A Minicomputer-Compatible Microcomputer System: The DEC LSI-11
MARK J. SEBERN
In recent years, minicomputers have found application in a wide range of areas. In so doing, they have displaced larger computer systems in many traditional maxicomputer markets. At the same time, they have opened up many new markets, primarily because of their low cost, small size, and general ease of use. Still, in spite of this remarkable success, minicomputers are not without competition. In cost- sensitive areas, the minicomputer is being eased out of its dominant position by a new generation of LSI microcomputers; the new "processors on a chip" have found a warm reception from designers seeking inexpensive computing power. That warm reception sometimes cools, however, when the user finds himself with a collection of components, instead of a complete computing system. The discovery that he is largely on his own when it comes to software and debugging support has a similarly chilling effect. The entry into the world of programming PROMs, using FORTRAN cross-assemblers and simulators, and writing even simple software routines from scratch can be a traumatic experience indeed. Still, the advantages of LSI microcomputers are very real, and many users have found the difficulties well worthwhile. Even so, some cannot help but wonder why they cannot simply have the best of both worlds: the cost and size of the microcomputer, and the ease of use and performance of the minicomputer systems with which they are familiar.
Therefore, the appearance of a new LSI microcomputer system that is fully compatible with a line of 16-bit minicomputers is an event of some significance. This new microcomputer, the DEC LSI-l1 (see Figure 1), is a complete 4K PDP-11 on a 2l.6 cm X26.7 cm(8.5 inch X 10.5 inch) board; priced to compete with other LSI microcomputers, it offers true mini computer performance and maxicomputer support. The LSI-l1, while not meant to be yet another low-end minicomputer, does bring many minicomputer strengths to the new microcomputer applications for which it is intended.
To provide minicomputer performance at a microcomputer price, the LSI-l 1 was designed to optimize system costs, rather than component costs. A one-chip central processor, then, was not necessarily superior to a four-chip