On Dec 23, 1:22, Pete Turnbull wrote: I've just realised something. The machine was used in a real-time process control system in the pharmaceutical industry. If that system was designed as a "hard" real-time system, then interrupts would be verboten, as then it would be impossible to calculate the worst-case execution times for scheduling routines. I guess this was built not very long after the Flixborough disaster in 1974 -- an event still used as an example in safety-critical systems design courses. My guess is the designers just eliminated all the unlikely problems they could, as well as the likely ones. -- Pete Peter Turnbull Network Manager University of York