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files and data bases; and personnel familiar with the programming, use, and operation of the systems. For example, to preserve this investment a major computer manufacturer just recently abandoned a major effort for new computer architectures in favor of evolving its current architectures [McLean, 1977].

The second, less tangible area is the preservation of those attributes (other than price) that make minicomputer systems attractive. These include approachability, understandability, and ease of use. Preservation of these attributes suggests that simply modeling 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 more 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 extension of PDP-11 architecture (Chapter 9) [Bell and Strecker, 1976]. The most distinctive feature of VAX-11 is the extension of the virtual address from i6 bits as provided on the PDP-11 to 32 bits. With the 8-bit byte as 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 primary goal, early VAX-11 design efforts focused on literally extending the PDP-l1: preserving the existing instruction formats and instruction set and fitting the virtual address extension around them. The objective 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-l1 instruction set without privileged instructions (such as HALT) and floating-point instructions (which are optional on most PDP-11 processors and not required by most PDP-l1 software).

In addition to compatibility mode, a number of other features to preserve PDP-l 1 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 the following.

1. The 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 are very close to those on the PDP-i 1. As a consequence, VAX-11 native mode assembly language programming is quite similar to PDP-11 assembly language programming.

2. The VAX-11/780 uses the same peripheral buses (Unibus and Massbus) and the same peripherals as the PDP-l1.

3. The VAX/VMS operating system is an evolution of the PDP-11 RSX-11M and IAS operating systems. It offers a similar although extended set of system services and uses the same command languages. Additionally, VAX/VMS supports most of the RSX-11M/IAS system service requests issued by programs executing in compatibility mode.

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