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{{Short description|Line of computers sold by Digital Equipment Corporation}}
{{Redirect|Vax}}
{{Use mdy dates|date=October 2022}}
{{Infobox CPU architecture
| name       = VAX
| image      = VAX logo.svg
| designer   = [[Digital Equipment Corporation]]
| bits       = 32-bit
| introduced = {{Start date and age|1977}}
| design     = [[Complex instruction set computer|CISC]]
| type       = {{ubl | Register–register | Register–memory | Memory–memory }}
| encoding   = Variable <small>(1 to 56 bytes)</small>
| endianness = Little
| extensions = PDP-11 compatibility mode, VAX Vector Extensions,<ref name="macro">{{cite web |url=http://www.openvms.compaq.com/doc/73final/4515/4515pro_012.html#basic_arch_sect |title=VAX MACRO and Instruction Set Reference Manual |at=8.1 Basic Architecture |date=April 2001 |work=OpenVMS documentation |archive-url=https://web.archive.org/web/20010906051514/http://www.openvms.compaq.com/doc/73final/4515/4515pro_012.html#basic_arch_sect |archive-date=September 6, 2001 |url-status=dead}}</ref> VAX Virtualization Extensions<ref>{{cite book|url=http://www.bitsavers.org/pdf/dec/vax/archSpec/EL-00032-00-decStd32_Jan90.pdf|title=DEC STD 032 – VAX Architecture Standard|date=January 5, 1990|publisher=Digital Equipment Corp|page=12-5|access-date=2022-08-01}}</ref>
| open       = No
| branching  = [[Status register|Condition code]]
| page size  = 512 bytes
| predecessor = [[PDP-11]]
| successor = [[DEC Alpha|Alpha]]
| gpr        = 16 × 32-bit
| fpr        = not present, uses the GPR
| vpr        = 16 × 4096-bit (64 elements of 64 bits each)
}}

'''VAX''' (an acronym for '''virtual address extension''') is a series of computers featuring a [[32-bit computing|32-bit]] [[instruction set architecture]] (ISA) and [[virtual memory]] that was developed and sold by [[Digital Equipment Corporation]] (DEC) in the late 20th century. The [[VAX-11/780]], introduced October 25, 1977, was the first of a range of popular and influential computers implementing the VAX ISA. The VAX family was a huge success for DEC, with the last members arriving in the early 1990s. The VAX was succeeded by the [[DEC Alpha]], which included several features from VAX machines to make [[porting]] from the VAX easier.

==Description==
VAX was designed by [[Digital Equipment Corporation]] (DEC) as a successor to the [[16-bit computing|16-bit]] [[PDP-11]], one of the most successful [[minicomputer]]s in history{{r|bellstreckerpdp11vaxalpha}} with approximately 600,000 units sold. The system was designed to offer [[backward compatibility]] with the PDP-11 while extending the memory to a full [[32-bit computing|32-bit]] implementation and adding [[Paging|demand paged]] [[virtual memory]]. The name VAX refers to its ''virtual address extension'' concept that allowed programs to make use of this newly available memory while still being compatible with unmodified user mode PDP-11 code. The name "VAX-11", used on early models, was chosen to highlight this capability. The VAX ISA is considered a [[complex instruction set computer]] (CISC) design.

DEC quickly dropped the −11 branding as PDP-11 compatibility was no longer a major concern. The line expanded to both high-end [[mainframe]]s like the [[VAX 9000]] as well as to the [[workstation]]-scale systems like the [[VAXstation]] series. [[List of VAX computers|The VAX family]] ultimately contained ten distinct designs and over 100 individual models in total. All of them were compatible with each other and normally ran the [[OpenVMS|VAX/VMS]] [[operating system]].

VAX has been perceived as the quintessential CISC ISA,<ref>{{cite web |last1=Bistriceanu |first1=Virgil |title=Computer Architecture – Class notes |url=http://www.cs.iit.edu/~virgil/cs470/Book/chapter3.pdf |publisher=Illinois Institute of Technology |access-date=April 15, 2022}}</ref> with its very large number of [[assembly language]] programmer-friendly [[addressing mode]]s and machine instructions, highly [[orthogonal instruction set]] architecture, and instructions for complex operations such as [[Queue (data structure)|queue]] insertion or deletion, number formatting, and [[polynomial]] evaluation.<ref>{{cite journal |last1=Payne |first1=Mary |last2=Bhandarkar |first2=Dileep |year=1980 |title=VAX floating point: a solid foundation for numerical computation |journal=ACM SIGARCH Computer Architecture News |publisher=ACM |volume=8 |issue=4 |pages=22–33 |issn=0163-5964 |doi=10.1145/641845.641849 |s2cid=15021135|doi-access=free }}</ref>

== Name ==
[[File:VAX 11-780 intero.jpg|thumb|VAX-11/780]]

The name "VAX" originated as an [[acronym]] for ''virtual address extension'', both because the VAX was seen as a 32-bit extension of the 16-bit PDP-11{{r|bellstreckerpdp11vaxalpha}} and because it was (after [[Prime Computer]]) an early adopter of [[virtual memory]] to manage this larger address space.

Early versions of the VAX processor implement a "compatibility mode" that emulates many of the PDP-11's instructions, giving it the 11 in VAX-11 to highlight this compatibility. Later versions offloaded the compatibility mode and some of the less used CISC instructions to emulation in the operating system software.

== Instruction set ==
The VAX instruction set was designed to be powerful, [[Orthogonal instruction set|orthogonal]],<ref name="Levy">{{Cite book|url=https://books.google.com/books?id=3LijBQAAQBAJ|title=Computer Programming and Architecture: The Vax|last1=Levy|first1=Henry|last2=Eckhouse|first2=Richard|date=June 28, 2014|publisher=Digital Press|isbn=9781483299372|language=en}}</ref> and "compiler-friendly".{{r|bellstreckerpdp11vaxalpha}} When it was introduced, many programs were written in assembly language, so having a "programmer-friendly" instruction set was important.<ref>{{cite web
|title=Another Approach to Instruction Set Architecture—VAX
|url=http://www.weblearn.hs-bremen.de/risse/RST/docs/COD2e/WebExtns.ion/webext3.pdf
|quote=... instruction set architectures, we chose the VAX as programmer-friendly instruction set, an asset
|access-date=October 3, 2018
|archive-date=June 10, 2017
|archive-url=https://web.archive.org/web/20170610060129/http://www.weblearn.hs-bremen.de/risse/RST/docs/COD2e/WebExtns.ion/webext3.pdf
|url-status=dead
}}</ref><ref>{{cite web |title=VAX |quote=Esp. noted for its large, assembler-programmer-friendly instruction set --- an asset that|url=http://www.lysator.liu.se/hackdict/split/vax.html}}</ref> In time, as more programs were written in [[high-level programming language]]s, the instruction set became less visible, and the only ones much concerned about it were compiler writers.

One unusual aspect of the VAX instruction set is the presence of register masks<ref>{{cite web |url=http://www.openvms.compaq.com/doc/73final/4515/4515pro_020.html#index_x_790 |title=VAX MACRO and Instruction Set Reference Manual |at=9.2.5 Procedure Call Instructions |date=April 2001 |work=OpenVMS documentation |archive-url=https://web.archive.org/web/20020330111145/http://www.openvms.compaq.com/doc/73final/4515/4515pro_020.html#index_x_790 |archive-date=March 30, 2002 |url-status=dead}}</ref> at the start of each subprogram. These are arbitrary bit patterns that specify, when control is passed to the subprogram, which registers are to be preserved. On most architectures, it is up to the compiler to produce instructions to save out the needed data, typically using the [[call stack]] for temporary storage. On the VAX, with 16 registers, this might require 16 instructions to save the data and another 16 to restore it. Using the mask, a single 16-bit value performs the same operations internally in hardware, saving time and memory.<ref name="Levy"/>

Since register masks are a form of data embedded within the executable code, they can make linear parsing of the machine code difficult. This can complicate optimization techniques that are applied on machine code.<ref name="MCO">{{cite thesis |last=Goss |first=Clinton F. |date=August 2013 |title=Machine Code Optimization: Improving Executable Object Code |type=PhD |orig-date=First published June 1986 |volume=Computer Science Department Technical Report No. 246 |publisher=Courant Institute, New York University |url=http://www.ClintGoss.com/mco/Goss_1986_MachineCodeOptimization.pdf |access-date=August 22, 2013 |arxiv=1308.4815 |bibcode=2013arXiv1308.4815G}}
*{{cite thesis |degree=PhD |author=Clinton F. Goss |date=2013 |orig-date=1986 |title=Machine Code Optimization – Improving Executable Object Code | publisher=Courant Institute, New York University |url=http://www.ClintGoss.com/mco/}}</ref>

== Operating systems ==
[[File:VAX VMS logo.svg|thumb|right|Stylized "VAX/VMS" used by Digital]]

The native VAX [[operating system]] is Digital's VAX/VMS (renamed to [[OpenVMS]] in 1991 or early 1992 when it was ported to [[DEC Alpha|Alpha]], modified to comply with [[POSIX]] standards, and branded as compliant with [[XPG4]] by the [[X/Open]] consortium).<ref name="vaxvmsat20">{{Cite web |editor1-last=Rainville |editor1-first=Jim |editor2-last=Howard |editor2-first=Karen |year=1997 |url=http://h41379.www4.hpe.com/openvms/20th/ |title=VAX/VMS at 20 |publisher=Digital Equipment Corporation |access-date=July 20, 2018 |archive-url=https://web.archive.org/web/20180720225439/http://h41379.www4.hpe.com/openvms/20th/ |archive-date=July 20, 2018 |url-status=dead}}</ref> The company wanted to avoid VAX having many incompatible operating systems like PDP-11;<ref name="bellstreckerpdp11vaxalpha">{{Cite report |url=https://archive.computerhistory.org/resources/text/DEC/alpha/dec.alpha.bell_stecker.pdp-11_vax_and_alpha.1998.102630388.pdf |title=What Have We Learned from the PDP-11 - What We Have Learned from VAX and Alpha |last1=Bell |first1=Gordon |author-link1=Gordon Bell |last2=Strecker |first2=W.D. |access-date=2025-06-26}}</ref> VAX and VMS were "[[Concurrent Engineering#Concurrent engineering workflow|engineered concurrently]]" to take maximum advantage of each other, as was the initial implementation of the [[VMScluster|VAXcluster]] facility.

During the 1980s, a [[hypervisor]] for the VAX architecture named ''VMM'' (Virtual Machine Monitor), also known as the ''VAX Security Kernel'', was developed at Digital with the aim of allowing multiple isolated instances of VMS and ULTRIX to be run on the same hardware.<ref>{{cite conference|url=https://www.scs.stanford.edu/nyu/04fa/sched/readings/vmm.pdf|title=A VMM security kernel for the VAX architecture|author1=Paul A. Karger|author2=Mary Ellen Zurko|author3=Douglas W. Benin|author4=Andrew H. Mason|author5=Clifford E. Kahnh|date=May 7–9, 1990|conference=Proceedings. 1990 IEEE Computer Society Symposium on Research in Security and Privacy|access-date=2021-01-31|publisher=IEEE|doi=10.1109/RISP.1990.63834}}</ref> VMM was intended to achieve [[Trusted Computer System Evaluation Criteria|TCSEC]] A1 compliance. By the late 1980s, it was operational on [[VAX 8000]] series hardware, but was abandoned before release to customers.

Other VAX operating systems have included various releases of [[Berkeley Software Distribution]] (BSD) [[UNIX]] up to [[4.3BSD]], [[Ultrix]]-32, [[VAXELN]], and [[Xinu]]. More recently, [[NetBSD]]<ref>{{cite web|url=http://wiki.netbsd.org/ports/vax/|title=NetBSD/vax}}</ref> and [[OpenBSD]]<ref>{{cite web|url=http://www.openbsd.org/vax.html|title=OpenBSD/vax}}</ref> have supported various VAX models and some work has been done on porting [[Linux]] to the VAX architecture.<ref>{{cite web|url=http://vax-linux.org|title=Porting Linux to the VAX}}</ref> OpenBSD discontinued support for the architecture in September 2016.<ref name="obsd60releasenotes">{{Cite web |url=https://www.openbsd.org/60.html |title=OpenBSD 6.0 |year=2016 |access-date=June 20, 2017}}</ref>

== History ==
[[File:DEC-VAX-8350-front-0a.jpg|thumb|VAX 8350 front view with cover removed]]

VAX design began in 1975, about when DEC recognized that PDP-11's 16-bit architecture was too limiting in the amount of addressable memory.{{r|bellstreckerpdp11vaxalpha}} The first VAX model sold was the [[VAX-11/780]], introduced on October 25, 1977 at DEC's annual shareholder meeting.<ref>{{cite web |url=https://www.old-computers.com/history/detail.asp?n=20&t=3 |title=VAX 11/780, The First VAX System (October 1977)}}</ref> Bill Strecker, [[C. Gordon Bell]]'s doctoral student at [[Carnegie Mellon University]], was responsible for the architecture.<ref>{{Cite book |url=https://books.google.com/books?id=aWTtMyYmKhUC&q=%22Bill+Strecker%22+vax+designer |title=Portraits in Silicon |last=Slater |first=Robert |publisher=MIT Press |year=1987 |page=[https://books.google.com/books?id=aWTtMyYmKhUC&pg=PA213&lpg=PA213&dq=%22Bill+Strecker%22+vax+designer 213] |isbn=978-0-262-69131-4}}</ref> Like PDP-11, VAX was very successful; it provided the majority of DEC's sales, sales growth, and profit from the early 1980s to early 1990s. VAX and VMS became DEC's only actively developed computer architecture.{{r|bellstreckerpdp11vaxalpha}} Many different models with different prices, performance levels, and capacities were subsequently created. VAX [[superminicomputer]]s were very popular in the early 1980s.

For a while the VAX-11/780 was used as a standard in [[central processing unit|CPU]] [[Benchmark (computing)|benchmark]]s. It was initially described as a one-[[million instructions per second|MIPS]] machine, because its performance was equivalent to an [[IBM System/360]] that ran at one MIPS, and the System/360 implementations had previously been de facto performance standards. The actual number of instructions executed in 1 second was about 500,000, which led to complaints of marketing exaggeration.  The result was the definition of a "VAX MIPS", 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 Performance]]) 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 [[VMScluster|VAXcluster]]. (The performance of the VAX-11/780 still serves as the baseline metric in the [[BRL-CAD]] Benchmark, a performance analysis suite included in the BRL-CAD solid modeling software distribution.) The VAX-11/780 included a subordinate stand-alone [[PDP-11|LSI-11]] computer that performed microcode load, booting, and diagnostic functions for the parent computer. This was dropped from subsequent VAX models. Enterprising VAX-11/780 users could therefore run three different Digital Equipment Corporation operating systems: VMS on the VAX processor (from the hard drives), and either RSX-11S or RT-11 on the LSI-11 (from the single density single drive floppy disk).

The VAX went through many different implementations. The original VAX 11/780 was implemented in [[Transistor-transistor logic|TTL]] and filled a four-by-five-foot cabinet<ref name=chm11780>{{cite web |title=VAX 11/780 Computer: CPU |url=http://www.computerhistory.org/revolution/mainframe-computers/7/182/736 |publisher=Computer History Museum |access-date=October 24, 2012}}</ref> with a single [[Central processing unit|CPU]]. Through the 1980s, the high-end of the family was continually improved using ever-faster discrete components, an evolution that ended with the introduction of the [[VAX 9000]] in October 1989. This design proved too complex and expensive and was ultimately abandoned not long after introduction. CPU implementations that consisted of multiple [[emitter-coupled logic]] (ECL) [[gate array]] or [[macrocell array]] chips included the [[VAX 8000|VAX 8600 and 8800]] superminis and finally the VAX 9000 [[mainframe computer|mainframe]] class machines. CPU implementations that consisted of multiple [[MOSFET]] custom chips included the 8100 and 8200 class machines. The VAX 11-730 and 725 low-end machines were built using [[AMD Am2900|AMD Am2901]] [[bit-slice]] components for the ALU.

The [[MicroVAX]] I represented a major transition within the VAX family. At the time of its design, it was not yet possible to implement the full VAX architecture as a single [[VLSI]] chip (or even a few VLSI chips as was later done with the [[DEC V-11|V-11]] CPU of the VAX 8200/8300). Instead, the MicroVAX I was the first VAX implementation to move some of the more complex VAX instructions (such as the packed decimal and related opcodes) into emulation software. This partitioning substantially reduced the amount of [[microcode]] required and was referred to as the "MicroVAX" architecture. In the MicroVAX I, the [[arithmetic logic unit|ALU]] and registers were implemented as a single [[Gate array|gate-array]] chip while the rest of the machine control was conventional logic.

A full [[VLSI]] ([[microprocessor]]) implementation of the MicroVAX architecture arrived with the [[MicroVAX 78032|MicroVAX II's 78032]] (or DC333) CPU and 78132 (DC335) FPU. The 78032 was the first microprocessor with an on-board [[memory management unit]]<ref>{{cite web|url=http://simh.trailing-edge.com/semi/uvax.html|website=Computer History and Simulation|title=MicroVAX II (1985)}}</ref> The MicroVAX II was based on a single, quad-sized processor board which carried the processor chips and ran the [[OpenVMS#Origin and name changes|MicroVMS]] or [[Ultrix]]-32 [[operating system]]s. The machine featured 1 MB of on-board memory and a [[Q-Bus|Q22-bus]] interface with [[Direct memory access|DMA]] transfers. The MicroVAX II was succeeded by many further MicroVAX models with much improved performance and memory.

Further VLSI VAX processors followed in the form of the V-11, [[CVAX]], CVAX SOC ("System On Chip", a single-chip CVAX), [[Rigel (microprocessor)|Rigel]], Mariah and [[NVAX]] implementations. The VAX microprocessors extended the architecture to inexpensive [[workstation]]s and later also supplanted the high-end VAX models. This wide range of platforms (mainframe to workstation) using one architecture was unique in the computer industry at that time. Sundry graphics were etched onto the CVAX microprocessor die. The phrase ''CVAX... when you care enough to steal the very best'' was etched in broken Russian as a play on a [[Hallmark Cards]] slogan, intended as a message to Soviet engineers who were known to be both purloining DEC computers for military applications and [[reverse engineering]] their chip design.<ref>{{cite web|website=micro.magnet.fsu.edu|url=http://micro.magnet.fsu.edu/creatures/pages/russians.html|title=Steal the best|access-date=January 30, 2008}} The Russian phrase was: {{lang|ru|СВАКС... Когда вы забатите довольно воровать настоящий лучший}}</ref><ref>{{cite web|url=http://simh.trailing-edge.com/semi/cvax.html|website=Computer History and Simulation|title=CVAX (1987)|access-date=January 30, 2008}}</ref> By the late 1980s, the VAX microprocessors had grown in power to be competitive with discrete designs. This led to the abandonment of the 8000 and 9000 series and their replacement by Rigel-powered models of the [[VAX 6000]], and later by NVAX-powered [[VAX 7000]] systems.

Extrapolating from [[Moore's Law]], DEC expected that VAX's 32-bit design would be a viable architecture until about 1999. The company did not foresee that [[RISC]] would, during the 1980s, usurp traditional computing architectures with significantly more performance per cost.{{r|bellstreckerpdp11vaxalpha}} As [[Unix]] RISC systems from [[Sun Microsystems]] and others lured VAX customers,<ref name="marshall19890206">{{Cite magazine |last=Marshall |first=Martin |date=1989-02-06 |title=The Year of the Workstation |url=https://books.google.com/books?id=OzoEAAAAMBAJ&pg=PT42 |access-date=2025-06-27 |magazine=InfoWorld |pages=43–44 |volume=11 |issue=6}}</ref> in 1989 DEC introduced a range of RISC workstations and servers that ran [[Ultrix]], the [[DECstation]] and [[DECsystem]] respectively, using processors from [[MIPS Computer Systems]]. In 1992 DEC introduced its own RISC instruction set architecture, the [[DEC Alpha|Alpha AXP]] (later renamed Alpha), and their own Alpha-based microprocessor, the [[Alpha 21064|DECchip 21064]], a high performance [[64-bit]] design capable of running OpenVMS.

In August 2000, Compaq announced that the remaining VAX models would be discontinued by the end of the year,<ref>{{cite web|url=http://www.compaq.com/alphaserver/vax/vax_letter_final.html |title=VAX Systems: A letter from Jesse Lipcon |archive-url=https://web.archive.org/web/20000815201016/http://www.compaq.com/alphaserver/vax/vax_letter_final.html |archive-date=August 15, 2000 |url-status=dead}}</ref> but old systems remain in widespread use.<ref>{{Cite web|url=https://www.pcworld.com/article/468250/if-it-aint-broke-dont-fix-it-ancient-computers-in-use-today.html|title=If It Ain't Broke, Don't Fix It: Ancient Computers in Use Today|website=PCWorld|access-date=October 11, 2021}}</ref> The Stromasys [[Charon (software)|CHARON-VAX]] and [[SIMH]] software-based VAX emulators remain available. VMS is now developed by VMS Software Incorporated, albeit only for the [[DEC Alpha|Alpha]], [[HPE Integrity Servers|HPE Integrity]], and [[x86-64]] platforms.

== Processor architecture ==
[[File:Microvax 3600 (2).jpg|thumb|MicroVAX 3600 (left) with printer (right)]]

{| role="presentation" style="font-size:88%;border: 1px solid #a2a9b1;	border-spacing: 3px; background-color: var(--background-color-neutral-subtle, #f8f9fa); color: var(--color-base, black); margin: 0.5em 0 0.5em 1em; padding: 0.2em; float: right; clear: right;"
|-
|+ '''DEC VAX registers'''
|-
|
{| style="font-size:88%;width:39em;"
|-
| style="width:10px; text-align:left"  | <sup>3</sup><sub>1</sub>
| style="width:60px; text-align:center"| ...
| style="width:10px; text-align:right" | <sup>2</sup><sub>3</sub>
| style="width:60px; text-align:center"| ...
| style="width:10px; text-align:center"| <sup>1</sup><sub>5</sub>
| style="width:60px; text-align:center"| ...
| style="width:10px; text-align:center"| <sup>0</sup><sub>7</sub>
| style="width:60px; text-align:center"| ...
| style="width:10px; text-align:center"| <sup>0</sup><sub>0</sub>
| style="width:100px; background: var(--background-color-base, white); color: var(--color-base, black);" | ''(bit position)''
|-
! colspan="10" style="text-align:left;" | General registers
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R0
| style="background: var(--background-color-base, white); color: var(--color-base, black);" colspan="3"| Register 0
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R1
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 1
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R2
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 2
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R3
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 3
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R4
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 4
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R5
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 5
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R6
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 6
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R7
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 7
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R8
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 8
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R9
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 9
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R10
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 10
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R11
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 11
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R12 / AP
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 12  / Argument Pointer
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R13 / FP
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 13 / Frame Pointer
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R14 / SP
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 14 / Stack Pointer
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| R15 / PC
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| Register 15 / Program Counter
|-
! colspan="10" style="text-align:left;" | Processor Status Longword
|- style="background: var(--background-color-disabled, silver); color: var(--color-base, black);"
| style="text-align:center" colspan="9"| (See adjacent table for bit definitions)
| style="background: var(--background-color-base, white); color: var(--color-base, black);"| PSL
|}
|}

=== Virtual memory map ===
The VAX virtual memory is divided into four sections. Each is one gigabyte (in the context of addressing, 2<sup>30</sup> bytes) in size:

{|class="wikitable"
!Section
! Address range
|-
|P0
| <code>0x00000000</code> – <code>0x3fffffff</code>
|-
|P1
| <code>0x40000000</code> – <code>0x7fffffff</code>
|-
|S0
| <code>0x80000000</code> – <code>0xbfffffff</code>
|-
|S1
| <code>0xc0000000</code> – <code>0xffffffff</code>
|}
For VMS, P0 was used for user process space, P1 for process stack, S0 for the operating system, and S1 was reserved.

=== Privilege modes ===
The VAX has four hardware implemented privilege modes:

{|class="wikitable"
! No.
! Mode
! VMS use
! Notes
|-
| 0
| Kernel
| OS kernel
| Highest [[privilege level]]
|-
| 1
| Executive
| [[File system]]
| 
|-
| 2
| Supervisor
| Shell (DCL)
| 
|-
| 3
| User
| Normal programs
| Lowest privilege level
|-
|}

=== Processor status longword ===
The process status longword contains 32 bits:

{|class="wikitable"
! width=3% | CM
! width=3% | TP
! width=6% | MBZ
! width=3% | FD
! width=3% | IS
! width=6% | cmod
! width=6% | pmod
! width=3% | MBZ
! width="15%" |IPL
! width="24%" |MBZ
! width=3% | DV
! width=3% | FU
! width=3% | IV
! width=3% | T
! width=3% | N
! width=3% | Z
! width=3% | V
! width=3% | C
|- style="text-align:center"
|31
|30
|29:28
|27
|26
|25:24
|23:22
|21
|20:16
|15:8
|7
|6
|5
|4
|3
|2
|1
|0
|}

{|class="wikitable"
! Bits || Meaning || Bits || Meaning
|-
| align="center"|  31 || PDP-11 compatibility mode                     || align="center"| 15:8|| MBZ (must be zero)
|-
| align="center"|  30 || trace pending                                 || align="center"|  7  || decimal overflow trap enable
|-
| align="center"|29:28|| MBZ (must be zero)                            || align="center"|  6  || floating-point underflow trap enable
|-
| align="center"|  27 || first part done (interrupted instruction)     || align="center"|  5  || [[integer overflow]] trap enable
|-
| align="center"|  26 || interrupt stack                               || align="center"|  4  || trace
|-
| align="center"|25:24|| current privilege mode                        || align="center"|  3  || negative
|-
| align="center"|23:22|| previous privilege mode                       || align="center"|  2  || zero
|-
| align="center"|  21 || MBZ (must be zero)                            || align="center"|  1  || overflow
|-
| align="center"|20:16|| IPL (interrupt priority level)                || align="center"|  0  || carry
|}

== VAX-based systems ==
{{See also|List of VAX computers}}
[[File:SPEC-1 VAX 05.jpg|thumb|The SPEC-1 VAX, a VAX 11/780 used for [[benchmark (computing)|benchmarking]], showing internals]]

The first VAX-based system was the [[VAX-11/780]], a member of the [[VAX-11]] family. The high-end [[VAX 8600]] replaced the VAX-11/780 in October 1984 and was joined by the entry-level [[MicroVAX]] minicomputers and the [[VAXstation]] workstations in the mid-1980s. The MicroVAX was superseded by the [[VAX 4000]], the VAX 8000 was superseded by the [[VAX 6000]] in the late 1980s and the mainframe-class [[VAX 9000]] was introduced. In the early 1990s, the [[Fault tolerance|fault-tolerant]] [[VAXft]] was introduced, as were the [[DEC Alpha|Alpha]] compatible [[VAX 7000/10000]]. A variant of various VAX-based systems were sold as the [[VAXserver]].

=== SIMACS ===
''[[System Industries]]'' developed an ability to give more than one DEC CPU, but not at the same time, write access to a shared disk. They implemented an enhancement named [[SIMACS]] (simultaneous machine access),<ref>{{cite web |url=http://www.atnf.csiro.au/observers/memos/d95f80~1.pdf |title=Computing Requirements for AT Software Development |last1=Wand |first1=R. |last2=Kesteven |first2=M. |last3=Rayner |first3=P. |date=February 24, 1984}}</ref><ref name=SIMACS.HC84>{{cite magazine |last1=Joshi |first1=Prem |last2=Delacroix |first2=Jacques |date=September 1984 |title=New Flexibility For Multiple VAX/VMS |magazine=[[HARDCOPY]] |pages=64–68}}</ref> which allowed their special disk controller to set a [[Semaphore (computing)|semaphore flag]] for disk access, allowing multiple WRITES to the same files; the disk is shared by multiple DEC systems. ''SIMACS'' also existed on PDP-11 [[RSTS/E|RSTS]] systems.

=== Canceled systems ===
Canceled systems include the ''BVAX'', a high-end [[emitter-coupled logic]] (ECL) based VAX, and two other ECL-based VAX models: ''Argonaut'' and ''Raven''.<ref>{{cite web |title=Who are the Computer Architects? |url=http://www.cs.clemson.edu/~mark/architects.html |author=Mark Smotherman |date=July 19, 2008 |access-date=September 30, 2008}}</ref> Raven was canceled in 1990.<ref>{{cite web |url=http://simh.trailing-edge.com/semi/raven.html |website=Computer History and Simulation |title=Raven |last=Supnik |first=Bob |date=2007 |access-date=March 1, 2019}}</ref> A VAX named ''Gemini'' was also canceled, which was a fall-back in case the LSI-based ''Scorpio'' failed. It never shipped.

=== Clones ===
A number of VAX clones, both authorized and unauthorized, were produced. Examples include:

* [[Systime Computers Ltd]] of the United Kingdom produced clones of early VAX models such as the Systime 8750 (equivalent to the VAX 11/750).<ref>{{cite web |url=http://www.chilton-computing.org.uk/inf/literature/inf_reports/p001.htm |title=RAL Informatics Report 1984-85 |access-date=October 15, 2007}}</ref>
* Norden Systems produced the ruggedized, Military-specification MIL VAX series.<ref name="vaxvmsat20"/>
* The Hungarian Central Research Institute for Physics (KFKI) produced a series of clones of early VAX models, the TPA-11/540, 560 and 580.<ref>{{cite web |url=http://hampage.hu/tpa/e_index.html |title=The TPA story |access-date=October 15, 2007}}</ref>
* The SM 52/12<ref>{{cite journal |last1=Dujnic |first1=J. |last2=Fristacky |first2=N. |last3=Molnar |first3=L. |last4=Plander |first4=I. |last5=Rovan |first5=B. |year=1999 |title=On the history of computer science, computer engineering, and computer technology development in Slovakia |journal=IEEE Annals of the History of Computing |volume=21 |issue=3 |pages=38–48 |doi=10.1109/85.778981}}</ref> from [[Czechoslovakia]], developed at VUVT [[Žilina]] (today [[Slovakia]]) and produced from 1986 at ZVT [[Banská Bystrica]] (today [[Slovakia]]).
* The East German [[VEB Robotron]] [[Robotron K&nbsp;1840|K&nbsp;1840]] (SM 1710) is a clone of the VAX-11/780 and [[Robotron K 1820]] (SM 1720) is a copy of the MicroVAX II.
* The ''SM-1700'' is a Soviet clone of the VAX-11/730, ''SM-1702'' was a clone of MicroVAX II and ''SM-1705'' was a clone of VAX-11/785.<ref>{{cite journal|author1=Laimutis Telksnys|author2=Antanas Zilinskas|title=Computers in Lithuania|journal=IEEE Annals of the History of Computing|volume=21|number=3|pages=31–37|date=July 1999|url=http://www.computer.org/csdl/mags/an/1999/03/00778980.pdf|doi=10.1109/85.778980|s2cid=16240778}}</ref> These systems ran a variety of clone operating systems - [[DEMOS]] (based on BSD Unix), [[OpenVMS#Influence|MOS VP]] (based on VAX/VMS) or MOS VP RV (based on VAXELN).<ref>{{cite journal|url=http://www.swsys.ru/index.php?page=article&id=1480#|journal=Software Systems Journal|volume=1988|issue=3|title=Basic software for 32-bit SM computer models|author1=Prokhorov N.L.|author2=Gorskiy V.E.|language=Russian|access-date=2021-09-15}}</ref>
* The NCI-2780 Super-mini, also sold as Taiji-2780, is a clone of the VAX-11/780 developed by North China Institute of Computing Technology in Beijing.<ref>{{cite book|url=https://books.google.com/books?id=E0LoOH-GqagC&q=%22NCI-2780%22&pg=PA96|title=Technology transfer to China|author=[[U.S. Congress]], [[Office of Technology Assessment]]|publisher=[[U.S. Government Printing Office]]|date=July 1987|page=96|isbn=9781428922914|id=OTA-USC-340}}</ref><ref>{{cite book|chapter-url=https://books.google.com/books?id=u7HItO03SlwC&q=%22NCI-2780%22&pg=PA244|title=Space Commercialization: Launch Vehicles and Programs|editor1=F. Sharokhi|editor2=J. S. Greenberg|editor3=T. Al-Saud|chapter=Satellite Launch and TT&C Systems of China and Their Roles in International Cooperation|author1=Xia Nanyin|author2=Chan Laixing|year=1990|page=244|publisher=[[American Institute of Aeronautics and Astronautics]]|isbn=0-930403-75-4}}</ref>

== References ==
{{reflist|30em}}

== Further reading ==
* {{cite news |last1=Coy |first1=Peter |title=Who Remembers the VAX Minicomputer, Icon of the 1980s? |url=https://www.bloomberg.com/news/articles/2021-01-06/who-remembers-the-vax-minicomputer-icon-of-the-1980s |access-date=January 9, 2021 |publisher=[[Bloomberg News]] |date=January 6, 2021 |language=en}}

== External links ==
{{commons category}}
* {{Webarchive|url=https://web.archive.org/web/20041207213429/http://h18002.www1.hp.com/alphaserver/vax/|title=HP: VAX Systems|date=December 7, 2004}}
* [http://simh.trailing-edge.com/dsarchive.html DEC Microprocessors]
* [http://simh.trailing-edge.com/ SimH VAX] Open source emulator that supports VAX architecture
*[http://www.dtjcd.vmsresource.org.uk/ The complete Digital Technical Journals]

{{Digital Equipment Corporation}}
{{DEC hardware}}
{{Processor technologies}}

[[Category:Minicomputers]]
[[Category:32-bit computers]]
[[Category:Computer-related introductions in 1977]]
[[Category:DEC mainframe computers]]