How to address more peripheral RAM than controller RAM?

[Steven Kan <steven@kan.org>]

We are developing CompactPCI data acquisition cards with large on-board
memories. The way things are set up right now, the entire on-board
memory is directly addressable by the controller, which means that our
system controller has to have more RAM than all the peripheral boards
combined (to leave room for silly things like an OS :))

We don't have to read the memories in real-time. We capture from the
outside world to the on-board memory on each card, and then DMA from
there to the host RAM once the acquisition has completed.

We are running out of room this way, as we are trying to build systems
with 4 or 8 cards, each with 512 MB on-board, and we can't find
controllers with 4 and 8 GB of RAM.

What are the other options for doing this? Is there any way to use
virtual memory? The OS is NT4, but all the address space assignment
happens before the OS loads, right?

Thanks!