Section 3

Minicomputers

A major attribute of computers below the class of maxi is their use in dedicated applications areas. The minicomputer evolved from a conceptual view of design wherein a programmable controller was perceived to be the cheapest, fastest way to implement a special-purpose function. The minicomputer did not require the generality of larger computers and hence required less software and less overhead. Thus minicomputers were leaner and more responsive than their cousins.

The need for minicomputers evolved from several areas including control, switching, and data processing. IBM's first minicomputer was the 1401 (c. 1962). As initially conceived, the 1401 was a stored-program replacement for the former hardwired controllers used to interconnect card readers, magnetic tape units, and line printers in an offline batch support system.

The CDC 160, introduced in 1960 at a price of $60,000, was the first high-performance, low-cost, real time computer. Like the 1401 it was designed as a support computer to a larger machine and as a computer to test peripherals. Although it was not intended to be sold as a programmable computer, it was subsequently applied to scientific and commercial computations.

The DEC PDP-5 was introduced in 1964 for real time data collection and control. The PDP-5 had a single 12-bit accumulator, a 1-bit link for overflow and multiple-precision arithmetic, and a 1-bit interrupt enable. The program counter was held in primary memory, and an analog-to-digital converter was built directly into the accumulator.

The immediate successor of the PDP-5, the PDP-8, can be credited with triggering the minicomputer revolution. Its small size (half a cabinet) and cost ($18,000) brought the computer into the region where it was cost-effective in dedicated real time applications, especially since it could be packaged as part of a larger system. By 1980 over 100,000 PDP-8's had been sold since their introduction in 1966.

From these origins, where the minicomputer was considered to be the minimal-complexity computer, the minicomputer has grown in functionality and performance to the point where it rivals the higher-cost, general-purpose computers of a decade ago. This section describes four minicomputers: the PDP-8, the PDP-11, the HP 2116, and the IBM System/38.
 

The PDP-8

The 12-bit PDP-8 is described in a top-down fashion in Chap. 8. The description is carried from the PMS and ISP levels to register-transfer, gate, and circuit levels, illustrating the hierarchy of design. Since the PDP-8 is conceptually simple, it is possible to provide substantial details of the design in terms of the mid-1960s discrete technology used to implement the original PDP-8. A Kiviat graph for the original PDP-8 is shown in Fig. 1.

Chapter 15 illustrates how the PDP-8 might be implemented by using contemporary bit-sliced microprogrammed chip sets. The design illustrates the use of ISP to describe the hardware building blocks (the Am2901 and 2909) and microcode to emulate other ISPs. PDP-8 programs have been successfully executed by using the ISP simulator on this bit-sliced PDP-8. After Chap. 15, machines are discussed only at the register-transfer level or above. However, the reader should have enough working knowledge about technology at this point to use Am2900 chips and/or ISP in design exercises completing the details in lower-level descriptions of other machines in this book. We encourage the reader to try at least a paper exercise of some other machine.

Finally, Chap. 46 summarizes the evolution of the PDP-8 family of implementations over a decade of technological change ranging from discrete logic to microcomputer implementations.
 

The PDP-11

The need was felt to increase the functionality of minimal computers, especially by providing a larger address space. This, coupled with a change from 6-bit (e.g., two characters per PDP-8

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