Control Block
A control block is a structured area of main storage (RAM) used by the z/OS operating system and its components to store critical information about system resources, tasks, data, and their current state. It acts as a central repository for control data that the system needs to manage and process various operations efficiently.
Key Characteristics
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- Predefined Structure: Each control block has a fixed, predefined format, often mapped by a
DSECT(Dummy Section) in assembler language, specifying the layout and meaning of its fields. - Memory Resident: Control blocks reside in main storage, making their data readily accessible to the operating system and privileged programs for quick lookup and updates.
- System-Critical Information: They contain vital data necessary for z/OS to manage tasks, I/O operations, virtual storage, security, and other core system functions.
- Hierarchical and Chained: Many control blocks are linked together in complex chains or tree structures (e.g., a
TCBpoints toRBs, which might point toDEBs), allowing the OS to navigate related information. - Privileged Access: Accessing or modifying control blocks typically requires specific system privileges (e.g., supervisor state,
KEY 0) due to their critical nature and potential to compromise system integrity. - Dynamic Creation/Deletion: Control blocks are created, updated, and deleted dynamically by z/OS as resources are allocated, tasks are initiated, or events occur.
- Predefined Structure: Each control block has a fixed, predefined format, often mapped by a
Use Cases
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- Task Management: The
TCB(Task Control Block) is fundamental for managing individual tasks, storing information like task status, priority, and resource ownership. - Program Execution:
RBs (Request Blocks) are used to manage the execution of programs within a task, tracking program status and control flow. - I/O Operations:
DCBs (Data Control Blocks) andDEBs (Data Extent Blocks) store information about datasets, I/O buffers, and device characteristics for data access. - Virtual Storage Management: Control blocks like the
ASCBE(Address Space Control Block Extension) and various page/segment tables manage the allocation and attributes of virtual storage. - Security Context: The
ACEE(Accessor Environment Element) holds security-related information for a user or address space, used for authorization checks.
- Task Management: The
Related Concepts
Control blocks are the backbone of z/OS internal operations. They are intimately tied to Assembler Language because their structures are often defined using DSECTs, and many system services (implemented as macros) manipulate them. System services like ATTACH, OPEN, GETMAIN, and SVC calls directly interact with or create control blocks to perform their functions. They are fundamental to virtual storage management, task management, and I/O processing, forming the data structures that underpin these core z/OS functionalities.
- Utilize System Services: Always use provided z/OS system services (macros or callable services) to interact with and modify control blocks indirectly, rather than attempting direct manipulation, to ensure system integrity and compatibility.
- Understand DSECTs for Debugging: For system programmers and advanced problem determination, understanding the
DSECTs of key control blocks is essential for interpreting dumps and diagnosing system issues. - Exercise Extreme Caution: Direct modification of control blocks should only be performed by highly experienced system programmers with a deep understanding of z/OS internals, as incorrect changes can lead to system crashes or data corruption.
- Security Awareness: Be mindful that control blocks contain sensitive system and security information; ensure any custom code accessing them operates with appropriate authorization and adheres to security best practices.
- Performance Considerations: While control blocks are optimized for access, avoid unnecessary traversal of long control block chains in performance-critical applications, as this can introduce overhead.