Coding the callable services

A callable service is a programming interface that uses the CALL macro to access system services. To code a callable service, code the CALL macro followed by the name of the callable service, and a parameter list; for example:

           CALL service,(parameter list)

The syntax diagram for the sample callable service SCORE:

Considerations for coding callable services are:

• You must code all the parameters in the parameter list because parameters are positional in a callable service interface. That is, the function of each parameter is determined by its position with respect to the other parameters in the list. Omitting a parameter, therefore, assigns the omitted parameter's function to the next parameter in the list.
• You must place values explicitly into all input parameters, because callable services do not set default values.
• You can use the list and execute forms of the CALL macro to preserve your program's reentrancy.