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Chapter 46

The PDP-8 Family1

C. G. Bell / J. E. McNamara

Figure 1 depicts the PDP-8 family tree. The family ancestry began with the Laboratory Instrument Computer (LINC) initially built at the MIT Lincoln Laboratory in 1962, which, incidentally, we believe was the earliest personal computer. DEC began manufacturing LINC's in 1965. Eventually a PDP-8 and LINC were combined in a dual processor called the LINC-8.

In 1962, the need arose to produce a replacement for an analog monitoring system as a front end to a reactor control complex. A 12-bit real time control computer, the PDP-5, was constructed.

The analog nature of the initial application was addressed by building an analog-to-digital converter into the Accumulator. The concept of an I/O bus was introduced instead of the radial I/O structure of previous DEC designs. The I/O Bus permitted equipment options to be added incrementally from a zero base instead of having the pre-allocated space, wiring, and cable drivers that characterized the radial structure. This lowered the entry cost of the system and simplified the later reconfiguring of machines in the field.

Although the design was optimized around the 4-Kword memory, the PDP-5 ultimately evolved to 32-Kword configurations using a memory extension unit. Similarly, although the base machine design did not include built-in multiply and divide functions, these were added later in the form of an Extended Arithmetic Element.

While the PDP-5 had been a reasonably successful computer, it soon became evident that a new machine capable of far greater performance was required. New logic technology promised a substantial speed improvement, and new core memory technology was becoming available that would permit the memory cycle

1Abstracted from C. G. Bell, J. C. Mudge, and J. E. McNamara, Computer Engineering; A DEC View of Hardware Systems Design, Digital Press, Maynard, Mass., 1978, pp. 175-208.


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