Extending OpenVMS for 64-bit addressable virtual memory
Michael S. Harvey, Leonard S. Szubowicz · Digital technical journal · 1996
ported a 32-bit virtual address space that maximized compatibility for OpenVMS VAX users as they ported their applications from the VAX platform to the Alpha platform. Providing access to the 64-bit virtual memory capability defined by the Alpha architecture was always a goal for the OpenVMS operating system. An early consideration was the eventual use of this technology to enable a transition from a purely 32-bitoriented context to a purely 64-bit-oriented native context. OpenVMS designers recognized that such a fundamental transition for the operating system, along with a 32-bit VAX compatibility mode support environment, would take a long time to implement and could seriously jeopardize the migration of applications from the VAX platform to the Alpha platform. A phased approach was called for, by which the operating system could evolve over time, allowing for quicker time-to-market for significant features and better, more timely support for binary compatibility. In 1989, a strategy emerged that defined two fundamental phases of OpenVMS Alpha development. Phase 1 would deliver the OpenVMS Alpha operating system initially with a virtual address space that faithfully replicated address space as it was defined by the VAX architecture. This familiar 32-bit environment would ease the migration of applications from the VAX platform to the Alpha platform and would ease the port of the operating system itself. Phase 1, the OpenVMS Alpha version 1.0 product, was delivered in 1992. For Phase 2, the OpenVMS operating system would successfully exploit the 64-bit virtual address capacity of the Alpha architecture, laying the groundwork for further evolution of the OpenVMS system. In 1989, strategists predicted that Phase 2 could be delivered approximately three years after Phase 1. As planned, Phase 2 culminated in 1995 with the delivery of OpenVMS Alpha version 7.0, the first version of the OpenVMS operating system to support 64-bit virtual addressing. This paper discusses how the OpenVMS Alpha Operating System Development group extended the OpenVMS virtual address space to 64 bits. Topics covered include compatibility for existing applications, the options for extending the address space, the