VSAM provides a set of macros, GENCB, TESTCB, MODCB, and SHOWCB, to let you manipulate the contents of control blocks at execution time. Use these macros to generate, test, modify, and display the contents of fields in the access method control block, the exit list, and the request parameter list. You do not have to know the format of the control block when you use these macros.

The GENCB, MODCB, TESTCB, and SHOWCB macros build a parameter list that describes, in codes, the actions indicated by the parameters you specify. The parameter list is passed to VSAM to take the indicated actions. An error can occur if you specify the parameters incorrectly.

If you issue a MODCB, SHOWCB, or TESTCB for a non-VSAM ACB, unpredictable results occur.