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Chapter 30

The SYMBOL Computer

A Large Experimental System Exploring Major Hardware Replacement of Software1

William B. Smith / Rex Rice / Gilman D. Chesley / Theodore A. Laliotis / Stephen F. Lundstrom / Myron A. Calhoun / Lawrence D. Gerould / Thomas C. Cook


The SYMBOL system is the result of a major developmental effort to increase the functional capability of hardware. Part of the charter of the broad based project was to reexamine the traditional division between hardware and software, to reexamine the respective roles of program instruction and data storage, and to reduce the overall complexity and cost of computing [Rice and Smith, 1971]. In order to adequately evaluate the concepts that had been developed it was concluded that an experimental, usable, real system must be built. The SYMBOL system, now operational, is the embodiment of this effort.

The system was developed in an environment with hardware and software design considered in common. Virtually no one associated with the project could refer to himself as a hardware or software specialist exclusively. As an example, the logic design of the field process units was done by an individual with a basic programming background [Mazor, 1968]. The wire routing automation was developed by an engineer who was formerly a pure logic design specialist.

Even before the system became operational much had been learned about the practical aspects of building highly capable hardware, No claim is made that SYMBOL represents an optimum general purpose, time-sharing, multiprocessing system. In contrast, numerous simplifying assumptions were made in the system where they did not serve the goals of the project. Certain modularity restrictions are examples of this. It Is claimed that SYMBOL represents a significant advance in systems technology and provides the foundation for a significant reduction in the cost of computing. As the system moves into an intensive evaluation phase it should prove to be a real asset for advanced systems research.

This paper represents an overview of the SYMBOL organization. An attempt is made to give simplified examples of various key features in contrast to a broad brush treatment of many topics.

Gross Organization

The system has eight specialized processors that operate as autonomous units. Each functional unit is linked to the system by the Main Bus. See Fig. 1. Consider some of the features of the system and their relationship to the gross processor organization as outlined in the following sections.

Dynamic Memory Management

Direct hardware memory management is perhaps the most unique feature of the SYMBOL system. The memory management centers around a special purpose processor called the Memory Controller (MC). The MC effectively isolates the main memory from the main bus and the other processors and in turn provides a more sophisticated storage function for the various processors. In contrast to simple read/write memory operations the MC has a set of fifteen operations that are available to the

1AFIPS SJCC, 1971, pp. 601-616.


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