DOCUMENT:Q101349  13-JUL-1993  [W_NT]
TITLE   :INF: How DMA Device Access Memory Greater then 16 MB
PRODUCT :Windows NT
PROD/VER:3.10
OPER/SYS:WINDOWS
KEYWORDS:

--------------------------------------------------------------------
The information in this article applies to:
 
 - Microsoft Windows NT operating system version 3.1
 - Microsoft Windows NT Advanced Server version 3.1
--------------------------------------------------------------------
 
Enabling DMA devices to access memory beyond 16 MB is handled by the
hardware abstraction layer (HAL). When Windows NT boots, the HAL
reserves some memory in the lower 16 MB. When a DMA device
initializes, it informs the HAL of its characteristics and
limitations, such as only 24 address lines (maximum 16 MB of address
space). Later, when the DMA device needs to do DMA, it requests the
physical address of the buffer using the IoMapTransfer call. The HAL
checks to see if this buffer is accessible to the device. If the
buffer cannot be reached by the device the HAL uses one if its pages
instead. If the operation is a write, then the data is copied from
high memory to the lower memory. When the DMA device has completed the
data transfer, it notifies the HAL using the IoFlushAdapterBuffers
call. If the operation was a read, data is copied from low memory to
high memory. Using this interface the driver never has to be concerned
about where the memory is located.
 
A few additional notes: The buffer allocated by the HAL is contiguous
so that it can also be used by devices that do not support
scatter/gather such as floppy drives. The buffer is shared by all
devices that need to use it and grows according to expected usage and
memory size. Finally, the data is only copied when necessary.
 
Additional reference words: 3.10
KBCategory:
KBSubCategory: hrdwr

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1993.