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Chapter 43

The structure of SYSTEM/3601

Part I-Outline of the logical structure

G. A. Blaauw / F. P. Brooks, Jr.

Summary A general introductory description of the logical structure of SYSTEM/360 is given. In addition, the functional units, the principal registers and formats, and the basic addressing and sequencing principles of the system are indicated.

In the SYSTEM/360 logical structure, processing efficiency and versatility are served by multiple accumulators, binary addressing, bit-manipulation operations, automatic indexing, fixed and variable field lengths, decimal and hexadecimal radices, and floating-point as well as fixed-point arithmetic. The provisions for program interruption, storage protection, and flexible CPU states contribute to effective operation. Base-register addressing, the standard interface between channels and input/output control units, and the machine-language compatibility among models contribute to flexible configurations and to orderly system expansion.

SYSTEM 360 is distinguished by a design orientation toward very large memories and a hierarchy of memory speeds, a broad spectrum of manipulative functions, and a uniform treatment of input/output functions that facilitates communication with a diversity of input/output devices. The overall structure lends itself to program-compatible embodiments over a wide range of performance levels.

The system, designed for operation with a supervisory program, has comprehensive facilities for storage protection, program relocation, nonstop operation, and program interruption. Privileged instructions associated with a supervisory operating state are included. The supervisory program schedules and governs the execution of multiple programs, handles exceptional conditions, and coordinates and issues input/output (I/O) instructions. Reliability is heightened by supplementing solid-state components with built-in checking and diagnostic aids. Interconnection facilities permit a wide variety of possibilities for multisystem operation.

The purpose of this discussion is to introduce the functional units of the system, as well as formats, codes, and conventions essential to characterization of the system.

Functional structure

The SYSTEM/360 structure schematically outlined in Fig. 1 has seven announced embodiments. Six of these, namely, Models 30. 40, 50, 60, 62, and 70, will be treated here.1 Where requisite I/O devices, optional features, and storage capacity are present, these six models are logically identical for valid programs that contain explicit time dependencies only. Hence, even though the allowable channels or storage capacity may vary from model to model (as discussed in Chap. 44), the logical structure can be discussed without reference to specific models.

Input/out put

Direct communication with a large number of low-speed terminals and other I/O devices is provided through a special multiplexor channel unit. Communication with high-speed I/O devices is accommodated by the selector channel units. Conceptually, the input/output system acts as a set of subchannels that operate concurrently with one another and the processing unit. Each subchannel, instructed by its own control-word sequence, can govern a data transfer operation between storage and a selected I/O device. A multiplexor channel can function either as one or as many subchannels; a selector channel always functions as a single subchannel. The control unit of each I/O device attaches to the channels via a standard mechanical-electrical-programming interface.


The processing unit has sixteen general purpose 32-bit registers used for addressing, indexing, and accumulating. Four 64-bit floating-point accumulators are optionally available. The inclusion of multiple registers permits effective use to be made of small high-speed memories. Four distinct types of processing are pro-

1IBM Sys. J, vol. 3, no. 2, pp. 119-135, 1964.

1A seventh embodiment, the Model 92, is not discussed in this paper. This model does not provide decimal data handling and has a few minor differences arising from its highly concurrent, speed-oriented organization. A paper on Model 92 is planned for future publication in the IBM Systems Journal.



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