GENCB     AM operand = VTAM
          BLK operand = control block type (ACB|EXLST|RPL|NIB)
          control block field name operand = value set in field
          COPIES operand = number of copies desired
          WAREA operand → work area where blocks are built
          LENGTH operand = length of 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 generated control blocks (if register 15
                       indicates successful completion)
          Register 1 → generated control blocks (if register 15
                       indicates successful completion)
          Register 15 = general return code