The device number is a 3-digit or 4-digit hexadecimal number assigned to the device when it is installed. In JCL statements, always precede a 4-digit number with a slash (/). A 3-digit number can be specified with or without a slash.
Note that the slash before a 4-digit device number distinguishes it from a device type, which is also 4 digits, but cannot contain a slash or be preceded by a slash.
For example, UNIT=/3490 is the device number for a specific device.
Do not specify a device by its number unless absolutely necessary. When you specify a device number, the system can assign only that specific device. Specifying a device number will delay a job if another job is using the device.
Specifying device type: Requesting a device type allows the system to assign any available device of that type. For example, UNIT=3390 indicates that you want the system to assign any available 3390 Direct Access Storage device. For more information on specifying device types, see z/OS HCD Planning.
Specifying group name: During system initialization, the installation can define group names for a group of devices. The devices in a group may or may not all be the same type. Requesting a group name allows the system to assign any available device in the group. For example, if the group named DISK includes 3380 and 3390 Direct Access Storage Devices, the system assigns an available 3380 and 3390 device when UNIT=DISK is coded. If the group named 3390A includes three particular 3390 devices, the system assigns one of these 3390 devices when UNIT=3390A is coded.
Groups with Several Types of Devices: If the group contains more than one type of device and the DD statement requests more than one device, the system allocates devices of the same type from the group. For example, if the group named TAPE includes both 3400-5 and 3400-6 devices and the DD statement specifies UNIT=(TAPE,2), the system assigns either two 3400-5s or two 3400-6s. If the system does not have enough devices of one type to satisfy the request, the system terminates the job.
If a group contains more than one type of device, do not code the group name when requesting an existing data set or a specific volume. The system may assign one type of device while the data set resides on another type. For example, if SYSSQ contains all tape and direct access devices, do not code UNIT=SYSSQ for an existing data set on tape; the system might assign a direct access device.
Groups with Devices with Special Features: This rule also applies if the data set resides on a device that has required feature such as more cache, connectivity to another system or PPRC, Peer-to-Peer Remote Copy.
If a nonspecific volume request requires more than one tape device from a group that contains both single and dual density tape drives, the system assigns the devices so that the single density drive is the first one used. The default density is the density of the single density drive. The operator may be requested to mount the volumes in a different order than assigned by the system.
Concurrent allocation of devices: Only direct access devices can be allocated to different jobs executing concurrently. Teleprocessing equipment cannot be allocated more than once in the same job step. If a printer, punch, teleprocessing equipment, or graphics device is designated as a console, it cannot be allocated to a job.
Allocating a teleprocessing device with a group name: If you request that the system allocate one or more lines of a line group by using a group name, the system attempts to allocate the lines within the line group, starting with the lowest teleprocessing (TP) line address and continuing in ascending order. If the first eligible line in the line group is already allocated, the system fails the request to allocate from that line group.
Definition of UNIT parameters in system initialization: The installation describes each device to the system during system initialization. During this process, the installation defines the device types and group names to be coded in the DD UNIT parameter.
The installation should maintain a list of the device types and group names. For more information, see z/OS HCD Planning.