Memory-mapped I/O

[NCOHEN@IBM.COM ("Norman H. Cohen")]

I agree with Harold Rabbie:  The best way to do memory-mapped I/O is
through LOW_LEVEL_IO.  If your compiler does not have an appropriate
SEND_CONTROL procedure, the best alternative is a code procedure.
This is one of the few circumstances in which a code procedure is both
appropriate and palatable.  It is appropriate because the intent is to
emit a specific short sequence of machine-code instructions.  It is
palatable because the sequence is only one instruction long.

A case study that SofTech did for CECOM (Ft. Monmouth) in 1982 pointed
out another problem, in addition to those mentioned by Rabbie, with the
use of assignment statements:  An assignment to a variable that is
located at an I/O address is typically not followed by a reference to
that variable.  Thus the variable looks "dead" to data-flow analysis and
the assignment is likely to be eliminated as useless by an optimizing
compiler.  (A strong case can be made that a compiler for a machine with
memory-mapped I/O must never treat an assignment as useless if the
target variable is subject to an address clause, but a case can also be
made to justify this behavior.)

Norman Cohen