Instructional experience with an XPL-implemented operating system model

An operating system model, programmed in assembly language and executed under a Sigma 9 simulator, has been employed for some time at the University of Southern Mississippi in an undergraduate operating systems course. Due to student inexperience and the inherent difficulties of writing and debugging assembly language, models developed in this way had to be group efforts, and either never worked or did not achieve any degree of sophistication. XPL, which was already being used in USM compiler courses, seemed a natural way to improve this situation, but presented several difficulties: 1) code generated by the XPL compiler required runtime support which in turn could depend on the "real" operating system (CP-V); and, 2) linkages for internal (XPL) procedures and external (assembly language) procedures had to be used to effect interrupt, trap and supervisor call control transitions. A special runtime package which flagged certain "support" calls as errors and provided the privileged instructions solved these problems.