The SSI is a way for one routine to call another routine. There are a number of other ways that a routine can call another routine, such as:

• Branch and link register (BALR) 14,15
• LINK or LINKX macro
• Program call (PC)
• SVC

The SSI is different from these linkage interfaces, however, in that:

• The called routine does not have to be there. That is, when a routine calls the subsystem, the SSI checks to see if the subsystem either is not interested in the request or does not exist. The caller then receives an appropriate return code.
• A caller's request can be routed to multiple subsystem routines.