Call Gate

A call gate is a mechanism in Intel's x86 architecture for changing the privilege level of a process when it executes a predefined function call using a CALL FAR instruction.

Call gates are intended to allow less privileged code to call code with a higher privilege level. This type of mechanism is essential in modern operating systems that employ memory protection since it allows user applications to use kernel functions and system calls in a way that can be controlled by the operating system.

Call gates use a special selector value to reference a descriptor accessed via the Global Descriptor Table or the Local Descriptor Table, which contains the information needed for the call across privilege boundaries. This is similar to the mechanism used for interrupts.

Assuming a call gate has been set up already by the operating system kernel, code simply does a CALL FAR with the necessary segment selector (the offset field is ignored). The processor will perform a number of checks to make sure the entry is valid and the code was operating at sufficient privilege to use the gate. Assuming all checks pass, a new CS/EIP is loaded from the segment descriptor, and continuation information is pushed onto the stack of the new privilege level (old SS, old ESP, old CS, old EIP in that order). Parameters may also be copied from the old stack to the new stack if needed. The number of parameters to copy is located in the call gate descriptor.

The kernel may return to the user space program by using a RET FAR instruction which pops the continuation information off the stack and returns to the outer privilege level.

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