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Manufacturer: Digital Equipment Corporation
Byte size: 8 bits
Address bus size: 32 bits
Peripheral bus: Unibus, Qbus
Architecture: CISC, virtual memory
Operating systems: VAX/VMS, Ultrix, BSD UNIX

VAX is a 32-bit computing architecture that supports an orthogonal instruction set (machine language) and virtual addressing (i.e. demand paged virtual memory). It was developed in the mid-1970s by Digital Equipment Corporation (DEC). DEC was later purchased by Compaq, which in turn was 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.


The name

"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.

VAX is also a brand of wet-dry vacuum cleaners, invented in the 1970s by Alan Brazier. That brand's 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 historical context, when many industrial electronics firms were involved in development of large computer systems, this seemed much less ridiculous than today.

Operating systems

VAX computer systems (informal plural is VAXen) can run several operating systems, usually BSD UNIX or DEC's VMS (but even Linux and NetBSD are running on some VAXen nowadays). 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") connected via star couplers but later over Ethernet as well.


The first VAX model sold was the VAX-11/780, which became available in 1978. The architect of this model was Bill Strecker. Many different models with different prices, performance levels, and capacities were subsequently created. 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. By 2005 all manufacturing of VAX computers had ceased, but old systems remained in widespread use.

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 related term cluster VUPs was informally used to describe the aggregate performance of a VAXcluster, a group of VAXen interconnected and operating as a cluster.

The VAX went through many different implementations. The original VAX was implemented in TTL and filled more than one rack for a single CPU. CPU implementations that consisted of multiple ECL gate array or macrocell array chips included the 8600, 8800 superminis and finally the 9000 mainframe class machines. CPU implementations that consisted of multiple MOSFET custom chips included the 8100 and 8200 class machines. There were also microprocessor implementations which included the MicroVAX-II, CVAX, Rigel, and NVAX chips. The VAX microprocessors extended the architecture to inexpensive workstations. This wide range of platforms (mainframe to workstation) using one architecture was unique in the computer industry at that time.

The VAX processor was superseded in 1992 by the DEC Alpha (originally named AXP), a high performance 64-bit RISC architecture.

VAX models

Working titles in parentheses.

Non-LSI VAXen (in roughly chronological order):

  • VAX-11/780 ("Star")
  • VAX-11/782 ("Atlas", Dual-processor /780)
  • VAX-11/784 ("VAXimus", Four /780 CPUs sharing a single MA780 memory unit. Very rare)
  • VAX-11/785 ("Superstar", Faster /780)
  • VAX-11/788 ("VISQ")
  • VAX-11/750 ("Comet", More-compact, lower-performance gate array-based implementation)
  • VAX-11/730 ("Nebula", Still-more-compact, still-lower-performance bit slice implementation)
  • VAX-11/725 ("LCN", Low-Cost Nebula)
  • VAX 8600    ("Venus")
  • VAX 8650    ("Morningstar", a faster 8600)
  • VAX 8X00    ("Gemini", Fall-back in case the LSI-based "Scorpio" failed; Never shipped)
  • VAX 8500    ("Flounder" (TBC), Single-processor, deliberately-slowed 8800)
  • VAX 8530    ("Skipjack", Single-processor, less-slowed 8800)
  • VAX 8550    ("Skipjack", Single-processor 8800, unexpandable)
  • VAX 8700    ("Nautilus", Single-processor Nautilus, expandable to full 8800)
  • VAX 8800    ("Nautilus", Macrocell array-based implementation)
  • VAX 9000    ("Aridus", Air-cooled. Originally designed to be water-cooled, named "Aquarius")

LSI VAXen (in roughly chronological order):

  • MicroVAX/VAXstation-I ("Seahorse")
  • MicroVAX-II/VAXstation-II ("Mayflower")
  • MicroVAX-3100/VAXstation-3100 (Desktop form-factor, many models)
  • MicroVAX-3300/3400 ("Mayfair II", used KA640 CPU card)
  • MicroVAX-3500/3600 ("Mayfair", used KA650 CPU card)
  • MicroVAX-3800-3900 ("Mayfair-III", used KA655 CPU card)
  • VaxStation 4000 Model 60 and 90
  • VAX 4000 (many models)
  • VAX 8200/8300 (Single- and dual-processor "Scorpio")
  • VAX 8250/8350 (Faster "Scorpio"s)
  • VAX 62X0    ("CVAX")
  • VAX 63X0    ("Calypso")
  • VAX 64X0    ("Rigel")
  • VAX 65X0    ("NVAX")
  • VAX 7000 models 610-640 ("Laser/Neon")
  • VAX 7000 models 710-760 ("Laser/Krypton")
  • VAX 7000 models 810-860 ("Laser/Krypton")
  • VAX 10000 models 610-640 ("Blazer")
  • VAX XXXX    ("BVAX", High-end VAX; Never shipped)

External links

it:VAX nl:VAX


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