A prerequisite for any access to a tape file is that the associated volumes (magnetic tapes or magnetic tape cartridges (MTC)) and devices (tape or MTC drives) are available. Tapes and tape devices can be requested explicitly using the CREATE-FILE, ADD-FILE-LINK and SECURE-RESOURCE-ALLOCATION commands, the FILE macro or implicitly at OPEN time or, if the next tape is to be mounted on a different device, when tapes are swapped.
A tape request via the SECURE-RESOURCE-ALLOCATION or ADD-FILE-LINK command or by means of the FILE macro causes a PREMOUNT message to be sent to the console unless the tapes were reserved offline. A tape request at OPEN time causes a MOUNT message to be sent to the console. MOUNT=0 means that no PREMOUNT message is output on the console. The device is reserved offline. DMS issues the MOUNT message only when the device is actually required. The device is allocated to the requesting job.
By default, DMS does not issue MOUNT messages if the requested tapes are already mounted. In this case an implicit device allocation is made. For the processing of MF/MV sets, BS2000 issues PREMOUNT messages so that the operator can mount further tapes of the set in good time.
The operator can accept the MOUNT message for a specific device or he can suggest the use of another device. When he confirms that the volume has been mounted, the requested device is allocated to the job.
If devices are reserved implicitly (e.g. by specifying a device type), the volume can be moved to another device if a device error occurs on the first device. If the device was reserved explicitly, this is not possible.
Device management should be informed whenever a volume is to be moved to another device. The system then suggests which device to use. After the volume has been moved, device management carries out the necessary positioning of the tape so that it can be processed further.