360 Assembly/360 Instructions/SVC

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SVC - Supervisor Call - Opcode 0A

[edit] Example usage

SVC 11

[edit] Purpose of Instruction

This instruction is used to allow a program to call the operating system in order to request services. The operand is the Supervisor Call number, which is in the range 0 to 255 (decimal). The value of the operand is operating system dependent. Typically, a supervisor call is made after loading certain machine registers with values, usually register 1, sometimes register 0, and in some cases, register 2. Supervisor Calls may or may not return information. They may return the information in registers, or in specific fields which were passed in the original call.

In some cases, a supervisor call will never return. Either the call is invalid and the program is terminated, or the call is in fact the operating system call to terminate the program. Some operating systems have privileged SVCs which are not available to user programs, and not all operating systems use all SVC values.

[edit] Typical Usage

Typically a supervisor call is used as part of an Operating System macro. For example, under OS/VS1 the WTO macro issues an SVC 35.