Device Number
In z/OS, a device number (also known as a Unit Address or UCB address) is a unique hexadecimal identifier assigned to a specific hardware device (such as a disk drive, tape drive, printer, or communication controller) attached to the mainframe. It allows the operating system and applications to directly address and control physical I/O devices.
Key Characteristics
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- Format: Typically a 3-digit or 4-digit hexadecimal number (e.g.,
0A0,180,2000), often reflecting the channel, control unit, and device address. - Uniqueness: Each physical device connected to a z/OS system has a unique device number within that system's I/O configuration.
- UCB Association: Every device number is associated with a Unit Control Block (UCB), an in-memory data structure that z/OS uses to store all relevant information about that specific device.
- Configuration: Device numbers are defined and managed within the I/O configuration of the z/OS system, typically through the Hardware Configuration Definition (HCD) utility.
- Static/Dynamic: While historically static, modern z/OS environments with FICON and dynamic I/O allow for certain device numbers to be dynamically added or removed without requiring a system IPL.
- Logical vs. Physical: A device number represents a logical address that maps to a physical hardware path, enabling the OS to manage I/O requests.
- Format: Typically a 3-digit or 4-digit hexadecimal number (e.g.,
Use Cases
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- JCL Allocation: Specifying a device for a dataset in JCL using the
UNIT=parameter (e.g.,UNIT=3390for a DASD volume, which resolves to an available device number of that type). - Operator Commands: Displaying device status, varying devices online/offline, or performing other control actions using commands like
D U,,,0A0(Display Unit) orV 0A0,ONLINE. - System Programming: Low-level I/O operations where system programs or device drivers need to interact directly with a specific hardware device using its address.
- Problem Determination: Identifying the exact hardware component involved in an I/O error, performance bottleneck, or hardware failure by its unique device number.
- Storage Management: Identifying specific DASD volumes or tape drives by their device number for allocation, deallocation, or migration tasks.
- JCL Allocation: Specifying a device for a dataset in JCL using the
Related Concepts
Device numbers are fundamental to z/OS I/O operations, working in conjunction with Unit Control Blocks (UCBs), which are the in-memory representations of devices. They are defined and managed through the Hardware Configuration Definition (HCD) and stored in the Input/Output Definition File (IODF), which dictates the system's I/O configuration. JCL uses device numbers (often implicitly via generic unit names like 3390 or TAPE) to request specific types of devices for dataset allocation, and the system then assigns an available device number.
- Standardization: Implement consistent naming conventions for device numbers, especially for shared or critical resources, to enhance clarity and simplify management.
- Documentation: Maintain comprehensive and up-to-date documentation of the I/O configuration, mapping device numbers to physical hardware, logical functions, and associated UCBs.
- Dynamic I/O Utilization: Leverage dynamic I/O capabilities where appropriate to add or remove devices without requiring a system IPL, minimizing downtime and improving agility.
- Monitoring: Regularly monitor device usage and performance associated with specific device numbers to proactively identify bottlenecks, I/O errors, or potential hardware issues.
- Security: Restrict access to commands and utilities that can modify I/O configuration or vary device status (e.g., HCD, operator commands) to authorized personnel only.