Fundamentals of Mainframe Computing

An IBM Z mainframe is a high-performance computer platform engineered for exceptional reliability, availability, and scalability. It is designed to be a highly secure and connected system that manages a large percentage of the world's mission-critical business data and transactions. The hardware platform, known as IBM Z, provides the foundation upon which operating systems like z/OS run to deliver these enterprise-grade capabilities.

z/OS is a 64-bit operating system that serves as the software foundation for IBM Z mainframes. It provides a comprehensive suite of services for program management, data management, security, network communications, and distributed computing. As a mature and robust operating system, z/OS is designed to manage complex workloads, support thousands of concurrent users, and ensure the integrity and availability of critical business applications.

The z/OS operating system is specifically designed to work in conjunction with the IBM Z hardware, which follows the 64-bit z/Architecture. This tight integration allows z/OS to fully leverage the hardware's capabilities, such as its specialized processors for cryptography and its robust I/O subsystem. The services provided by z/OS, from security to data management, are built upon the reliability and scalability inherent in the z/Architecture.

A logical partition, or LPAR, is a way to divide a mainframe's physical hardware resources—such as processors and memory—into multiple, independent virtual machines. Each LPAR runs its own copy of an operating system and functions as a separate computer, though they all share the same physical hardware frame. The I/O Definition File (IODF) is a key configuration component that contains data defining the logical partitions on a server.

Mainframe virtualization works by using LPARs to logically divide the system's resources. During the system startup process, known as an Initial Program Load (IPL), z/OS locates the memory and other resources that have been allocated to its specific LPAR. This allows a single physical mainframe to host multiple, isolated system environments simultaneously, enabling different workloads (e.g., production, test, development) to run on the same hardware without interfering with one another.

The core principles of the z/Architecture are rooted in providing a highly reliable, available, and scalable 64-bit computing environment. This architecture is the foundation for the mainframe's ability to manage mission-critical data and high-volume transactions with exceptional stability. Key tenets include robust security, advanced I/O processing capabilities, and backward compatibility to protect investments in existing applications.

The z/OS system programmer is responsible for the installation, customization, and maintenance of the mainframe operating system and its related software. Their duties include planning the system configuration, managing daily operations, defining I/O configurations, ensuring security and integrity, and providing technical support to users. This role is critical for aligning the system's capabilities with business needs and maintaining high levels of availability and performance.

Virtual storage is a concept that allows the memory addresses available to a program (the address space) to be much larger than the physical memory installed on the computer. z/OS manages this through a process called Dynamic Address Translation (DAT), using components like the Virtual Storage Manager (VSM), Real Storage Manager (RSM), and Auxiliary Storage Manager (ASM). When data isn't in physical memory, z/OS moves it to and from disk in units called pages, a process known as paging and swapping. z/OS further optimizes this with a "logical swap

Multiprogramming in z/OS refers to the ability of the operating system to manage and execute multiple programs concurrently, giving each a slice of processor time to maximize resource utilization. Multiprocessing is the system's ability to use multiple physical processors (CPUs) simultaneously to execute instructions from different programs or tasks. Together, these concepts allow a mainframe to handle an immense number of concurrent workloads efficiently.

An Initial Program Load (IPL) is the process of loading the operating system into the computer's memory to begin operations, equivalent to "booting" on other platforms. The purpose of an IPL is to initialize the hardware and software environment, starting from a specific device called the SYSRES volume. During this process, the system reads configuration parameters from members in parmlib to customize the environment for that specific session.

The Data Facility Storage Management Subsystem (DFSMS) is a suite of products that automates and centralizes the management of data and storage on z/OS. Its key components provide functions for managing storage hardware and software, maintaining catalogs, and managing specific data types like VSAM data sets. The suite is an operating environment that manages storage based on defined policies for availability, performance, space, and security.

Specialized processors, such as the IBM z Integrated Information Processor (zIIP), are designed to run specific types of workloads, like Java applications or other designated tasks. The primary advantage of using them is economic: workloads running on a zIIP do not incur software license charges. This allows enterprises to run new or different types of applications on the mainframe more cost-effectively by offloading the work from general-purpose processors.

A sysplex, short for "system complex," is a configuration where multiple z/OS systems are connected and work together as a single, coordinated entity. A Parallel Sysplex allows these systems to share data and workloads, providing high availability and scalability. A Geographically Dispersed Parallel Sysplex (GDPS) extends this concept across different physical locations to provide a comprehensive disaster recovery solution.

The Job Entry Subsystem (JES), which comes in two versions (JES2 and JES3), is a core component of z/OS that manages batch jobs. Its primary function is to receive jobs into the system, schedule them for execution by z/OS, and control the processing of their output. JES uses a technique called spooling to manage job input and output efficiently on disk, decoupling the high speed of the CPU from slower peripheral devices.

Based on the products, trademarks, and company information presented, International Business Machines Corporation (IBM) is the primary manufacturer and developer. The hardware platform is referred to as IBM Z, the operating system is IBM z/OS, and nearly all key software components mentioned, such as RACF, DB2, CICS, and Parallel Sysplex, are IBM products or trademarks. The documentation itself is from IBM Redbooks.