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The PDP-8 was the first minicomputer. As with many early second-generation machines, the popularity and longevity of the basic ISP was greatly underestimated. The first applications of new technology focused on price reduction while maintaining constant performance, although the original PDP-8 had a significant performance increase over its predecessor, the PDP-5. The architecture was also subjected to several painful stretches from 1963 to 1979, including:

• Three changes in the physical I/O bus (from negative-polarity logic to positive-polarity logic, then to a unified bus, and finally to a reduced-width microcomputer bus)
• Memory expansion from 4 kilowords to 32 kilowords, and finally to 128 kilowords
• ISP enhancements, including multiply-divide, and, finally, floating-point instruction sets

The PDP-8 family history (Chap. 46) is an excellent example of the problems with maintaining a family with limited growth potential (primarily due to the simple ISP) through several technological generations.

As the System/360 was IBM's follow-on to the 7090 series, the PDP-11 was DEC's follow-on to PDP-8. The PDP-11 also represents a planned family, although one not as tightly controlled as the System/360 (see Chap. 47). The implementation tradeoffs among the PDP-11 models is discussed in Chap. 39, where a simple two-parameter model is developed which relates technology and implementation techniques to performance. The model fits PDP-11 data as well as System/360 and System/370 data.

It is interesting to note that a PDP-1l now fits on a small number of LSI chips (e.g., four) and DEC has developed a semantically richer ISP, the VAX-11/780 ISP (Chap. 42).

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