ASID - Address Space Identifier
An ASID (Address Space Identifier) is a unique 16-bit identifier assigned by z/OS to each active address space within the system. It serves as a crucial component for the operating system to manage, protect, and differentiate between the various independent virtual storage environments where programs execute.
Key Characteristics
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- Uniqueness: Each active address space on a z/OS system has a unique ASID, ensuring that the operating system can unambiguously refer to it.
- 16-bit Value: ASIDs are 16-bit unsigned integers, ranging from
X'0001'toX'FFFF'. This limits the maximum number of concurrently active address spaces to 65,535. - Dynamic Assignment: ASIDs are assigned dynamically by z/OS when an address space is created (e.g., when a job starts, a TSO user logs on, or a started task is initiated) and are reused once an address space terminates.
- Memory Protection: The ASID is fundamental to z/OS's memory protection mechanisms, allowing the system to isolate the virtual storage of one address space from another, preventing unauthorized access or corruption.
- Associated with ASCBs: Every address space is represented by an Address Space Control Block (ASCB), and the ASID is a key field within the ASCB, linking the identifier to the control block that defines the address space's attributes.
Use Cases
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- Process Identification and Monitoring: System programmers and operators use ASIDs in tools like
SDSForRMFto identify specific jobs, started tasks, or TSO users, facilitating monitoring and troubleshooting. - Security Contexts: In
RACF(Resource Access Control Facility) and other security products, ASIDs can be used in audit records or access rules to identify the originating address space of an access attempt. - Inter-Address Space Communication: While not directly used for data exchange, the ASID is implicitly involved in mechanisms like
Cross-Memory ServicesorProgram Call (PC)routines, where one address space needs to interact with another. - Problem Determination and Debugging: When analyzing system dumps or traces, the ASID is a critical piece of information to pinpoint which address space was involved in an error or abend.
- Workload Management (WLM): WLM uses ASIDs to track and manage the performance of individual address spaces according to defined service goals.
- Process Identification and Monitoring: System programmers and operators use ASIDs in tools like
Related Concepts
The ASID is inextricably linked to the concept of an Address Space itself, serving as its primary identifier. It is managed by the z/OS System Resource Manager (SRM), which handles the assignment and reuse of ASIDs. When the operating system performs a context switch between tasks, the ASID in the Program Status Word (PSW) is updated to reflect the currently active address space, ensuring that virtual storage references are correctly translated. Furthermore, the ASID is a key component in the Address Space Control Block (ASCB), which contains vital information about the address space, and works in conjunction with Task Control Blocks (TCBs) to manage individual tasks within an address space.