ASSIGN Clause

The format of the ASSIGN clause has become simpler. COBOL for OS/390 and VM may sometimes allow the DOS/VS COBOL format but may produce unexpected results at run-time. Refer to the COBOL for OS/390 and VM Compiler and

Run-Time Migration Guide for more information.

12.4.3 DATA DIVISION - FILE DESCRIPTION (FD)

BLOCK CONTAINS

In OS/390 it is recommended that you specify BLOCK CONTAINS 0 RECORDS or BLOCK CONTAINS 0 CHARACTERS in your program. For an output file, you specify the required information on the DD statement. If you omit the blocking information for an output file OS/390 will supply a System Determined Blocksize (SDB). For an input file, the information is obtained from information in the catalog and VTOC.

If you specify BLOCK CONTAINS n RECORDS and also supply the information on the

DDstatement, for an output file, the BLOCK CONTAINS n RECORDS takes precedence over the DD statement information.

If you specify BLOCK CONTAINS n RECORDS for an input file, and the VTOC information does not match, your program may ABEND or return a file status code of 39.

LABEL RECORDS

DOS/VS COBOL accepts the LABEL RECORD IS data-namefor non-sequential files. COBOL for OS/390 and VM does not, therefore you must change your program to remove LABEL RECORD IS data-namefor these files.

LINAGE Clause and END-OF-PAGE Phrase

Under DOS/VS COBOL the END-OF-PAGEphrase may be specified without a corresponding LINAGE clause in the file description entry.

Under COBOL for OS/390 and VM if the END-OF-PAGEphrase is specified then the FD entry for the file must contain a LINAGE clause. Even then, you may find that your printed page layout is not as you expect. You should use your own line count logic in your program, making use of the LINAGE-COUNTERspecial register.

12.4.4 PROCEDURE DIVISION - Input/Output

CLOSE Statement for Tapes

Under DOS/VS COBOL the CLOSE file-nameWITH LOCK statement closed and locked the file, and UNLOADed the tape reel or cartridge. Under COBOL for OS/390 and VM the file is closed and locked, but only rewound, not onloaded.

Similarly, for multi-volume tape files, DOS/VS COBOL rewinds and unloads each volume at end-of-volume. COBOL for OS/390 and VM only rewinds the tape, it does not unload it.

Note that this behavior may be different if you use a tape management system.

256VSE to OS/390 Migration Workbook

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IBM OS/390 Procedure Division Input/Output, Assign Clause, Linage Clause and END-OF-PAGE Phrase, Close Statement for Tapes

OS/390 specifications

IBM OS/390, a versatile operating system, was a cornerstone in enterprise environments and played a pivotal role in mainframe computing. Released in the mid-1990s, OS/390 combined the strengths of IBM's MVS (Multiple Virtual Storage) with new features and enhancements, targeting scalability, reliability, and performance in demanding business applications.

One of the key features of OS/390 was its robust support for multiple users and processes. The system allowed thousands of concurrent users to access applications and data, ensuring high availability and minimizing downtime—a critical requirement for many large organizations. This scalability was supported through various enhancements in memory management and processor scheduling, enabling optimal resource allocation across diverse workloads.

OS/390 was known for its superior workload management capabilities. The Workload Manager (WLM) component allowed administrators to define service policies, specifying how system resources would be allocated according to the priority of tasks. This ensured that critical business processes received the necessary resources while less critical tasks were managed more flexibly.

Another significant characteristic of OS/390 was its commitment to security. The operating system provided comprehensive security features, including user authentication, data encryption, and auditing capabilities. This focus on security was vital for organizations handling sensitive data, ensuring compliance with regulations and safeguarding against unauthorized access.

OS/390 also supported advanced technologies that facilitated integration and development. The system included features like the IBM CICS (Customer Information Control System) for transaction processing and IMS (Information Management System) for database management. These technologies allowed organizations to build robust, high-performance applications tailored to specific business needs.

The ease of network integration was another strength of OS/390. With the advent of the Internet and global connectivity, OS/390 systems could easily interface with various network protocols, enabling businesses to operate in a connected world. This inclusion paved the way for many organizations to expand their capabilities and offer new services, driving digital transformation.

In conclusion, IBM OS/390 represented a significant advancement in mainframe technology, combining scalability, security, and robust workload management. Its rich feature set and support for critical enterprise applications solidified its role as a vital component of many organizations' IT infrastructures, ensuring they could meet their operational challenges head-on while supporting future growth. As technology continues to evolve, the legacy of OS/390 remains influential in the realm of computing.