Basic IBM Mainframe Assembly Language Programming

The purposes of this book are to give the reader a basic understanding of the IBM mainframe architecture from the perspective of the instruction set and to serve as an assembly language text for introductory college courses in computer organization.Many computer architectures have come and gone in the past 50 years and only a relatively few designs remain. While most computer users are familiar with the ubiquitous Intel x86 microprocessor line, few have had much experience with another widely used architecture, the venerable IBM mainframe. In the not to distant past it was assumed that the rise of microprocessors and server centric computing would bring an end to the age of big iron (as mainframes were referred to). In the early 90s the rapid rise of companies such as Sun, Apollo, Silicon Graphics, MIPS, Digital Equipment Corporation (DEC), Tandem, Prime, Data General, SDS (also knows as XDS) and many more seemed to point to a world without the mainframes. All these companies, however, are now gone. Likewise vanished are the other mainframe manufacturers: NCR, Burroughs, Control Data Corporation, General Electric, Honeywell, RCA and UNIVAC.But the IBM mainframe architecture has endured and, rather than declining, is growing in popularity and becoming the platform of choice for such rapidly emerging technologies as cloud computing and virtualization. As the world chokes in an energy and space intensive microprocessor based server sprawl, a single mainframe system can deliver thousands of virtual Linux platforms for a fraction of the cost in dollars, energy and space. Now, however, the baby-boom generation programmers who wrote the billions of lines of legacy mainframe assembly code since the introduction of the IBM 360 in 1964 are beginning to retire. Consequently, the demand for replacement programmers to maintain, support and extend these applications as well as develop new ones is growing rapidly. To those looking to meet this need, this text will serve as a basic road map and introduction.The other purpose of this book is to provide an alternative assembly language platform for introductory college level courses in computer organization. From an academic perspective, the question of which architecture to use poses several challenges. Once, the answer was simple: the Intel x86 assembly language. However, as the Intel x86 architecture has grown into an intricate and unwieldy collection of exceptions and special cases, its value as a viable pedagogical platform in an introductory course has diminished. So, in many cases, the choice defaults to an assembly language based on a simulator for a non-existent, abstract RISC based architecture. Although this gives students a taste of a machine level architecture, it provides little real experience that can be used to actually further their careers and is often regarded by students themselves as an exercise in futility.For most, the basic impediment to teaching mainframe assembly language is actual access to a mainframe. While many colleges and universities use mainframes, students seldom have access to these. This text, however, is designed to be used in combination with a free, portable, open sourced, GPL licensed, Java based z390 mainframe emulator and macro assembler which permits a student to assemble and execute IBM mainframe programs on a Windows based PC2.This text concentrates on the basic architecture in order to keep to a manageable size suitable for a 5 week section in a computer organization course. While there are a number of instructions and features not discussed, these are, for the most part, logically intuitive extensions of the basic architecture discussed here.