There are notions about privilege that occur at the processor level and the operating system. With processors, there are instructions that can't be executed except in supervisor mode. Similarly, in the operating system, there are operations that cannot be performed without going through the kernel (syscalls). Question: How do these two concepts map to each other? Can we rewrite a system call entirely in user space? If not, is it because a system call hides the aforementioned privileged instructions? 
Rewrite a system call in userspace
linux;system calls