DCD - Device Characteristics Directory
The Device Characteristics Directory (DCD) is an in-memory control block structure within z/OS that contains detailed technical specifications and capabilities for each type of I/O device supported by the system. It serves as a central repository for the operating system to understand and manage the various characteristics of attached storage, network, and peripheral devices. The Device Characteristics Directory (DCD) is a system-defined table in z/OS that contains detailed technical specifications and capabilities for each I/O device configured in the system. It provides the operating system with essential information required to manage, allocate, and interact correctly with various hardware devices, such as DASD, tape drives, and printers.
Key Characteristics
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- Device-Specific Information: Stores attributes like device type (e.g.,
3390,3590), track capacity, block size limits, channel path requirements, and special features (e.g., caching, data compression). - System-Wide Repository: Contains information for *all* device types that the system is configured to recognize, not just currently online devices.
- Generated from HCD: Typically built during the Hardware Configuration Definition (HCD) process, which translates the I/O configuration into the
IODF(I/O Definition File) used by z/OS. - In-Memory Structure: Resides in system memory, allowing for fast access by the z/OS I/O Supervisor (IOS) and other components during device allocation and I/O operations.
- Static for Device Types: While individual device instances (UCBs) can change, the DCD itself describes the *types* of devices and is relatively static unless new device types are introduced or existing ones updated via an
IODFactivation.
- Device-Specific Information: Stores attributes like device type (e.g.,
Use Cases
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- Device Allocation: When a JCL statement requests a specific device type (e.g.,
UNIT=3390), z/OS uses the DCD to understand the requested device's characteristics and find a suitable available device. - I/O Supervisor (IOS) Operations: The IOS uses DCD information to correctly format Channel Command Words (CCWs) and manage data transfer operations, ensuring compatibility with the specific device's capabilities.
- Error Recovery: During I/O error handling, the DCD helps the system determine device-specific recovery procedures or alternative paths based on the device's known characteristics.
- Performance Optimization: System components can leverage DCD data to optimize I/O operations, such as selecting optimal block sizes or utilizing device-specific features like caching.
- Dynamic I/O Configuration: While the DCD itself describes device types, its underlying data (from HCD) is crucial for dynamically adding, removing, or modifying device instances and ensuring the system correctly recognizes their capabilities.
- Device Allocation: When a JCL statement requests a specific device type (e.g.,
Related Concepts
The DCD is intrinsically linked to the overall z/OS I/O configuration. It works in conjunction with the Unit Control Block (UCB), which is an in-memory control block representing a *specific instance* of an I/O device (e.g., a particular disk volume or tape drive). While the DCD describes the *type* of device, the UCB points to the DCD to retrieve the characteristics for its specific device type. Both are populated based on the I/O Definition File (IODF), which is created using the Hardware Configuration Definition (HCD) tool. The I/O Supervisor (IOS) heavily relies on DCD information to manage I/O requests and interact with devices.