Chapter 46 ½ The PDP-8 Family 775
somewhat between 1968 and 1977, as medium-scale integrated circuits continued to be the implementation technology, and the cost of packaging and connecting components continued to be controlled by the relatively wide bus structure.
During their evolution, the DEC 12-bit computers have significantly changed in physical structure, as can be seen from the block diagrams in Fig. 9. The machines up through the PDP-8/L had a relatively centralized structure with three buses to interface to memory, program-controlled I/O devices, and Direct Memory Access devices. The Omnibus-8 machines bundled these connections together in a simpler physical structure. The CMOS-8 avoids the wide bus problem by moving the bus to lines on a printed circuit board. The number of interconnection signals on the bus is then reduced by roughly a factor of 4 to about 25 signals which can be brought into and out of the chips within the number of pins available.
Figures 4 and 7 illustrate the oscillating price/performance history of the design evolution summarized below:
1 While the PDP-5 was designed to keep price at a minimum, the PDP-8 had additions to improve the performance while not increasing price significantly over that of a slower speed design. The cost per word was modestly higher with the PDP-8 than with the PDP-5, but the PDP-8 had 6 times the performance of a PDP-5. Thus, the PDP-8 crosses three lines of constant price/performance in Fig. 7.
2 The PDP-8/S was an attempt to achieve a minimum price by using serial logic and a minimum price memory design. However, the performance of the PDP-8/S was low.
3 The market pressures created by PDP-8/S performance probably caused the return to the PDP-8 design, but in an integrated circuit implementation, the PDP-8/I.
4 The PDP-8/I was relatively expensive, so the PDP-8/L was quickly introduced to reduce cost and bring the design into line with market needs and expectations.
5 The PDP-8/E was introduced as a high performance machine that would permit the building of systems larger than those possible with the PDP-8/L.
6 The PDP-8/Mwas a lower cost, smaller cabinet version of the PDP-8/E and was intended to meet the needs of the OEM market.
The design goal of machines subsequent to the PDP-8/M has been primarily one of price reduction. The PDP-8/A was introduced to further reduce cost from the level of the PDP-8/E and PDP-8/M, although some large system configurations are still built with PDP-8/E machines. The CMOS-8 chips represent a substantial cost reduction but also a substantial performance reduction. The CMOS-8 performance is one-third that of a PDP-8/A, so a stand-alone system using a CMOS-8 is less cost-effective than an PDP-8/A when the central processor is used as the only performance criterion. The main reason for using large-scale integration is the reduced cost and smaller package rather than performance. Obviously, the next step is increased performance or more memory, or both more performance and more memory on the same chip.