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VAX-11/780: A Virtual Address Extension to the DEC PDP-11 Family



Large Virtual Address Space Minicomputers

Perhaps the most useful definition of a minicomputer system is based on price: Depending on one's perspective, such systems are typically found in the $20 K to $200 K range. The twin forces of market pull - as customers build increasingly complex systems on minicomputers -and technology push - as the semiconductor industry provides increasingly lower cost logic and memory elements - have induced minicomputer manufacturers to produce systems of considerable performance and memory capacity. Such systems are typified by the DEC PDP 11/70. From an architectural point of view, the characteristic that most distinguishes many of these systems from larger mainframe computers is the size of the virtual address space: the immediately available address space seen by an individual process. For many purposes, the 65- Kbyte virtual address space typically provided on minicomputers (such as the PDP-l 1) has not been and probably will not continue to be a severe limitation. However, there are some applications whose programming is impractical in a 65-Kbyte virtual address space and, perhaps most importantly, others whose programming is appreciably simplified by having a large virtual address space. Given the relative trends in hardware and software costs, the latter point alone will ensure that large virtual address space minicomputers play an increasingly important role in minicomputer product offerings.

In principle, there is no great challenge in designing a large virtual address minicomputer system. For example, many of the large mainframe computers could serve as architectural models for such a system. The real challenge lies in two areas: compatibility - very tangible and important; and simplicity - intangible but nonetheless important.

The first area is preserving the customer's and the computer manufacturer's investment in existing systems. This investment exists at many levels: basic hardware (principally buses and peripherals); systems and applications software;


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