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Design Decisions for the PDP-11/60 Mid-Range Minicomputer



Design evolution of a minicomputer family usually proceeds along three basic dimensions: cost, functionality, and size. That is, the minicomputer becomes cheaper, more powerful, and smaller with time. The underlying hardware technology is the dominant factor in determining the evolution. In contrast to the evolution of large computers, market factors have less influence on the growth pattern of minicomputers. However, minicomputer software characteristics are affected by the market. These requirements rapidly feed down to modify the hardware, given that the technology will support user needs.

The DEC PDP-l1/60 serves to demonstrate minicomputer designing with improved technologies. Being a mid-range machine, i.e., neither the lowest in cost nor the highest in performance, its design is a rich source of tradeoff examples. Its cache design illustrates a price/performance trade; the decreasing cost of read-only memories (ROMs) show how hardware-microcode tradeoffs change over time, and its integral floating-point arithmetic unit exemplifies a software-hardware tradeoff.


Equipment history reveals that a member is added to a minicomputer family whenever technology advances by a factor of 2; for example, doubling of bit density on a memory chip. Over the past 15 years, such an improvement has occurred about every two years.

These advances in technology can be translated into either of two fundamentally different design styles. One provides essentially constant functionality at a minimal price (which de creases over time); the second keeps cost constant and increases functionality. (Here, and in the discussion to follow, the definition of functionality has been broadened from its conventional single component, speed, to include components such as extended instructions and self-checking.) Both design approaches coordinate with the basic marketing philosophy of the minicomputer industry: more computation for more users at less cost. There have been ten models, or implementations, of the PDP- 11 architecture since the unit was introduced in 1970 (Chapter 9). Figure 1 illustrates how the two de sign styles affected successive implementations within this minicomputer family.


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