View source for Von Neumann architecture

{{Short description|Computer architecture where code and data share a common bus}}
{{See also|Stored-program computer|Universal Turing machine#Stored-program computer}}
{{Distinguish|text=[[self-replicating machine|Von Neumann machines]]}}
{{Use American English|date = March 2019}}
{{Use mdy dates|date=September 2012}}
<!--By English typography, the letter "v" in "von" should never be in uppercase — except in titles of works (or at the beginning of a sentence).  See the disambiguation page [[Von Neumann]] — you will see that every article works that way. -->
[[File:Von Neumann Architecture.svg|thumb|upright=1.35|A von Neumann architecture scheme]]

The '''von Neumann architecture'''—also known as the '''von Neumann model''' or '''Princeton architecture'''—is a [[computer architecture]] based on the ''[[First Draft of a Report on the EDVAC]]'',<ref name ="FirstDraftReport" /> written by [[John von Neumann]] in 1945, describing designs discussed with [[John Mauchly]] and [[J. Presper Eckert]] at the University of Pennsylvania's [[Moore School of Electrical Engineering]]. The document describes a design architecture for an electronic [[digital computer]] made of "organs" that were later understood to have these components:
* A [[Central processing unit|processing unit]] with both an [[arithmetic logic unit]] and [[processor register]]s
* A [[control unit]] that includes an [[instruction register]] and a [[program counter]]
* [[Computer memory|Memory]] that stores [[Data (computing)|data]] and [[Instruction set|instructions]]
* External [[mass storage]]
* [[Input and output]] mechanisms<ref name="FirstDraftReport">{{Citation |author-last=von Neumann |author-first=John |author-link=John von Neumann |title=First Draft of a Report on the EDVAC |date=1945 |url=https://sites.google.com/site/michaeldgodfrey/vonneumann/vnedvac.pdf |access-date=2011-08-24 |archive-url=https://web.archive.org/web/20130314123032/http://qss.stanford.edu/~godfrey/vonNeumann/vnedvac.pdf |archive-date=2013-03-14}}.</ref><ref name="GanesanCh4">{{Harvnb|Ganesan|2009}}.</ref>

The attribution of the invention of the architecture to von Neumann is controversial, not least because Eckert and Mauchly had done a lot of the required design work and claim to have had the idea for stored programs long before discussing the ideas with von Neumann and [[Herman Goldstine]].<ref name="FiftyYearsOfArmyComputing">{{Citation |author-last=Bergin |author-first=Thomas J. |title=Fifty Years of Army Computing: From ENIAC to MSRC, U.S. Army Research Laboratory |date=2000 |page=34 |url=https://www.govinfo.gov/app/details/GOVPUB-D105-PURL-LPS58495 |access-date=2024-11-05}}</ref>

The term "von Neumann architecture" has evolved to refer to any [[stored-program computer]] in which an [[instruction fetch]] and a data operation cannot occur at the same time (since they share a common [[Bus (computing)|bus]]). This is referred to as the [[#von Neumann bottleneck|von Neumann bottleneck]], which often limits the performance of the corresponding system.<ref>{{Citation |author-last=Markgraf |author-first=Joey D. |title=The Von Neumann Bottleneck |date=2007 |url=http://aws.linnbenton.edu/cs271c/markgrj/ |archive-url=https://web.archive.org/web/20131212205159/http://aws.linnbenton.edu/cs271c/markgrj/ |archive-date=December 12, 2013 |url-status=dead |df=mdy-all }}.</ref>

The von Neumann architecture is simpler than the [[Harvard architecture]] (which has one dedicated set of address and data buses for reading and writing to memory and another set of address and data buses to fetch [[instruction fetch|instructions]]).

A [[stored-program computer]] uses the same underlying mechanism to encode both [[Computer program|program instructions]] and data as opposed to designs which use a mechanism such as discrete [[plugboard]] wiring or fixed control circuitry for instruction [[implementation]]. Stored-program computers were an advancement over the manually reconfigured or fixed function computers of the 1940s, such as the [[Colossus computer|Colossus]] and the [[ENIAC]]. These were programmed by setting [[switch]]es and inserting [[patch cable]]s to route data and control signals between various functional units.

The vast majority of modern computers use the same hardware mechanism to encode and store both data and program instructions, but have [[CPU cache|caches]] between the CPU and memory, and, for the caches closest to the CPU, have separate caches for instructions and data, so that most instruction and data fetches use separate buses ([[Modified Harvard architecture#Split-cache (or almost-von-Neumann) architecture|split-cache architecture]]).

==History==
The earliest computing machines had fixed programs.  Some very simple computers still use this design, either for simplicity or training purposes.  For example, a desk [[calculator]] (in principle) is a fixed program computer.  It can do basic [[mathematics]], but it cannot run a [[word processor]] or games.  Changing the program of a fixed-program machine requires rewiring, restructuring, or redesigning the machine.  The earliest computers were not so much "programmed" as  "designed" for a particular task.  "Reprogramming"—when possible at all—was a laborious process that started with [[flowchart]]s and paper notes, followed by detailed engineering designs, and then the often-arduous process of physically rewiring and rebuilding the machine. It could take three weeks to set up and debug a program on [[ENIAC]].<ref>{{Harvnb|Copeland|2006|p=104}}.</ref>

With the proposal of the stored-program computer, this changed. A stored-program computer includes, by design, an [[instruction set]], and can store in memory a set of instructions (a [[computer program|program]]) that details the [[computation]].

A stored-program design also allows for [[self-modifying code]]. One early motivation for such a facility was the need for a program to increment or otherwise modify the address portion of instructions, which operators had to do manually in early designs. This became less important when [[index register]]s and [[Addressing mode|indirect addressing]] became usual features of machine architecture. Another use was to embed frequently used data in the instruction stream using [[Addressing mode|immediate addressing]].

When von Neumann described the automatic computing systems using different terminology than is typically described with the model. In the ''[[First Draft of a Report on the EDVAC]]'',<ref name ="FirstDraftReport" /> the architecture was composed of "a high-speed memory M, a central arithmetic unit CA, an outside recording medium R, an input organ I, an output organ O, and a central control CC" <ref>{{cite book |last1=von Neumann |first1=Jon |title=Theory of Self-reproducing Automata |date=1966 |publisher=University of Illinois Press |isbn=978-0252727337 |page=10}}</ref> 

==Capabilities==
On a large scale, the ability to treat instructions as data is what makes [[Assembly language#Assembler|assemblers]], [[compiler]]s, [[Linker (computing)|linkers]], [[Loader (computing)|loaders]], and other automated programming tools possible. It makes "programs that write programs" possible.<ref>{{Citation |url=http://catb.org/~esr/jargon/html/M/MFTL.html |title=''MFTL'' (My Favorite Toy Language) entry Jargon File 4.4.7 |access-date=2008-07-11}}.</ref>  This has made a sophisticated self-hosting computing ecosystem flourish around von Neumann architecture machines.

Some [[High-level programming language|high-level languages]] leverage the von Neumann architecture by providing an abstract, machine-independent way to manipulate [[Executable|executable code]] at runtime (e.g., [[LISP]]), or by using runtime information to tune [[just-in-time compilation]] (e.g. languages hosted on the [[Java virtual machine]], or languages embedded in [[web browsers]]).

On a smaller scale, some repetitive operations such as [[Bit blit|BITBLT]] or [[High-Level Shader Language|pixel and vertex shaders]] can be accelerated on general purpose processors with just-in-time compilation techniques. This is one use of self-modifying code that has remained popular.

==Development of the stored-program concept==
The mathematician [[Alan Turing]], who had been alerted to a problem of mathematical logic by the lectures of [[Max Newman]] at the [[University of Cambridge]], wrote a paper in 1936 entitled ''On Computable Numbers, with an Application to the [[Entscheidungsproblem]]'', which was published in the ''Proceedings of the London Mathematical Society''.<ref name="Turing1936">{{Citation |author-last=Turing |author-first=Alan M. |author-link=Alan M. Turing |publication-date=1937 |date=1936 |title=On Computable Numbers, with an Application to the Entscheidungsproblem |periodical=Proceedings of the London Mathematical Society |series=2 |volume=42 |pages=230–265 |doi=10.1112/plms/s2-42.1.230|s2cid=73712 }} and {{Citation |author-last=Turing |author-first=Alan M. |author-link=Alan M. Turing |publication-date=1937 |title=On Computable Numbers, with an Application to the Entscheidungsproblem. A correction |periodical=Proceedings of the London Mathematical Society |series=2 |volume=43 |pages=544–546 |doi=10.1112/plms/s2-43.6.544 |date=1938 |issue=6}}.</ref> In it he described a hypothetical machine he called a ''universal computing machine'', now known as the "[[Universal Turing machine]]". The hypothetical machine had an infinite store (memory in today's terminology) that contained both instructions and data. [[John von Neumann]] became acquainted with Turing while he was a visiting professor at Cambridge in 1935, and also during Turing's PhD year at the [[Institute for Advanced Study]] in [[Princeton, New Jersey]] during 1936–1937. Whether he knew of Turing's paper of 1936 at that time is not clear.

In 1936, [[Konrad Zuse]] also anticipated, in two patent applications, that machine instructions could be stored in the same storage used for data.<ref>{{citation |title=Electronic Digital Computers |journal=Nature |date=September 25, 1948 |volume=162 |page=487 |doi=10.1038/162487a0 |df=mdy-all |last1=Williams |first1=F. C. |last2=Kilburn |first2=T. |issue=4117 |bibcode=1948Natur.162..487W |s2cid=4110351 |doi-access=free }}.</ref>

Independently, [[J. Presper Eckert]] and [[John Mauchly]], who were developing the [[ENIAC]] at the [[Moore School of Electrical Engineering]] of the [[University of Pennsylvania]], wrote about the stored-program concept in December 1943.
<ref name="Lukoff_1979">{{cite book |author-last=Lukoff |author-first=Herman |author-link=Herman Lukoff |title=From Dits to Bits: A personal history of the electronic computer |date=1979 |publisher=Robotics Press |location=Portland, Oregon|isbn=0-89661-002-0 |lccn=79-90567}}</ref><ref>ENIAC project administrator Grist Brainerd's December 1943 progress report for the first period of the ENIAC's development implicitly proposed the stored program concept (while simultaneously rejecting its implementation in the ENIAC) by stating that "in order to have the simplest project and not to complicate matters", the ENIAC would be constructed without any "automatic regulation".</ref> In planning a new machine, [[EDVAC]], Eckert wrote in January 1944 that they would store data and programs in a new addressable memory device, a mercury metal [[delay-line memory]]. This was the first time the construction of a practical stored-program machine was proposed.  At that time, he and Mauchly were not aware of Turing's work.

Von Neumann was involved in the [[Manhattan Project]] at the [[Los Alamos National Laboratory]]. It required huge amounts of calculation, and thus drew him to the ENIAC project, during the summer of 1944. There he joined the ongoing discussions on the design of this stored-program computer, the EDVAC. As part of that group, he wrote up a description titled ''First Draft of a Report on the EDVAC''<ref name ="FirstDraftReport"/> based on the work of Eckert and Mauchly. It was unfinished when his colleague Herman Goldstine circulated it, and bore only von Neumann's name (to the consternation of Eckert and Mauchly).<ref>{{Harvnb|Copeland|2006|p=113}}.</ref> The paper was read by dozens of von Neumann's colleagues in America and Europe, and influenced{{vague|date=March 2022}} the next round of computer designs.

[[Jack Copeland]] considers that it is "historically inappropriate to refer to electronic stored-program digital computers as 'von Neumann machines{{'"}}.<ref>{{Citation |author-last=Copeland |author-first=Jack |author-link=Jack Copeland |title=A Brief History of Computing: ENIAC and EDVAC |date=2000 |url=http://www.alanturing.net/turing_archive/pages/Reference%20Articles/BriefHistofComp.html#ACE |access-date=2010-01-27}}.</ref> His Los Alamos colleague [[Stan Frankel]] said of von Neumann's regard for Turing's ideas<ref>{{Citation |author-last=Copeland |author-first=Jack |author-link=Jack Copeland |title=A Brief History of Computing: ENIAC and EDVAC |date=2000 |url=http://www.alanturing.net/turing_archive/pages/Reference%20Articles/BriefHistofComp.html#ACE |access-date=2010-01-27}} (a work which cites {{Citation |author-last=Randell |author-first=Brian |author-link=Brian Randell |title=On Alan Turing and the Origins of Digital Computers |editor-last=Meltzer |editor-first=B. |editor-last2=Michie |editor-first2=D. |page=10 |place=Edinburgh |publisher=[[Edinburgh University Press]] |journal=Machine Intelligence |volume=7 |date=1972 |isbn=0-902383-26-4}}.</ref>
{{quote|
I know that in or about 1943 or '44 von Neumann was well aware of the fundamental importance of Turing's paper of 1936....
Von Neumann introduced me to that paper and at his urging I studied it with care. Many people have acclaimed von Neumann as the "father of the computer" (in a modern sense of the term) but I am sure that he would never have made that mistake himself. 
He might well be called the midwife, perhaps, but he firmly emphasized to me, and to others I am sure, that the fundamental conception is owing to Turing—in so far as not anticipated by Babbage.... Both Turing and von Neumann, of course, also made substantial contributions to the "[[reduction to practice]]" of these concepts but I would not regard these as comparable in importance with the introduction and explication of the concept of a computer able to store in its memory its program of activities and of modifying that program in the course of these activities.
}}

At the time that the "First Draft" report was circulated, Turing was producing a report entitled ''Proposed Electronic Calculator''. It described in engineering and programming detail, his idea of a machine he called the ''[[Automatic Computing Engine|Automatic Computing Engine (ACE)]]''.<ref>{{Harvnb|Copeland|2006|pp=108–111}}.</ref> He presented this to the executive committee of the British [[National Physical Laboratory, UK|National Physical Laboratory]] on February 19, 1946. Although Turing knew from his wartime experience at Bletchley Park that what he proposed was feasible, the secrecy surrounding [[Colossus computer|Colossus]], that was subsequently maintained for several decades, prevented him from saying so. Various successful implementations of the ACE design were produced.

Both von Neumann's and Turing's papers described stored-program computers, but von Neumann's earlier paper achieved greater circulation and the computer architecture it outlined became known as the "von Neumann architecture". In the 1953 publication ''Faster than Thought: A Symposium on Digital Computing Machines'' (edited by B.&nbsp;V. Bowden), a section in the chapter on ''Computers in America'' reads as follows:<ref>{{Harvnb|Bowden|1953|pp=176, 177}}.</ref>
<blockquote>
'''The Machine of the Institute For Advanced Study, Princeton'''

In 1945, Professor J. von Neumann, who was then working at the Moore School of Engineering in Philadelphia, where the E.N.I.A.C. had been built, issued on behalf of a group of his co-workers, a report on the logical design of digital computers. The report contained a detailed proposal for the design of the machine that has since become known as the E.D.V.A.C. (electronic discrete variable automatic computer). This machine has only recently been completed in America, but the von Neumann report inspired the construction of the E.D.S.A.C. (electronic delay-storage automatic calculator) in Cambridge (see p. 130).

In 1947, Burks, Goldstine and von Neumann published another report that outlined the design of another type of machine (a parallel machine this time) that would be exceedingly fast, capable perhaps of 20,000 operations per second. They pointed out that the outstanding problem in constructing such a machine was the development of suitable memory with instantaneously accessible contents. At first they suggested using a special [[vacuum tube]]—called the "[[Selectron tube|Selectron]]"—which the Princeton Laboratories of RCA had invented. These tubes were expensive and difficult to make, so von Neumann subsequently decided to build a machine based on the [[Williams tube|Williams memory]]. This machine—completed in June, 1952 in Princeton—has become popularly known as the Maniac. The design of this machine inspired at least half a dozen machines now being built in America, all known affectionately as "Johniacs".
</blockquote>

In the same book, the first two paragraphs of a chapter on ACE read as follows:<ref>{{Harvnb|Bowden|1953|p=135}}.</ref>
<blockquote>
'''Automatic Computation at the National Physical Laboratory'''

One of the most modern digital computers which embodies developments and improvements in the technique of automatic electronic computing was recently demonstrated at the National Physical Laboratory, Teddington, where it has been designed and built by a small team of mathematicians and electronics research engineers on the staff of the Laboratory, assisted by a number of production engineers from the English Electric Company, Limited. The equipment so far erected at the Laboratory is only the pilot model of a much larger installation which will be known as the Automatic Computing Engine, but although comparatively small in bulk and containing only about 800 thermionic valves, as can be judged from Plates XII, XIII and XIV, it is an extremely rapid and versatile calculating machine.

The basic concepts and abstract principles of computation by a machine were formulated by Dr.&nbsp;A.&nbsp;M. Turing, F.R.S., in a paper<sup>1</sup>. read before the London Mathematical Society in 1936, but work on such machines in Britain was delayed by the war. In 1945, however, an examination of the problems was made at the National Physical Laboratory by Mr.&nbsp;J.&nbsp;R. Womersley, then superintendent of the Mathematics Division of the Laboratory. He was joined by Dr. Turing and a small staff of specialists, and, by 1947, the preliminary planning was sufficiently advanced to warrant the establishment of the special group already mentioned. In April, 1948, the latter became the Electronics Section of the Laboratory, under the charge of Mr.&nbsp;F.&nbsp;M. Colebrook.
</blockquote>

==Early von Neumann-architecture computers==
The ''First Draft'' described a design that was used by many universities and corporations to construct their computers.<ref>{{cite web |url=http://www.ias.edu/people/vonneumann/ecp/ |title=Electronic Computer Project |date=September 11, 2009 |publisher=[[Institute for Advanced Study]] |access-date=2011-05-26}}</ref> Among these various computers, only ILLIAC and ORDVAC had compatible instruction sets.
* [[APEXC|ARC2]] ([[Birkbeck, University of London]]) officially came online on May 12, 1948.<ref name="birkbeck">{{cite journal |author-last1=Campbell-Kelly |author-first1=Martin |title=The Development of Computer Programming in Britain (1945 to 1955) |journal=IEEE Annals of the History of Computing |date=April 1982 |volume=4 |issue=2 |pages=121–139 |doi=10.1109/MAHC.1982.10016|s2cid=14861159 }}</ref>
* [[Manchester Baby]] ([[Victoria University of Manchester]], England) made its first successful run of a stored program on June 21, 1948.
* [[EDSAC]] ([[University of Cambridge]], England) was the first practical stored-program electronic computer (May 1949)
* [[Manchester Mark 1]] ([[University of Manchester]], England) Developed from the Baby (June 1949)
* [[CSIRAC]] ([[Commonwealth Scientific and Industrial Research Organisation|Council for Scientific and Industrial Research]]) Australia (November 1949)
* [[MESM]] at the [[Kiev Institute of Electrotechnology]] in [[Kyiv|Kiev]], [[Ukrainian Soviet Socialist Republic|Ukrainian SSR]] (November 1950)
* [[EDVAC]] ([[Ballistic Research Laboratory]], Computing Laboratory at [[Aberdeen Proving Ground]] 1951)
* [[IAS machine]]  at [[Institute for Advanced Study]] (1951)
* [[ORDVAC]] (University of Illinois) at [[Aberdeen Proving Ground]], Maryland (completed November 1951)<ref>{{citation |author-first=James E. |author-last=Robertson |title=Illiac Design Techniques |publisher=report number UIUCDCS-R-1955–146, Digital Computer Laboratory, University of Illinois at Urbana-Champaign |date=1955}}.</ref>
* [[MANIAC I]] at [[Los Alamos Scientific Laboratory]] (March 1952)
* [[ILLIAC]] at the [[University of Illinois Urbana-Champaign|University of Illinois]], (September 1952)
* [[BESM|BESM-1]] in Moscow (1952)
* [[AVIDAC]] at [[Argonne National Laboratory]] (1953)
* [[ORACLE (computer)|ORACLE]] at [[Oak Ridge National Laboratory]] (June 1953)
* [[BESK]] in Stockholm (1953)
* [[JOHNNIAC]] at [[RAND Corporation]] (January 1954)
* [[DASK]] in Denmark (1955)
* [[WEIZAC]] at the [[Weizmann Institute of Science]] in [[Rehovot]], Israel (1955)
* [[PERM (computer)|PERM]] in Munich (1956)
* [[SILLIAC]] in Sydney (1956)

==Early stored-program computers==
The date information in the following chronology is difficult to put into proper order. Some dates are for first running a test program, some dates are the first time the computer was demonstrated or completed, and some dates are for the first delivery or installation.
* The [[IBM SSEC]] had the ability to treat instructions as data, and was publicly demonstrated on January 27, 1948. This ability was claimed in a [[United States patent law|US patent]].<ref>{{US patent|2636672|src=uspto|Selective Sequence Electronic Calculator (USPTO Web site)}}.</ref><ref>{{US patent|2636672|Selective Sequence Electronic Calculator (Google Patents)}}.</ref> However it was partially [[Electromechanics|electromechanical]], not fully electronic. In practice, instructions were read from [[Punched tape|paper tape]] due to its limited memory.<ref name="herb">{{Citation |author-first=Herbert R. J. |author-last=Grosch |title=Computer: Bit Slices From a Life |date=1991 |publisher=Third Millennium Books |isbn=0-88733-085-1 |url=http://www.columbia.edu/acis/history/computer.html |author-link=Herb Grosch}}.</ref>
* The [[APEXC|ARC2]] developed by [[Andrew Donald Booth|Andrew Booth]] and [[Kathleen Booth]] at [[Birkbeck, University of London]] officially came online on May 12, 1948.<ref name="birkbeck"/> It featured the first [[drum memory|rotating drum storage device]].<ref>{{cite book |editor-last1=Lavington |editor-first1=Simon |title=Alan Turing and his Contemporaries: Building the World's First Computers |date=2012 |publisher=[[British Computer Society]] |location=London |isbn=978-1906124908 |page=61}}</ref><ref>{{cite web |author-last1=Johnson |author-first1=Roger |title=School of Computer Science & Information Systems: A Short History |url=http://www.dcs.bbk.ac.uk/site/assets/files/1029/50yearsofcomputing.pdf |website=Birkbeck College |publisher=[[University of London]] |access-date=2017-07-23 |date=April 2008}}</ref>
* The [[Manchester Baby]] was the first fully electronic computer to run a stored program. It ran a factoring program for 52 minutes on June 21, 1948, after running a simple division program and a program to show that two numbers were [[Coprime integers|relatively prime]].
* The [[ENIAC]] was modified to run as a primitive read-only stored-program computer (using the Function Tables for program [[Read-only memory|ROM]]) and was demonstrated as such on September 16, 1948, running a program by [[Adele Goldstine]] for von Neumann.
* The [[BINAC]] ran some test programs in February, March, and April 1949, although was not completed until September 1949.
* The [[Manchester Mark 1]] developed from the Baby project.  An intermediate version of the Mark 1 was available to run programs in April 1949, but was not completed until October 1949.
* The [[Electronic Delay Storage Automatic Calculator|EDSAC]] ran its first program on May 6, 1949.
* The [[EDVAC]] was delivered in August 1949, but it had problems that kept it from being put into regular operation until 1951.
* The [[CSIRAC|CSIR Mk I]] ran its first program in November 1949.
* The [[SEAC (computer)|SEAC]] was demonstrated in April 1950.
* The [[Pilot ACE]] ran its first program on May 10, 1950, and was demonstrated in December 1950.
* The [[SWAC (computer)|SWAC]] was completed in July 1950.
* The [[Whirlwind (computer)|Whirlwind]] was completed in December 1950 and was in actual use in April 1951.
* The first [[UNIVAC 1101|ERA Atlas]] (later the commercial ERA 1101/UNIVAC 1101) was installed in December 1950.

==Evolution==
[[File:Computer system bus.svg|thumb|Single [[system bus]] evolution of the architecture]]

Through the decades of the 1960s and 1970s computers generally became both smaller and faster, which led to evolutions in their architecture. For example, [[memory-mapped I/O]] lets input and output [[Peripheral|devices]] be treated the same as memory.<ref>{{Citation |title=A New Architecture for Mini-Computers – The DEC PDP-11 |author-first1=C. Gordon |author-last1=Bell |author-link1=Gordon Bell |author-first2=R. |author-last2=Cady |author-first3=H. |author-last3=McFarland |author-first4=J. |author-last4=O'Laughlin |author-first5=R. |author-last5=Noonan |author-first6=W. |author-last6=Wulf |work=Spring Joint Computer Conference |pages=657–675 |date=1970 |url=http://research.microsoft.com/en-us/um/people/gbell/CGB%20Files/New%20Architecture%20PDP11%20SJCC%201970%20c.pdf}}.</ref> A single [[system bus]] could be used to provide a modular system with lower cost{{clarify|date=November 2015}}. This is sometimes called a "streamlining" of the architecture.<ref>{{Citation |title=The essentials of computer organization and architecture |author-first1=Linda |author-last1=Null |author-first2=Julia |author-last2=Lobur |publisher=[[Jones & Bartlett]] Learning |date=2010 |isbn=978-1-4496-0006-8 |edition=3rd |pages=36, 199–203 |url=https://books.google.com/books?id=f83XxoBC_8MC&pg=PA36}}.</ref>
In subsequent decades, simple [[microcontrollers]] would sometimes omit features of the model to lower cost and size.
Larger computers added features for higher performance.

==Design limitations==
===von Neumann bottleneck===<!-- linked in [[Random-access memory]] -->
The use of the same bus to fetch instructions and data leads to the ''von Neumann bottleneck'', the limited [[throughput]] (data transfer rate) between the [[central processing unit]] (CPU) and memory compared to the amount of memory.  Because the single bus can only access one of the two classes of memory at a time, throughput is lower than the rate at which the CPU can work.  This seriously limits the effective processing speed when the CPU is required to perform minimal processing on large amounts of data.  The CPU is continually<!-- http://en.wiktionary.org/wiki/continual#Usage_notes --> [[Wait state|forced to wait]] for needed data to move to or from memory.  Since CPU speed and memory size have increased much faster than the throughput between them, the bottleneck has become more of a problem, a problem whose severity increases with every new generation of CPU.

The von Neumann bottleneck was described by [[John Backus]] in his 1977 ACM [[Turing Award]] lecture.  According to Backus:

<blockquote>Surely there must be a less primitive way of making big changes in the store than by pushing vast numbers of [[Word (computer architecture)|words]] back and forth through the von Neumann bottleneck. Not only is this tube a literal bottleneck for the data traffic of a problem, but, more importantly, it is an intellectual bottleneck that has kept us tied to word-at-a-time thinking instead of encouraging us to think in terms of the larger conceptual units of the task at hand. Thus programming is basically planning and detailing the enormous traffic of words through the von Neumann bottleneck, and much of that traffic concerns not significant data itself, but where to find it.<ref name="backus">{{cite journal |doi=10.1145/359576.359579 |s2cid=16367522 |title=Can Programming Be Liberated from the von Neumann Style? A Functional Style and Its Algebra of Programs |author-last=Backus |author-first=John W. |author-link=John Backus |journal=Communications of the ACM |volume=21 |issue=8 |date=August 1978 |pages=613–641 |doi-access=free }}</ref><ref>{{cite web |url=http://www.cs.utexas.edu/~EWD/transcriptions/EWD06xx/EWD692.html |title=E. W. Dijkstra Archive: A review of the 1977 Turing Award Lecture |access-date=2008-07-11 |author-first=Edsger W. |author-last=Dijkstra |author-link=Edsger W. Dijkstra}}</ref></blockquote>

====Mitigations====
{{update section|reason=Everything in the list has been implemented in ordinary desktop computers to some degree, often extensively.  NUMA has been common on multichip-module workstation processors for years now.  Are they helping this problem?  Has the problem space changed?|date=April 2025}}
There are several known methods for mitigating the Von Neumann performance bottleneck.  For example, the following all can improve performance:{{why|date=November 2015}}
* Providing a [[CPU cache|cache]] between the CPU and the [[main memory]].
* Providing separate caches or separate access paths for data and instructions (the so-called [[Modified Harvard architecture]]).
* Using [[branch predictor]] algorithms and logic.
* Providing a limited CPU stack or other on-chip [[scratchpad memory]] to reduce memory access.
* Implementing the CPU and the [[memory hierarchy]] as a [[System on a chip|system on chip]], providing greater [[locality of reference]] and thus reducing latency and increasing throughput between [[processor register]]s and main memory.

The problem can also be sidestepped somewhat by using [[parallel computing]], using for example the [[non-uniform memory access]] (NUMA) architecture—this approach is commonly employed by [[supercomputer]]s. It is less clear whether the ''intellectual bottleneck'' that Backus criticized has changed much since 1977. Backus's proposed solution has not had a major influence.{{citation needed|date=December 2010}} Modern [[functional programming]] and [[object-oriented programming]] are much less geared towards "pushing vast numbers of words back and forth"{{how?|reason=Objects are just vast numbers of words arranged into clear structures.  Functional languages potentially push even more data around since the functions themselves may be passed around as parameters and they'll eventually be working on data|date=April 2025}} than earlier languages like [[FORTRAN]] were, but internally, that is still what computers spend much of their time doing, even highly parallel supercomputers.{{citation needed|date=April 2025}}

===Self-modifying code===
{{main|Self-modifying code}}

Aside from the von Neumann bottleneck, program modifications can be quite harmful, either by accident or design.{{citation needed|date=September 2023}} In some simple stored-program computer designs{{which?|reason=Is this relevant on any modern machine?|date=April 2025}}, a malfunctioning program can damage itself, other programs, or the operating system, possibly leading to a computer crash. However, this problem also applies to conventional programs that lack [[bounds checking]]. [[Memory protection]] and various access controls generally safeguard against both accidental and malicious program changes.

==See also==
* [[CARDboard Illustrative Aid to Computation]]
* [[Interconnect bottleneck]]
* [[Little man computer]]
* [[Random-access machine]]
* [[Harvard architecture]]
* [[Turing machine]]

==References==
{{Reflist}}

==Further reading==
{{refbegin}}
* {{Citation |publication-date=1953 |editor-last=Bowden |editor-first=B. V. |title=Faster Than Thought: A Symposium on Digital Computing Machines |publication-place=London |publisher=Sir Isaac Pitman and Sons Ltd.}}
* {{Citation |publication-date=2000 |editor-last1=Rojas |editor-first1=Raúl |editor-last2=Hashagen |editor-first2=Ulf |title=The First Computers: History and Architectures |publisher=[[MIT Press]] |isbn=0-262-18197-5 |date=2000}}
* {{Citation |author-last=Davis |author-first=Martin  |author-link=Martin Davis (mathematician) |publication-date=2000 |title=The universal computer: the road from Leibniz to Turing |publication-place=New York |publisher=[[W. W. Norton & Company Inc.]] |isbn=0-393-04785-7 |date=2000}} republished as: {{Citation |author-last=Davis |author-first=Martin |author-link=Martin Davis (mathematician) |title=Engines of Logic: Mathematicians and the Origin of the Computer |place=New York |publisher=[[W. W. Norton & Company]] |date=2001 |isbn=978-0-393-32229-3}}
* ''Can Programming be Liberated from the von Neumann Style?''. [[John Backus|Backus, John]]. 1977 ACM Turing Award Lecture. Communications of the ACM, August 1978, Volume 21, Number 8 [http://www.stanford.edu/class/cs242/readings/backus.pdf Online PDF] {{Webarchive|url=https://web.archive.org/web/20070621162552/http://www.stanford.edu/class/cs242/readings/backus.pdf |date=June 21, 2007 }} see details at https://www.cs.tufts.edu/~nr/backus-lecture.html
* Bell, C. Gordon; Newell, Allen (1971), ''Computer Structures: Readings and Examples'', [[McGraw-Hill Book Company]], New York. Massive (668 pages)
* {{Citation |author-last=Copeland |author-first=Jack |publication-date=2006 |contribution=Colossus and the Rise of the Modern Computer |editor-last=Copeland |editor-first=B. Jack |title=Colossus: The Secrets of Bletchley Park's Codebreaking Computers |publication-place=Oxford |publisher=[[Oxford University Press]] |isbn=978-0-19-284055-4 |date=2006}}
* {{Citation |author-last=Ganesan |author-first=Deepak |title=The von Neumann Model |date=2009 |url=http://none.cs.umass.edu/~dganesan/courses/fall09/handouts/Chapter4.pdf |access-date=2011-10-22 |archive-date=April 25, 2012 |archive-url=https://web.archive.org/web/20120425083227/http://none.cs.umass.edu/~dganesan/courses/fall09/handouts/Chapter4.pdf |url-status=dead }}
* {{cite book |ref=SCOTT |author-last=McCartney |author-first=Scott |title=ENIAC: The Triumphs and Tragedies of the World's First Computer |publisher=Walker & Co. |date=1999 |isbn=0-8027-1348-3 |url=https://archive.org/details/eniac00scot }}
* {{cite book |ref=HHG |author-last=Goldstine |author-first=Herman H. |author-link=Herman Goldstine |title=The Computer from Pascal to von Neumann |publisher=[[Princeton University Press]] |date=1972 |isbn=0-691-08104-2 |url=https://archive.org/details/computerfrompasc00herm }}
* {{cite book |ref=JOEL |author-last=Shurkin |author-first=Joel |title=Engines of the Mind: A history of the Computer |publisher=[[W. W. Norton & Company]] |date=1984 |location=New York, London |isbn=0-393-01804-0}}
{{refend}}

==External links==
{{Commons category}}
* [http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka11516.html Harvard vs von Neumann]
* [https://web.archive.org/web/20080219131555/http://home.gna.org/vov/ A tool that emulates the behavior of a von Neumann machine]
* [http://sourceforge.net/projects/johnnysimulator/ JOHNNY: A simple Open Source simulator of a von Neumann machine for educational purposes]

{{CPU technologies}}

{{DEFAULTSORT:Von Neumann Architecture}}
[[Category:Computer architecture]]
[[Category:Flynn's taxonomy]]
[[Category:Reference models]]
[[Category:Classes of computers]]
[[Category:Department of Computer Science, University of Manchester]]
[[Category:Computer-related introductions in 1945]]
[[Category:John von Neumann]]
[[Category:Naming controversies]]