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A program running on the ECS system ran inside a subprocess
(protection domain) of a process. Through calls on the system,
it could store data in files, send signals to programs in other
processes via event channels and call programs in other
subprocesses of the same process.
Each subprocess had a C-list; a list of system provided,
unforgeable, pointers to various objects, such as files and event
channels. This C-list defined the set of objects which a program in
this subprocess could access.
Each physical I-O device was represented by a set of files and
event channels. A program could exchange data with the device through
the files, and exchange control signals through the event channels.