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The supervisor call (SVC) is a special machine instruction (hex 0A) that programs use to communicate with the operating system. The SVC passes a parameter list to the operating system to tell it what system service the calling program is requesting. Examples include data set allocate and file OPEN and CLOSE. z/OS components that perform these services must often run in supervisor state or the master storage key 0 so they can update system control blocks. The SVC instruction lets them do this, even when they are called from an unauthorized program. When the SVC returns control to the calling program, it relinquishes the authorized state. The programs that are invoked by the SVC instruction are also called SVCs.
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