“Yeah, but …”
	• No user can interrupt it while it is running • Windows, Linux routinely interrupt system calls • No user can feed it data to make it – violate access control policies – stop serving other users • No user can replace or alter any system call code • Except your average virus • No user can add functionality to the OS! • Except dynamically loaded device drivers • Data must NEVER be treated as code! • “One man’s code is another man’s data” A. Perlis