VAX is a 32-bit computing architecture that supports an orthogonal machine language and virtual addressing (i.e. demand paged virtual memory). It was developed in the mid-1970s by Digital Equipment Corporation (DEC) (later purchased by Compaq (later purchased by Hewlett-Packard)). The VAX has been perceived as the quintessential CISC processing architecture, with its very large number of addressing modes and machine instructions, including instructions for such complex operations as queue insertion/deletion and polynomial evaluation.

VAX computer systems (informal plural is VAXen) could run several operating systems, usually BSD UNIX or DECs VAX/VMS. The VAX architecture and VMS operating system were "engineered concurrently to take maximum advantage of each other, including sophisticated clustering, initially over special CI buses ("Computer Interconnect") but later over Ethernet as well.

"VAX" was originally an acronym for "Virtual Address eXtension" because the VAX was seen as a 32-bit extension of the older 16-bit PDP-11; early versions of the VAX processor implemented a "compatibility mode" that emulated many of the PDP-11's instructions. Later versions offloaded the compatibility mode and some of the less used CISC instructions to microcode or emulation in the operating system software.

The first VAX model sold was the 11-780 which became available circa 1978. Many different models with different prices, performance levels, and capacities were made. VAX superminis were very popular in the early 1980s. In 2001 there were still VAXen doing useful work, and Compaq was reportedly manufacturing and selling a tiny number of new ones.

For a while the VAX 11-780 was used as a baseline in CPU benchmarks because its speed was about one MIPS. Ironically enough, though, the actual number of instructions executed in 1 second was about 500,000. One VAX MIPS was the speed of a VAX 11-780; a computer performing at 27 VAX MIPS would run the same program roughly 27 times faster than the VAX 11-780. Within the Digital community the term VUP (VAX Unit of Processing) was the more common term, because MIPS do not compare well across different architectures.

The VAX went through many different implementations. The original VAX was implemented in TTL and filled more than one rack for a single CPU. The final versions were implemented in CMOS and ECL. The VAX processor was superseded in 1992 by the DEC Alpha (originally named AXP), a high performance 64-bit RISC architecture that could run VMS, Tru64 (DEC's UNIX), FreeBSD/NetBSD/OpenBSD and Linux.

The VAX Trademark

VAX is also a brand of Wet-Dry Vacuum Cleaners, invented in the 1970s by Alan Brazier. The advertising slogan "Nothing sucks like a Vax" was often applied wryly by users of VAX computers.

There are varied accounts of the legal interactions between DEC and the VAX corporation over the use of this trademark. The terms of the settlement involved a non-competition agreement between the companies -- DEC would not move into household appliances and the VAX corporation would stay out of computing. In the historial context, when many industrial electronics firms were involved in development of large computer systems, this seemed much less ridiculous than today.