Mainframe batch operations: Job Control Language (JCL) statements are primarily used by the operating system to indicate the start and end of a batch job and define its processing context—select the most accurate purpose.

Introduction / Context:
On IBM-style mainframes, JCL describes how batch jobs should run—what programs to execute, what datasets to use, and how outputs are handled. This question checks the essential role of JCL in identifying job boundaries and execution parameters rather than performing data transfers or CPU allocation itself.

Given Data / Assumptions:

• We are in a batch processing environment using JCL for job submission.
• The OS and subsystems (e.g., JES) interpret JCL statements.
• JCL conveys metadata and control, not low-level I/O copying logic.

Concept / Approach:
JCL statements such as JOB, EXEC, and DD define job start, program steps, resources, and outputs. They inform the OS where a job begins and ends and how to manage datasets and printers. JCL does not itself “allocate the CPU” (the scheduler does) or directly copy input from card readers to disks (utilities or programs do that work under OS control).

Step-by-Step Solution:
1) Identify JCL as a control language for job description, not a data-mover or CPU allocator.2) Match the option that states the core role: delineating job boundaries for the OS.3) Recognize that actual data transfers and CPU dispatching are managed by utilities and the scheduler, respectively.

Verification / Alternative check:
Operational manuals show JOB statements mark job start; end of input delineates job end. EXEC/DD statements provide step and dataset details. CPU allocation remains a scheduler function, separate from JCL descriptors.

Why Other Options Are Wrong:

• Reading card input to disk: Performed by utilities or programs, not JCL itself.
• Allocating the CPU: A kernel/scheduler responsibility, not a JCL feature.
• All of the above: Bundles incorrect roles with the correct one, so it is false.

Common Pitfalls:
Equating JCL with general-purpose scripting. JCL is declarative control metadata for batch jobs, not a procedural language for data copying or resource scheduling.

Final Answer:
Specify, to the operating system, the beginning and end of a job in a batch.