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NdisMAllocateSharedMemory function

NdisMAllocateSharedMemory allocates and maps a host memory range so that the memory range is simultaneously accessible from both the host system and a DMA NIC.


VOID NdisMAllocateSharedMemory(
  _In_  NDIS_HANDLE            MiniportAdapterHandle,
  _In_  ULONG                  Length,
  _In_  BOOLEAN                Cached,
  _Out_ PVOID                  *VirtualAddress,
  _Out_ PNDIS_PHYSICAL_ADDRESS PhysicalAddress


MiniportAdapterHandle [in]

Specifies the handle input to MiniportInitializeEx.

Length [in]

Specifies the number of bytes to allocate.

Cached [in]

This parameter is ignored (cached memory is always used on x86 and x64 systems).

VirtualAddress [out]

Pointer to a caller-supplied variable in which this function returns the base virtual address of the allocation for use by the miniport driver. If NdisMAllocateSharedMemory cannot satisfy its caller, it returns NULL to indicate that no memory was allocated.

PhysicalAddress [out]

Pointer to a caller-supplied variable in which this function returns a physical address, suitable for use by the NIC, that corresponds to that returned at VirtualAddress, or it returns NULL.

Return value



Note  A miniport driver must have already called NdisMRegisterScatterGatherDma or NdisMRegisterDmaChannel to initialize a scatter/gather DMA channel before calling NdisMAllocateSharedMemory.

Microsoft Windows Server 2003, Windows XP Service Pack 1, and later versions of Windows allow both bus-master DMA NICs and subordinate DMA NICs to call NdisMAllocateSharedMemory. Prior releases allow only bus-master DMA NICs to call NdisMAllocateSharedMemory. In these prior releases, if MiniportInitializeEx did not specify that the NIC is a bus master when it called NdisMSetMiniportAttributes, NdisMAllocateSharedMemory simply returns control without attempting to make an allocation.

NdisMAllocateSharedMemory provides both the mapped virtual address range that the driver uses to access the shared memory block and the NDIS_PHYSICAL_ADDRESS-type range that the NIC uses. A value returned at PhysicalAddress can be doubly mapped by the system. That is, a "physical" address range described by the value at PhysicalAddress and Length can be a range of mapped logical addresses that do not match the host physical addresses for the allocation in every possible platform.

NdisMAllocateSharedMemory can be called only from MiniportInitializeEx. How large an allocation to request depends on how the driver writer, knowing the capabilities and features of the NIC, decides to make the tradeoff between the following performance versus size dilemma:

  • In periods of high network traffic, a miniport driver cannot maintain high I/O throughput if it runs low on shared memory space for device-accessible data buffers.

    For example, the miniport driver could be indicating receive buffers in shared memory faster than such buffers are being returned from bound protocol drivers when a flood of receives come in to a NIC. If all its shared memory space is consumed by outstanding receive buffers, the miniport driver might have to disable receive interrupts on a NIC until it has some shared memory space available for receive buffers.

  • On the other hand, calling NdisMAllocateSharedMemory with a Length chosen to anticipate some maximum transfer demand makes the driver's image larger and its resource usage quite uneconomical, except for rare periods of very high I/O demand. Moreover, NdisMAllocateSharedMemory might not give the driver such a large block if insufficient system memory is available, forcing the driver to fail initialization.

A miniport driver that supplies a MiniportSharedMemoryAllocateComplete function has considerably more flexibility in resolving the preceding performance versus size dilemma. MiniportInitializeEx should allocate only enough shared memory with NdisMAllocateSharedMemory for a moderate demand for network transfer operations through the NIC if the driver has a MiniportSharedMemoryAllocateComplete function. Such a miniport driver can call NdisMAllocateSharedMemoryAsyncEx dynamically to allocate more shared memory in periods of heavier transfer demand on a NIC. When the high demand for transfers subsides, such a driver calls NdisMFreeSharedMemory to release the additional memory it allocated. Note that only bus-master DMA NICs can call NdisMAllocateSharedMemoryAsyncEx and export MiniportSharedMemoryAllocateComplete. This functionality is not supported for subordinate DMA NICs.

NdisMAllocateSharedMemory and NdisMAllocateSharedMemoryAsyncEx are the only NdisXxx functions that can be called to allocate host memory that is shared between the driver, which uses virtual addresses, and a NIC, which uses the corresponding logical addresses.

A miniport driver should align the buffers it allocates from shared cached memory on an integral of the host data-cache-line boundary to prevent cache-line tearing during DMA. Cache-line tearing can cause data-integrity problems in the driver or degrade the driver's (and the system's) I/O performance by requiring excessive data-cache flushing to maintain data integrity. MiniportInitializeEx can call NdisMGetDmaAlignment to determine the alignment boundary in the current platform for device-accessible buffers that the driver will set up within an allocated range of shared memory.

A miniport driver should set a limit on how much shared memory it can allocate. This limit is driver-specific and should be high enough so that the driver does not run out of buffers. Do not set a limit that is excessively high, as this could result in a wasteful consumption of shared memory that could reduce system performance.

MiniportInitializeEx also might call NdisSystemProcessorCount before it calls NdisMAllocateSharedMemory if the driver writer decides to allocate a larger shared memory block in multiprocessor machines on the assumption that any SMP machine is likely to be a network server with higher network-transfer demands on the NIC than a workstation.

If its call to NdisMAllocateSharedMemory fails, MiniportInitializeEx can call again requesting a smaller allocation. However, if MiniportInitializeEx cannot allocate sufficient shared memory for the NIC, it must release all resources it has already allocated and fail initialization.

If the miniport driver subsequently indicates receives with NdisMIndicateReceiveNetBufferLists, it must allocate some number of buffer descriptors from buffer pool that map the NIC's receive buffers in the shared memory block.

If the allocated memory is cached and, therefore, needs to be flushed on transfers, the miniport driver must call NdisAllocateMdl to allocate an NDIS_BUFFER-type descriptor for the shared memory range. The miniport driver must call KeFlushIoBuffers with this buffer descriptor to perform such a flush.

If a miniport driver calls NdisMAllocateSharedMemoryAsyncEx or NdisMAllocateSharedMemory, it must release all outstanding allocations with one or more calls to NdisMFreeSharedMemory when a NIC is removed, that is, when its MiniportHaltEx function is called.


Target platform



Supported in NDIS 6.0 and later.


Ndis.h (include Ndis.h)



See also




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