User jobs can request devices and volumes by means of the FILE macro and the commands CREATE-FILE, IMPORT-FILE, MODIFY-FILE-ATTIBUTES, ADD-FILE-LINK and SECURE-RESOURCE-ALLOCATION. In the case of tape files, volumes may also be requested implicitly during OPEN processing or automatic tape swapping.
By means of the CREATE-FILE and IMPORT-FILE commands, the user requests devices and volumes which are required for a specific file.
Using the SECURE-RESOURCE-ALLOCATION command, other resources such as files on public volumes can be reserved. If the SECURE-RESOURCE-ALLOCATION command is used to reserve a file which extends over several private volumes, DMS reserves all of the volumes and the necessary devices.
In a batch job, the SECURE-RESOURCE-ALLOCATION command causes the job to wait if one of the requested volumes is not free when the request is issued. The maximum wait time can be explicitly specified in the WAIT operand of the SECURE-RESOURCE-ALLOCATION command (for both batch and interactive mode).
Devices of a specific type can also be requested (in a CREATE-FILE, IMPORT-FILE or SECURE-RESOURCE-ALLOCATION command); the device manager then selects any free device of the specified type. Reservation by device type of disk devices in the SECURE-RESOURCE-ALLOCATION command can be used only for special applications (e.g. VOLIN).
A device can also be reserved via a mnemonic device name in the SECURE-RESOURCE-ALLOCATION command. NDM (Nucleus Device Management) reserves the device with the specified name if it is free. Once again, this is permitted only for special applications in the case of disk devices.
If all requested devices are free, they are reserved for the requesting job. If the user has also requested volumes, NDM sends a MOUNT message to the console, i.e. it requests the operator to mount the volumes. For files on private disks, all disks which contain parts of the file must be mounted.
If one or more of the requested devices is not free, no devices are reserved. In interactive mode, the job does not wait. In batch mode on the other hand, the job is entered in a queue (if the reservation was made using the SECURE-RESOURCE-ALLOCATION command) and is continued if all requested devices become free during the predefined wait period. If the devices were requested by means of the CREATE-FILE command, or if not all devices are available, a spin-off is initiated and DMS branches to the next SET-JOB-STEP or LOGOFF command.