Retrospective: What Have We Learned from the PDP-11 - What We Have Learned from VAX and Alpha

Abstract

The PDP-11, VAX-11, and Alpha can be considered members of an architecture family starting in the 1960's and extending to the present. The PDP-11 was a huge commercial success for DEC. The PDP-11 was the defacto standard 16-bit minicomputer in the 1970's. The basic PDF'-11's design was extremely elegant and it significantly influenced future computer architecture. However, the PDP-11's 16-bit virtual address space and the inability to efficiently and consistently extend the architecture, led to its successor - VAX - being designed only 6 years after its introduction. The VAX was similarly a huge commercial success for DEC. VAX and its closely related software system - VMS - became the defacto standard for 32-bit virtual memory networked computing in the 1980's. However, VAX, driven by its initial design goals and constraints, was a complex architecture, and was particularly challenged (internally and externally) by the RISC concept that competitively emerged in the mid-1980's. DEC's internal situation in the second half of the 1980's made it impossible to achieve a timely, rational response to the RISC challenge. By the time the Alpha strategy emerged in 1992/1993, DEC had lost momentum in the market and other vendors had established defacfo standards in RISC and UNIX. This situation would impact the commercial success of Alpha despite its superior technical attributes. However, the Alpha story awaits completion of the industry transition from 32 to 64 bits, starting - as we predicted in 1975 - in about 1999.