SystemCall

A system call introduces non-determinism into a process by pausing the deterministic execution of its code and yielding control to the RuntimeEnvironment (ultimately, an OperatingSystem or sometimes the bare Hardware), which then invokes effects on the outside world, modifies the process's State in some way, and (usually) resumes deterministic execution. System calls are encoded in the program and so occur at predictable ExecutionTimes (as opposed to, say, Interrupts, which also introduce non-determinism).

Examples of actions that usually invoke system calls include:

• reading or writing a file
• getting or sending data over a network
• getting the current time
• generating a random number (though pseudorandomness can be achieved without system calls, by seeding a random number generator like a MersenneTwister with in-process State)
• writing to the user's terminal or drawing graphics on the screen
• reading input from the user
• sleeping

I don't consider system calls to be a variant of ProcessGlobalState manipulation because they access state that can change for reasons not discoverable by inspecting the source code of the program. See also the discussion on State.