As to how to determine whether an instruction is present on a processor, this is a REALLY deep subject that I will defer to a subsequent thread.
As to whether SEARCH STRING (SRST) is available on a processor: As Mr. Weinhold correctly noted, you get an operation exception if an instruction is not present on a processor ... not a specification exception. You will notice in the instruction description of SRST in any z/Architecture Principles of Operation, there is no operation exception described. This is because SRST is standard on all z/Architecture-capable processors. As Mr. Farley noted, SRST recognizes a specification exception if any of bits 32-55 of general register 0 contain nonzero values. One potential reason for this is if the programmer used general register zero for both a string character and an operand address. See programming note 3 in SRST for details.
