Subprocess call stack

At most one subprocess of a process could be in execution at a time. A facility was available for code in one subprocess to call a fixed entry point in another subprocess. This would create an entry on the call stack to facilitate a return. The called subprocess would be swapped into CM if necessary, and begin execution at a predetermined location. The call action would also transfer some data items from the calling subprocess to the called subprocess, and transfer some capabilities between their local C-lists. (More on this under operations.) Saving and restoring the hardware registers was up to the called subprocess. ECS system actions were available to save the registers in a specified location, as well as restore them. They were not automatically saved during the call action.