to place data in a well known location in memory and then causes a fault, a type of error. When a system is first started, its kernel is set up to be the "handler" of all faults; thus, when a program causes a fault, the kernel takes over, examines the information passed to it, then carries out the instructions.

Under Mach, the IPC system was used for this role instead. To call system functionality, a program would ask the kernel for access to a port, then use the IPC system to send messages