Supervisor Call instruction

This article covers the specific instruction on the IBM System/360 and successor mainframe computers, and compatible machines. For the general concept of an instruction for issuing calls to an operating system, see System call. A Supervisor Call instruction (SVC) is a hardware instruction used by the System/360 family of IBM mainframe computers up to contemporary zSeries, the Amdahl 470V/5, 470V/6, 470V/7, 470V/8, 580, 5880, 5990M, and 5990A, and others; Univac 90/60, 90/70 and 90/80, and possibly others; the Fujitsu M180 (UP) and M200 (MP), and others; and is also used in the Hercules open source mainframe emulation software. It causes an interrupt to request a service from the operating system. The system routine providing the service is called an SVC routine. SVC is a system call. IBM mainframes in the System/360 and successor families operate in one of two states: problem state or supervisor state and in one of sixteen storage access keys (0 to 15). In problem state, a large set of general purpose non-privileged instructions are available to a user program. In supervisor state, system programs are additionally able to use a small set of privileged instructions which are generally intended for supervisory functions. These functions may affect other users, other processors, or the entire computer system. In storage key 0 a program is able to access all addressable storage, otherwise it is limited to storage areas with a matching key. A program is only allowed to access specific supervisory functions after thorough authorization checking by the operating system: DEBCHK (SVC 117), TESTAUTH (SVC 119), and possibly additional tests. Programs which fail any of these tests are ABENDed, that is abnormally terminated and immediately cease processing. Some of these tests were not available in OS/360, but were added in OS/VS1, SVS or MVS/370, but all were available in MVS/370 or subsequent releases, and are still available to this day.