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The PDP-8 and Other 12-Bit Computers



Since the Laboratory Instrument Computer (LINC) was one of the machines that had a great influence on the design of the PDP-4 and the PDP-5, a discussion of the DEC 12-bit machines must start with the LINC.

The LINC was designed by Clark and Molnar [Clark and Molnar, 1964; 1965], who were in turn influenced by Control Data Corporation's (CDC) 160, designed by Seymour Cray. The relationship of these early computers is shown in Figure 1. The first version of the LINC was built at the M.I.T. Lincoln Laboratory where it was demonstrated in March 1962 (Figure 2). In 1963 the LINC was redesigned for production at a special M.I.T. laboratory, the Center Development Office.

While the LINC contributed to DEC history primarily as a forerunner of the PDP-4 and PDP-5, it also generated a number of other developments. The LINC tape unit and the system ideas that permitted a user to have personal files were later incorporated directly into the DEC tape design and programs. The tape system and a powerful CRT-based console made possible the first complete personal computer available to a user, in this case the researcher, at a reasonable price. The LINC machines had been constructed mainly from DEC Systems Modules, a convenience when DEC subsequently manufactured LINC machines directly from the 1963 design. Later, Wes Clark with Dick Clayton designed the LINC-8, a two-processor machine (LINC + PDP-8) which executed both instruction sets in parallel. Clayton also designed the PDP-12, a single physical processor that executed either PDP-8 or LINC instructions sequentially by switching modes.

Some of the characteristics of the LINC Family machines are given in Table 1, and photographs appear in Figures 3, 4, and 5. Note that the size remained essentially constant at one cabinet throughout the life of this computer family.

On machines prior to the LINC, DEC had been stressing design flexibility and modularity, providing many ways to interconnect computer components in order to create a variety of structures. This detracted from having a base system configuration complete with software. In contrast, the LINC was quite constrained, with


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