Processor Mode and Context Switching
A system call is processed in kernel mode, which is accomplished by changing the processor execution mode to a more privileged one, but no process context switch is necessary —although a privilege context switch does occur. The hardware sees the world in terms of the execution mode according to the processor status register, and processes are an abstraction provided by the operating system. A system call does not require a context switch to another process, it is processed in the context of whichever process invoked it.
Read more about this topic: System Call
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