS_NEEDED - 1))
      {
             Status = NdisMAllocateMapRegisters(
                            Adapter->AdapterHandle,   // handle input to MiniportInitialize.
                            0,
                            NDIS_DMA_32BITS,          // address size that the NIC uses for DMA operations
                            Adapter->MapRegisters,    // maximum number of map registers
                            MAX_PHYSICAL_MAPPINGS);   // maximum number of bytes that the NIC can
                                                      // transfer in a single DMA operation


              // If Call succeeded, Maximum number of map registers
              // requested have been allocated... else the requested number
              // of map registers could not be allocated due to system

              // resource constraints.

              if(Status == NDIS_STATUS_SUCCESS)
              {
                    break;
              }

              // Else Reduce the request size and try again till you succeed or are out of resources.
              Adapter->MapRegisters--;
      }

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