The operating system requires several types of processing information to ensure proper control of your I/O operations. You must specify the forms of macros in the program, buffering requirements, and the addresses of your special processing routines during either the assembly or the execution of your program. The DCB parameters specifying buffer requirements are discussed in Managing SAM Buffer Space.
Because macros are expanded during the assembly of your program, you must supply the macro forms to be used in processing each data set in the associated DCB macro. You can supply buffering requirements and related information in the DCB and DCBE macro, the DD statement, or by storing the applicable data in the appropriate field of the DCB or DCBE before the end of your DCB exit routine. If the addresses of special processing routines (EODAD, SYNAD, or user exits) are omitted from the DCB and DCBE macro, you must complete them in the DCB or DCBE before they are required.