Extended Precision, Multitasking, File Organization

15.1.2 Extended Precision

Available with the MVS version of the PL/I compiler, extended precision floating point allows working with variables of two double-words. This extended precision is requested by specifying a precision greater than 16 for decimal float variables and 53 for binary float variables. DOS PL/I does not support extended precision floating point arithmetic.

15.1.3 Multitasking

Multitasking support in MVS introduces in PL/I a number of new statements. It is worth noting that these new functions are only invoked if the program calls them explicitly. For example, it is possible in a CALL macro to associate an

EVENT-type control variable. It is then possible, at the end of the program, to test the termination of the associated task with a procedure invoked by a WAIT or with the function COMPLETION. Similarly, it is possible to modify with a single statement the relative priority of a sub-task:

PRIORITY(T1) = PRIORITY(TI) + 2;

These new functions are additions to the DOS version and do not affect the compatibility of the compilers.

15.1.4 Dynamic Loading of Dependent Programs

It is possible to dynamically load sub-programs written in PL/I separately from a main PL/I program. This is done by using the FETCH statement. This statement, as the RELEASE statement with which it is associated, is only available on MVS. FETCH causes loading of a LOAD module from LINKLIB or an execution library, while RELEASE frees the space occupied by the module. It is necessary to pass control to the sub-program by a CALL statement. Certain restrictions exist in its use due to the fact that there must not be any external references between the main program and the sub-program loaded dynamically. Particular attention must be paid to the use in the sub-program of files and controlled variables declared in the main program. Since FETCH and RELEASE statements do not occur in DOS PL/I programs, their presence in the MVS implementation is purely additive and has no impact on DOS-to-MVS conversion.

15.1.5 File Organization

The file organizations supported in DOS by the PL/I Optimizer are:

REGIONAL (1) and REGIONAL (3)

CONSECUTIVE

VSAM

INDEXED

In MVS, these are all supported and so are REGIONAL(2) and TCAM (TRANSIENT). A new feature in the use of REGIONAL or INDEXED files is the capability of requesting a lock at the record level to control simultaneous updating of the same file by several programs or tasks. This is the attribute EXCLUSIVE. If this attribute is applied to a PL/I file, then at the time of a READ, the record will be locked until a REWRITE or UNLOCK has been issued. It is equally possible to request the reading of a record without locking it: READ NOLOCK. This support uses the ENQ and DEQ facilities of MVS, and implies that all the programs sharing the file be written in PL/I such that the RNAME and QNAE generated are identical for the same record. This function is not available for VSAM.

334VSE to OS/390 Migration Workbook

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.

IBM OS/390 manual Extended Precision, Multitasking, Dynamic Loading of Dependent Programs

Models: OS/390