Udo van den Heuvel wrote in a message to Richard Drapeau:

 UvdH> OK, but how can I safely let the chips write to a buffer,
 UvdH> somewhere below the 16 meg mark? How do I get a buffer below
 UvdH> the 16 meg mark?
 UvdH> When I set up the DMA chips to write to a buffer, how does
 UvdH> this work when another task is being serviced?
 UvdH> Is the memory protection a problem?
 UvdH> What other caveats are to be taken care of? (this is OS/2,
 UvdH> not MS-DOS) 

Under OS/2, you can only do DMA from a device driver running in Ring 0. 
I'll assume you understand that.

In general, a device driver sees a virtual address to memory, which is
16:16.  These addresses may have other aliases, especially when they
reflect buffers from applications in linear 0:32 space.  A device driver
uses system API calls to lock ranges of memory into some physical RAM and
then passes these physical addresses to the DMA hardware.  In some cases,
the RAM buffers may be physically discontigous, and this is compensated for
by scatter-gather hardware to do the DMA operation.

Since device drivers cannot be preempted, concurrency is not an issue.
 
-- Mike


---