The VAX (Virtual ADDRESS eXtension) is a network architecture of the Digital Equipment corporation.

History

The first VAX with the type designation 11/780 came in October 1977 on the market. In February 1978 a special operating system for the VAX with the designation VMS (Virtual MEMORY system) was finished, its development was at the same time begun. VAX computers were sold up to the year 2000.

Characteristics

One of the principal purposes with the specification of VAX architecture was it to extend the 16-Bit-Adressraum of the predecessor PDP-11 to 32 bits thus an extension of directly addressable 64 KB up for conditions at that time future-safe - 4 GB. Originally VAX architecture should be only a modified PDP-11 architecture with hardware extension for the support of virtual store management, therefore the designation VAX (extension in virtual addresses). In the course of the initial development was however decided to create a new architecture those compared with PDP-11 incompatible, in addition, additional instructions, data types and further addressing modes offers.

With the Design of VMS source code compatibility to older operating systems one respected, in order to be able to convert existing programs and data at little expenditure on the new system. Since VMS is considered as particularly stable and matured operating system, it found to a very large spreading particularly in the financial area and as OpenVMS was still developed further.

Processor

The 32-Bit-VAX-Hauptprozessoren was based on a CISC command sentence, which resembles the command set of the 68k-Prozessoren of the company Motorola, however additional instructions due to the common PDP-11-Vorbildes for operating system support, approximately for queue administration offered. The processor family was replaced from the alpha processor likewise developed by DEC, a 64-Bit-RISC-Prozessor.

VAX computers were after the PDP computers the first computers, on which the operating system Unix was portiert. Digital Equipment offered its own UNIX derivative named ULTRIX beside the company-owned VMS as operating system. Meanwhile also the open SOURCE operating systems Linux and NetBSD on VAX computers were portiert.

Originally the computers from standard construction units, were developed about TTL Logikschaltkreisen. Later digital developed own VAX microprocessors (MicroVAX processor), which were not sold however on the free market. They were available only together with computers of the MicroVAX family.

The software SIMH can simulate a complete MicroVAX 3900 system.

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