Summary
Knowledge of the relationship of processes to threads and the function of the system call interface will help us as we expand our study into the structural depths of the kernel. Remember that at any one instant in time a processor is simply executing a single instruction, and the processor is in either user mode or kernel mode. There are only two ways in which the kernel mode may be entered: a user-initiated system call or as the result of a system interruption (interrupt, fault, or check). The kernel always attempts to return to the user mode upon completion of its tasks. Along the way it evaluates which user thread is the most deserving to run.
Now that we understand the mechanics of moving between the two modes, let's examine in depth ...
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