Chapter 13. Assembler

13.1 Assembler Products

In OS/390, the High-Level Assembler for MVS and VM Program Product

(5696-234) is required for system generation (SYSGEN) and maintenance activities. It can also be used for application programming projects, and must be used when assembler routines are designed for 31-bit addressing facilities. See High-Level Assembler for MVS and VM General Information, GC26-4943 and OS/390 MVS Extended Addressability Guide, GC28-1769 for more information on this subject.

AGuide to Using MVS/XA Interface Facilities, SR21-1468 and SR21-1469, is

recommended for installations that wish to extend or customize system functions provided by OS/390. The ²interface facilities² described will use either exit or

macro instructions to provide customization.

Recommendation

In converting assembler programs from VSE to MVS no attempt should be made initially to use 31-bit programming techniques. The main objective should be to get the programs ²converted to MVS programs as expediently as possible²; that is, don¢t add new facilities (31-bit) at the outset. Once the programs are converted and operate successfully in MVS, they could then be reworked for 31-bit addressing if the need exists; for example, to address a VSCR problem.

13.2 General Assembler Conversion Comments

One of the most challenging tasks in moving from VSE to MVS may be the modification of application programs at the assembler language level. Coding at the assembler level includes control program macros and user-written machine language instructions. The machine language instructions are identical in both systems. The control program (or supervisor) macros and input/output macros of the systems are different, even though some have the same name. All base register usage, supervisor macros, and input/output logic of VSE assembler language programs must be checked for conformity to MVS conventions. A

one-for-one mechanical replacement is possible in many cases. The complexity of the program and its use of supervisor functions is proportionate to the effort required to convert the programs. Simple programs are usually easy to convert.

Registers are an important factor in performance. When a VSE program is to be used under MVS, there may be mandatory changes in the use of registers because of macro expansions. One way to handle this problem is to add additional instructions to shift the MVS register contents to make them correspond to the VSE register contents. However, this approach may cause MVS to run slower because of unnecessary loading and storing of registers. The alternative is to change the register usage within the program to conform to MVS requirements, using the symbolic register notation through equates. Correct register usage in complex programs prevents problems requiring extensive programmer debugging effort.

© Copyright IBM Corp. 1998

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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.