How To Use
Assuming a call gate has been set up already by the operating system kernel, code simply does a CALL FAR (LCALL in AT&T Syntax) 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 (LRET) instruction which pops the continuation information off the stack and returns to the outer privilege level.
For information on the exact format of the call gate descriptor, please see the IA-32 manuals.
Read more about this topic: Call Gate
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