1. A method of promoting a current privilege level of a processor of a computer system controlled by an operating system, wherein the current privilege level controls application instruction execution in the computer system by controlling accessibility to system resources, the method comprising: performing a privilege promotion instruction by the operating system, the privilege promotion instruction being stored in a first page of memory not writeable by application instructions at a first privilege level, the privilege promotion instruction including: reading a stored previous privilege level state; comparing the read previous privilege level state to the current privilege level; and if the previous privilege level state is equal to or less privileged than the current privilege level, promoting the current privilege level to a second privilege level which is higher than the first privilege level.