OpenVMS was designed from the outset to be a secure, multiuser system with full networking functionality. In fact, the VAX and VMS were designed concurrently at DEC by a hardware/software engineering team. The VAX is a CISC and included many instructions designed expressly to support VMS. The Alpha is a reduced instruction set computer (RISC). As OpenVMS evolved, new features such as symmetric multiprocessing (SMP) and clustering were added.

The conversion of the PDP-11 customer base to the VAX was accomplished by moving much of the PDP-11 operating system, RSX-11, to the VAX. RSX could be invoked as a DCL command, and the VAX hardware was designed to execute the PDP-11 instruction set. Even so, the new operating system, VAX/VMS, was part of the VAX-11 ^[3] release. Many OpenVMS concepts find their roots in RSX-11 (e.g., user identification code [UIC]; the name of the root directory, [000000]; and process privileges, to name a few). The DEC proprietary networking protocol, DECnet, is still compatible between a PDP-11/RSX-11 computer and an Alpha/OpenVMS computer.

Since 1993, OpenVMS has also been running on the 64-bit Alpha platform, a CPU pioneered by DEC and today, the primary OpenVMS host. About 50 Alpha models are supported, although many are no longer manufactured. Unlike the PDP-11-to-VAX transition, the operating system was unchanged when the Alpha was introduced. To expedite the switchover, a conversion routine was written so that any VAX executable could be converted to run on the Alpha. This...