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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.