For a magnetic tape device, the system constructs a CCW chain to set the mode specified in the DEB and pass control to the real version of your channel program. (You cannot set the mode yourself.) For cartridge tape devices, the mode byte also prohibits supervisor channel command words such as the mode set command. If your program opens a tape for input or read backwards and you do not have RACF® authority to write on the volume, the system normally prevents writes. With a reel tape the system does this by requiring the operator to remove the write ring. With a cartridge tape, the system does this using the Mode Set command. See z/OS DFSMS Using Magnetic Tapes for more information on tape handling.