Privilege promotion based on check of previous privilege level

A method of executing instructions in a computer system controlled by an operating system, the method comprising:

• executing application instructions in a processor of the computer system at a current privilege level of the processor equal to a first privilege level, wherein the application instructions are stored in a first page of memory, and wherein the current privilege level controls application instruction execution in the computer system by controlling accessibility to system resources;
• performing a call instruction to a second page of memory not writeable by the application instructions at the first privilege level, the call instruction including:
• storing a return address to the first page of memory;
• and
• storing the first privilege level in a previous privilege level state;
• and
• performing a privilege promotion instruction by the operating system, the privilege promotion instruction being stored in the second page of memory, the privilege promotion instruction including:
• reading the 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.

A computer system comprising:

• a processor having a current privilege level which controls application instruction execution in the computer system by controlling accessibility to system resources and having a previous privilege level state;
• a memory having a plurality of memory pages including a first memory page storing a privilege promotion instruction, wherein the first memory page is not writeable by application instructions at a first privilege level;
• and
• an operating system stored in the memory for controlling the processor and memory, and performing the privilege promotion instruction as follows:
• reads the previous privilege level state;
• compares 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, promotes the current privilege level to a second privilege level which is higher than the first privilege level.

A computer system comprising:

• a processor having a current privilege level which controls application instruction execution in the computer system by controlling accessibility to system resources;
• a memory having a plurality of memory pages including a first memory page storing application instructions and a second memory page storing a higher privileged routine and a privilege promotion instruction, wherein the second memory page is not writeable by the application instructions at a first privilege level;
• an operating system stored in the memory for controlling the processor and memory;
• wherein the processor executes the application instructions with the current privilege level equal to the first privilege level and the application instructions perform a call instruction to the second memory page as follows:
• stores a return address to the first memory page;
• and
• stores the first privilege level in a previous privilege level state;
• and
• wherein the operating system performs the privilege promotion instruction as follows:
• reads the stored previous privilege level state;
• compares 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, promotes the current privilege level to a second privilege level which is higher than the first privilege level.

A computer readable medium containing a privilege promotion instruction for controlling a computer system to perform a method of promoting a current privilege level of a processor of the computer system, wherein the current privilege level controls application instruction execution in the computer system by controlling accessibility to system resources, the method comprising:

• 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 privilege level which is higher than the current privilege level.