sysexit

Sysexit refers to the SYSEXIT assembly instruction used in the IA-32 x86 fast system call mechanism. It is part of a pair with SYSENTER that provides a faster way for the kernel to return control to user space after a system call, complementing the entry path created by SYSENTER.

Origin and purpose: The SYSENTER/SYSEXIT mechanism was introduced by Intel to speed up system calls on 32-bit x86 processors, offering a faster alternative to the traditional software interrupt path (such as int 0x80). The mechanism relies on dedicated model-specific registers to configure the transition between user mode and kernel mode, reducing the overhead associated with privilege-level changes.

How it works: A process uses SYSENTER to enter the kernel, where the requested service is performed. When the kernel has completed the work, it executes SYSEXIT to return to user space. During this transition, the kernel restores the user-mode segment selectors and stack context configured by the SYSENTER setup, and the return value of the system call is typically placed in the EAX register. The exact behavior is defined by the calling conventions and the operating system in use.

Modern context: In 64-bit x86 environments, the preferred fast path is typically the SYSENTER/SYSEXIT pair less commonly used today, with many implementations adopting the 64-bit syscall/sysret mechanism that leverages different instructions and conventions. Linux and Windows support both mechanisms on 32-bit systems, while 64-bit systems generally rely on the native 64-bit system call path.

See also: System call, SYSENTER, SYSRET, SYSCALL, privilege levels, interrupt handling.