Virtual address extension

from Wikipedia, the free encyclopedia
SPEC-1 VAX. VAX 11/780 used for benchmarking at DEC.
Processor core (die) photo of a DEC MicroVAX (78032, DC333R)
The photo of the associated DEC MicroVAX FPU (78132, DC337C)

The VAX ( V irtual A ddress e X tension ) is a computer architecture of the Digital Equipment Corporation .


The first VAX with the type designation 11/780 came onto the market in October 1977. In February 1978 a special operating system for the VAX called VMS ( Virtual Memory System ) was completed, the development of which had started at the same time. VAX computers were sold until 2000. Today, these computer architecture still happens in the military sector, for example in the fighter aircraft F-15 and F / A-18 from manufacturer McDonnell Douglas (now Boeing ) or in the Minuteman intercontinental ballistic missile system .

The main architect was William D. Strecker , a former graduate student of Gordon Bell .


One of the main goals in the specification of the VAX architecture was to expand the 16-bit address space of the predecessor PDP-11 to 32 bits , i.e. an expansion from directly addressable 64 kB to 4 GB, which was future-proof for the time. Originally, the VAX architecture was only intended to be a modified PDP-11 architecture with hardware expansion to support virtual memory management, hence the name VAX (expansion to virtual addresses). In the course of the initial development, however, it was decided to create a new architecture that, compared to PDP-11, offers incompatible, but also additional instructions, data types and other addressing modes.

When designing VMS, attention was paid to source code compatibility with older operating systems in order to be able to convert existing programs and data to the new system with little effort. As an option, some VAXs had a binary compatibility mode in which PDP-11 programs could be executed directly. Since VMS is considered to be a particularly stable and mature operating system, it has become very popular, especially in the financial sector and in air surveillance (civil and military) and is still being further developed as OpenVMS (also for other hardware platforms). The current license holder HP ( Hewlett Packard Enterprise ) announced on June 10, 2013 that it will continue to support OpenVMS until the end of 2020. In July 2014, HP and VMS Software, Inc. (VSI) announced that VSI would license the further development of OpenVMS.


32-bit main processor 78034 (CVAX) from a MicroVAX 3100

The 32-bit VAX main processors were based on a CISC instruction set, which, based on the common PDP-11 model, is similar to the instruction set of the Motorola m68k processors , but offered additional instructions for operating system support, for example for queue management . The processor family was replaced by the Alpha processor , a 64-bit RISC processor, also developed by DEC .

After the PDP computers, VAX computers were the first computers to which the Unix operating system was ported. Digital Equipment offered its own UNIX derivative called ULTRIX in addition to the company's own VMS as the operating system. Meanwhile, were open source - Operating Systems NetBSD , OpenBSD and Linux on VAX computers ported.

The computers were originally built from standard components such as bit slices and TTL logic circuits. Later Digital developed its own VAX microprocessors , the so-called MicroVAX processors such as the MicroVAX 78032 or CVAX , which however were not sold on the open market. They were only available together with computers from the MicroVAX family.

The SIMH software can emulate a complete MicroVAX-3900 system.


Surname Current
system platform License demand
SIMH 3.9-0 0May 3, 2012 various very old computers platform independent modified X11 license
CHARON -VAX 4.0 Dec 28, 2010 DEC VAX Windows commercially
eVAX 1.1 Jan 28, 2000 DEC VAX platform independent GPL

Web links

Commons : VAX  - collection of pictures, videos and audio files

Individual evidence

  1. Press release HP
  2. HP and VMS Software, Inc. Collaborate to Extend OpenVMS Solutions. (No longer available online.) In: Archived from the original on August 8, 2014 ; accessed on August 1, 2014 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  3. ^ VMS Software, Inc. Named Exclusive Developer of Future Versions of OpenVMS Operating System. In: Accessed August 1, 2014 .
  4. OpenVMS SIG. In: Retrieved August 1, 2014 .
  5. ^ The Computer History Simulation Project. Retrieved November 18, 2012 .
  6. VAX Emulation ( Memento from March 10, 2012 in the Internet Archive )
  7. Forest Edge Software and eVAX… (No longer available online.) Formerly in the original ; Retrieved November 18, 2012 .  ( Page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link /