SHOWCB     AM operand = VTAM
           control block type operand → control block
           FIELDS operand = control block field to be extracted
           AREA operand → work area where contacts of blocks
                     will be placed
           LENGTH operand = length of the work area
           MF operand = list, generate, or execute form parameters
 
           Register 0 = error return code (if register 15 indicates an error)
           or
           Register 0 = length of the control block field extracted
                     (if register 15 indicated successful completion)
           Register 15 = general return code