Re: Language runtime requirements (was Re: DoD

[fsi-ssd!mikeb@uunet.uu.net (Mike Bates)]

VAXELN, DEC's "real-time" OS for VAXen, does a nice job of mapping Ada
tasks to the OS; perhaps it's more accurate to say that the OS's
structure coincides neatly with Ada's requirements.  An Ada program
corresponds to a VAXELN job.  Tasks map to processes within the job;
the main task corresponds to the master process.  Scheduling is
preemptive, based on job priority, then on priority of processes
within the active job.

VAXELN Ada has a very useful, "thick" binding for system calls -- I
imagine it helps that VAXELN is written in a variant of Pascal.  The X
and Motif bindings were fairly easy to work with as well.  (X and
Motif themselves weren't so easy...)

We had a few problems with the environment -- device drivers were
written for transaction-processing "real-time," not for hard
real-time.  With the Pascal source for their drivers and some
assistance from DEC's customer support, we were able to write the
drivers we needed in Ada.  The other big problem is that VAXELN's
user community is relatively small; we uncovered several previously
undiscovered bugs in the Motif library and in the user shell.  It's a
strange feeling when the customer support engineer says, "We've never
known anyone to try that before."





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Mike Bates               FlightSafety International, Simulation Systems Div.
Principal Engineer       2700 North Hemlock Circle
Computer Systems Group   Broken Arrow, Oklahoma 74012
mikeb@ssd.fsi.com        918-251-0500 ext. 581