Executable

Enhanced Definition

In the context of IBM z/OS, an executable refers to a program in a machine-readable format, typically a **load module** or **program object**, that is ready to be loaded into memory and executed by the operating system. It is the final output of the compilation and linkage-editing (or binding) process, containing machine code, data, and control information necessary for its execution.

Key Characteristics

- Machine Code: Contains the actual machine instructions that the z/Architecture processor can directly understand and execute.
- Load Module/Program Object: Stored in a load library (a PDS or PDSE) as a member. A load module is the traditional format, while a program object (PDSE member) offers more flexibility, such as larger sizes and shared access.
- Linkage Edited/Bound: Created by the Linkage Editor or Binder from one or more object modules and resolved external references to other programs or system services.
- Entry Point: Has a defined entry point, which is the starting address where execution begins when the program is invoked.
- Re-entrant/Reusable: Can be designed to be re-entrant (multiple users can share a single copy in memory without interference) or reusable (can be loaded once and executed multiple times by the same user).

Use Cases

- Batch Job Execution: Invoked via a JCL EXEC statement (e.g., //STEP1 EXEC PGM=MYPROG) to perform tasks like data processing, report generation, or utility functions.
- Online Transaction Processing: Programs executed under CICS or IMS TM to handle real-time transactions from terminals or other applications.
- TSO Command Processors: Executables that extend TSO functionality, allowing users to issue custom commands from the TSO READY prompt.
- System Utilities and Services: Core operating system components, utility programs (like IEBGENER, IDCAMS), and system exits are often executables.
- Subroutines and Functions: Executables that are called by other programs to perform specific tasks, promoting modularity and code reuse.

Related Concepts

An executable is the culmination of the software development lifecycle on z/OS. It is derived from source code (e.g., COBOL, PL/I, Assembler) which is first translated into object modules by a compiler or assembler. These object modules are then combined and resolved by the Linkage Editor or Binder to form the final executable (load module or program object). It resides in a load library and is loaded into an address space for execution, typically initiated by a JCL EXEC statement or an online transaction manager like CICS or IMS.

Best Practices:

Version Control: Always manage executables and their corresponding source code under a robust Software Configuration Management (SCM) system to track changes and facilitate rollbacks.
Library Management: Organize executables into appropriate load libraries, using STEPLIB or JOBLIB in JCL judiciously to control program search order, or placing frequently used programs in the system LINKLIB or LPALIB for better performance.
Re-entrancy Design: For programs that will be heavily used or shared across multiple users/tasks (e.g., CICS programs), design them to be re-entrant to optimize memory usage and improve system throughput.
Performance Optimization: Ensure executables are compiled with appropriate optimization levels and consider factors like program size, I/O patterns, and CPU usage during development and testing.
Security: Implement proper RACF (or equivalent) security profiles on load libraries and individual load modules to control who can execute, modify, or delete them, preventing unauthorized access or tampering.