Memory-mapped device IO

[compata at cup.portal.com]

I wish to add an IO board to my Xenix system.  The only way to address this
board is via dual-ported memory.  The precise addresses are switch-selectable
on the board.  The board does not generate any interrupts although it is
possible to cause an interrupt to the board by writing to an output port.

I understand that /dev/mem would allow access to all of physical memory.
However, I don't know what physical memory is not already in use.  More
importantly, I can't be sure that the same physical memory will always be
available.  I would like to find a way to address this board in a portable way,
so the same technique could be later used on SysV, and possibly on non-Intel
processor machines.

Any suggestions will be very welcome.  Email gratefully accepted.

Dave Close, Compata, Arlington, Texas - compata at cup.portal.com