19.3  How PCI Direct DMA Works in OpenVMS Version 7.2

  Galaxy and memory hole considerations force us to possibly
  change the placement of the Direct DMA window.

  Memory:                                        PCI
          |        |
          |        |
          |        |
  IOC$K_DIRECT_DMA_BASE_PA:
          __________-\
          |        |   \                      |        |
          |        |    \                     |        |
          |        |     \                    |        |
          |        |      \----------------  |------  |1Gb  (IOC$K_DDMA_BASE_BA)
          |-------  |-\                       |        |    \
          |        |   \                      |        |     \
          |        |    \                     |        |       IOC$K_DDMA_WIN_SIZE

          |        |     \                    |        |     /
          |        |      \----------------  |_______|2Gb-1

  It is unknown from the drivers perspective where in memory
  the base of the Direct DMA window will be.  Simply com-
  paring a buffer address against the length of the window
  will no longer be sufficient to determine if a buffer is within
  the Direct DMA window.  Also, comparing against mmg$gl_
  maxpfn will no longer guarantee that all of pool is within the
  window.  The correct cell to check is mmg$gl_max_node_
  pfn.  Additionally, alignment concerns may require an slightly
  different offset be incorporated into physical bus address
  calculations.