Systems Reference Library IBM System/360 Principles of Operation

The <strong>IBM</strong> <strong>System</strong>/<strong>360</strong> is a solid-state, program compatible, data processing system providing the speed, precision, and data manipulating versatility demanded by the challenge <strong>of</strong> commerce, science, and industry. <strong>System</strong>/<strong>360</strong>, with advanced logical design implemented by microminiature technology, provides a new dimension <strong>of</strong> performance, flexibility, and reliability. This dimension makes possible a new, more efficient systems approach to all areas <strong>of</strong> information processing, with economy <strong>of</strong> implementation and ease <strong>of</strong> use. <strong>System</strong>/<strong>360</strong> is a single, coordinated set <strong>of</strong> new data processing components intended to replace the old logical structure with an advanced creative design for present and future application. The logical design <strong>of</strong> <strong>System</strong>/<strong>360</strong> permits efficient use at several levels <strong>of</strong> performance with the preservation <strong>of</strong> upward and downward program compatibility. Extremely high performance and reliability requirements are met by combining several models into one multisystem using the multisystem feature. General-Purpose Design <strong>System</strong>/<strong>360</strong> is a general-purpose system designed to be tailored for commercial, scientific, communications, or control applications. A Standard instruction set provides the basic computing function <strong>of</strong> the system .. To this set a decimal feature may be added to provide a Commercial instruction set or a floating-point feature may be added to provide a Scientific instruction set. When the storage protection feature is added to the commercial and scientific features, a Universal set is obtained. Direct control and timer features may be added to satisfy requirements for TELE-PROCESSINC@ systems to allow load-sharing or to satisfy real-time needs. <strong>System</strong>/<strong>360</strong> can accommodate large quantities <strong>of</strong> addressable storage. The markedly increased capacities over other present storage is provided by the combined use <strong>of</strong> high-speed storage <strong>of</strong> medium size and large-capacity storage <strong>of</strong> medium speed. Thus, the requirements for both performance and size are satisfied in one system by incorporating a heirarchy <strong>of</strong> storage units. The design also anticipates future development <strong>of</strong> even greater storage capacities. <strong>System</strong>/<strong>360</strong> incorporates a standard method for attaching input/output devices differing in function, data rate, <strong>IBM</strong> <strong>System</strong>/<strong>360</strong> and access time. An individual <strong>System</strong>/<strong>360</strong> is obtained by selecting the system components most suited to the applications from a wide variety <strong>of</strong> alternatives in internal performance, functional ability, and input/output (I/O). Models <strong>of</strong> <strong>System</strong>/<strong>360</strong> differ in storage speed, width (the amount <strong>of</strong> data obtained in each instruction access), register width, and capability <strong>of</strong> simultaneous processing. Yet these differences do not affect the logical appearance <strong>of</strong> <strong>System</strong>/<strong>360</strong> to the programmer. Several cpu's permit a wide choice in internal performance. The range is such that the ratio <strong>of</strong> internal performances between the largest and the smallest model is approximately 50 for scientific computation and 15 for commercial processing. Compatibility All models <strong>of</strong> <strong>System</strong>/<strong>360</strong> are upward and downward program compatible, that is, any program gives identical results on any model. Compatibility allows for ease in systems growth, convenience in systems backup, and simplicity in education. The compatibility rule has three limitations. 1. The systems facilities used by a program should be the same in each case. Thus, the optional CPU features and the storage capacity, as well as the quantity, type, and priority <strong>of</strong> I/O equipment, should be equivalent. 2. The program should be independent <strong>of</strong> the relation <strong>of</strong> instruction execution times and <strong>of</strong> I/O data rates, access times, and command execution times. 3. The compatibility rule does not apply to detail functions for which neither frequency <strong>of</strong> occurrence nor usefulness <strong>of</strong> result warrants identical action in all models. These functions, all explicitly identified in this manual, are concerned with the handling <strong>of</strong> invalid programs and machine malfunctions. <strong>System</strong> Program Interplay <strong>of</strong> equipment and program is an essential consideration in <strong>System</strong>/<strong>360</strong>. The system is designed to operate with a supervisory program that coordinates and executes all I/O instructions, handles exceptional conditions, and supervises scheduling and execution <strong>of</strong> multiple programs. <strong>System</strong>/<strong>360</strong> provides for <strong>IBM</strong> <strong>System</strong>/<strong>360</strong> 5