Entry Point

Enhanced Definition

In IBM mainframe systems, an entry point is the specific memory address within a load module or program where execution begins when that program or routine is invoked. It represents the designated starting instruction for a callable unit of code, allowing control to be transferred to a specific function or procedure. In z/OS, an **entry point** is the specific virtual storage address within a load module or program where execution begins when control is transferred to it. It serves as the designated starting instruction for a program or a specific routine within a larger program. In the context of IBM z/OS, an **Entry Point** is the specific memory address within a load module or program where execution begins when the program or a particular routine within it is invoked. It serves as the starting address for the CPU to fetch and execute instructions for that program or routine.

Key Characteristics

- Symbolic Name: An entry point is typically associated with a symbolic name, such as a PROGRAM-ID in COBOL, an ENTRY statement name in Assembler or PL/I, or a specific label within a program.
- Load Module Structure: A single load module can contain multiple entry points, enabling different routines or functions within the same physical module to be called independently by other programs.
- Address Resolution: During the link-edit (or bind) process, the linker resolves external references and assigns relative addresses to entry points. When the program is loaded into memory, these relative addresses are converted into absolute memory addresses.
- Invocation Mechanism: Programs are invoked via their entry points using mechanisms like the CALL statement in COBOL, EXEC CICS LINK or EXEC CICS XCTL in CICS, or the LINK macro in Assembler.
- Parameter Passing: Parameters are commonly passed to an entry point through a parameter list, which is a contiguous block of addresses pointing to the actual data. The address of this list is typically passed in a general-purpose register (e.g., R1).
- Control Transfer: Invoking an entry point involves transferring control of execution by setting the program counter (or instruction address register) to the entry point's resolved memory address.

Use Cases

- Main Program Execution: The PROGRAM-ID name of a COBOL program serves as its primary entry point when it is executed as a batch job step via JCL or invoked directly by the operating system.
- Subroutine/Subprogram Calls: A CALL statement in COBOL invokes a subprogram at its defined entry point, promoting modular programming and code reuse across different applications.
- CICS Program Invocation: In a CICS environment, EXEC CICS LINK PROGRAM('PROGNAME') or EXEC CICS XCTL PROGRAM('PROGNAME') transfers control to the specified program's entry point, enabling transaction processing.
- Dynamic Call Resolution: When a program is dynamically called, the system locates the corresponding load module and transfers control to its designated entry point.
- Alternate Entry Points: A single load module might contain multiple ENTRY statements (e.g., in Assembler or PL/I) to expose different functionalities or initialization routines within the same compiled unit.

Related Concepts

An entry point is a core concept closely tied to Load Modules, which are the executable units containing the compiled code and entry point information. The Link-Editor (Binder) is responsible for resolving external references and creating the load module, defining these entry points. When a program is loaded into memory, the system uses the entry point address to initiate execution. Calling Conventions dictate how parameters are passed to and from an entry point, often involving specific registers (like R1 for the parameter list address) and stack management.

Best Practices:

Clear Naming Conventions: Use descriptive and consistent naming for PROGRAM-IDs and ENTRY points to enhance program readability, maintainability, and debugging.
Single Primary Entry Point: For most COBOL programs, define a single, clear primary entry point (the PROGRAM-ID) to simplify program flow and reduce complexity, especially for maintenance.
Document Alternate Entry Points: If using alternate ENTRY points within a module, thoroughly document their purpose, expected parameters, and specific behavior to avoid misuse and ensure clarity.
Parameter Validation: Always validate input parameters received at an entry point to prevent data corruption, program abends, and potential security vulnerabilities.
Consistent Calling Conventions: Adhere to standard mainframe calling conventions (e.g., passing parameter list address in R1, return address in R14) to ensure seamless interoperability between different programming languages and modules.