An older mechanism is the call gate; originally used in Multics and later, for example, see call gate on the Intel x86. It allows a program to call a kernel function directly using a safe control transfer mechanism, which the operating system sets up in advance. This approach has been unpopular on x86, presumably due to the requirement of a far call (a call to a procedure located in a different segment than the current code segment) which uses x86 memory segmentation and the resulting lack of portability it causes, and existence of the faster instructions mentioned above.