IA32 System Programming - Part VI

Fast System Calls

Fast system calls are provided by the IA32 architecture with a low overhead mechanism for system software.

• The “sysenter” instruction is for use by user code running at privilege level 3 to access system procedures running at privilege level 0.
• The “sysexit” instruction is for use by system procedures running at privilege level 0 for fast returns to user code running at privilege level 3.

The target procedure entry point and tack pointer are predefined in fixed MSR (model specific register) addresses. MSRs could be accessed with the “rdmsr” and “wrmsr” privileged instructions. The overhead of complex privilege checks is simplified, and memory access to descriptor tables is then eliminated.