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Chapter 42

VAX-11 /780¾ A Virtual Address Extension to the DEC PDP-1 1 Family1

W. D. Strecker


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 $20K to $200K 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 which 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 65K byte virtual address space typically provided on minicomputers (such as the PDP-11) has not been and probably will not continue to be a severe limitation. However, there are some applications whose programming is impractical in a 65K byte 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 insure 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: compatibility¾ very tangible and important; and simplicity¾ 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 busses and peripherals); systems and applications software; files and data bases; and personnel familiar with the programming, use, and operation of the systems. For example, just recently a major computer manufacturer abandoned a major effort for new computer architectures in favor of evolving its current architectures [McLean, 1977].

The second intangible area is the preservation of those attributes (other than price) which make minicomputer systems attractive. These include approachability, understandability, and ease of use. Preservation of these attributes suggests that simply modelling an extended virtual address minicomputer after a large mainframe computer is not wholly appropriate. It also suggests that during architectural design, tradeoffs must be made between snore than just performance, functionality, and cost. Performance or functionality features which are so complex that they appreciably compromise understanding or ease of use must be rejected as inappropriate for minicomputer systems.

VAX-11 Overview

VAX-11 is the Virtual Address eXtention of PDP-11l architecture [Bell et al., 1970; Bell and Strecker, 1976]. The most distinctive feature of VAX-11 is the extension of the virtual address from 16 bits as provided on the PDP-11 to 32 bits. With the 8-bit byte the basic addressable unit, the extension provides a virtual address space of about 4.3 gigabytes which, even given rapid improvement in memory technology, should be adequate far into the future.

Since maximal PDP-11 compatibility was a strong goal, early VAX-11 design efforts focused on literally extending the PDP-11: preserving the existing instruction formats and instruction set and fitting the virtual address extension around them. The objective here was to permit, to the extent possible, the running of existing programs in the extended virtual address environment. While realizing this objective was possible (there were three distinct designs), it was felt that the extended architecture designs were overly compromised in the areas of efficiency, functionality, and programming ease.

Consequently, it was decided to drop the constraint of the PDP-11 instruction format in designing the extended virtual address space or native mode of the VAX-11 architecture. However, in order to run existing PDP-11 programs, VAX-11 includes a PDP-11 compatibility mode. Compatibility mode provides the basic PDP-11 instruction set less only privileged instructions (such as HALT) and floating point instructions (which are optional on most PDP-11 processors and not required by most PDP-11 software).

In addition to compatibility mode, a number of other features to preserve PDP-11 investment have been provided in the VAX-11 architecture, the VAX-11 operating system VAX/VMS, and the VAX-11/780 implementation of the VAX-11 architecture. These features include:

1 The equivalent native mode data types and formats are identical to those on the PDP-11. Also, while extended, the VAX-11 native mode instruction set and addressing modes


1AFIPS Proc. NCC, 1978, pp. 967-980.


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