13.2.6.14 PIOCS

EXCP is often used in VSE in association with card files (for example, SYSIPT), print files (for example, SYSLST) or the operator console (for example, SYSLOG). In MVS, EXCP can not be used for spool files or to dialog with the operator. VSE programs that use EXCP should be converted to use standard access method such as QSAM or the WTO/WTOR macros.

If IBM-supplied access methods are used in MVS, provide a DCB, issue an OPEN macro for the data set, and then process it by appropriate I/O macros. It is the function of the access routines to:

Provide for CCWs.

Construct the IOB (input/output block). Construct the ECB (event control block).

Issue an EXCP (execute channel program) macro.

Subsequently, the I/O supervisor schedules the request, issues the START I/O instruction, handles interrupts, posts results in the form of completion codes, and, if necessary, executes error recovery routines. Therefore, if the usual access methods do not apply to a specific problem, you must include in your program the coding necessary to provide for the preceding access method functions.

Under VSE, only one control block, the CCB, is needed and this control block may be built by a macro. Under MVS, however, you must allocate and partially fill the input/output control block and the DECB through the use of DCs.

Overview of Programming Elements

In VSE, certain job control statements, system macros, and control blocks are used. For each of these major programming elements (except SEOV), there is a corresponding major MVS element. (See Figure 47 on page 320)

CCB Macro

Figure 53 shows the relationship between the VSE CCB macro operands and their MVS equivalents:

CCB OPERAND

MVS EQUIVALENT

 

 

SYSnnn

DDNAME field of DCB macro. After the OPEN macro has been

 

executed, the unit control block address for the actual device

 

is in the DEB.

 

 

Command-list-name

CCW address field of IOB. See discussion on CCB Fields.

Xnnnn

 

(transmission

 

bytes)

 

 

 

Sense Address

No corresponding element. The system normally provides the

 

first two bytes in the IOB.

 

 

Figure 53. Relationship between CCB operands and MVS Equivalents

Chapter 13. A s s e m b l e r 327

Page 351
Image 351
IBM OS/390 manual Piocs, Overview of Programming Elements, CCB Macro

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.