= Von Neumann architecture =

======================================================================
= Von Neumann architecture =
======================================================================

Introduction
======================================================================
The von Neumann architecture�also known as the von Neumann model or
Princeton architecture�is a computer architecture based on a 1945
description by the mathematician and physicist John von Neumann and
others in the 'First Draft of a Report on the EDVAC'. That document
describes a design architecture for an electronic digital computer
with these components:
* A processing unit that contains an arithmetic logic unit and
processor registers
* A control unit that contains an instruction register and program
counter
* Memory that stores data and instructions
* External mass storage
* Input and output mechanisms

The term "von Neumann architecture" has evolved to mean any
stored-program computer in which an instruction fetch and a data
operation cannot occur at the same time because they share a common
bus. This is referred to as the von Neumann bottleneck and often
limits the performance of the system.

The design of a von Neumann architecture machine is simpler than a
Harvard architecture machine�which is also a stored-program system but
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
instructions.

A stored-program digital computer keeps both program instructions and
data in read-write, random-access memory (RAM). Stored-program
computers were an advancement over the program-controlled computers of
the 1940s, such as the Colossus and the ENIAC. Those were programmed
by setting switches and inserting patch cables to route data and
control signals between various functional units. The vast majority of
modern computers use the same memory for both data and program
instructions. The von Neumann vs. Harvard distinction applies to the
cache architecture, not the main memory (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 flowcharts 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.

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 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 registers and indirect addressing became usual
features of machine architecture. Another use was to embed frequently
used data in the instruction stream using immediate addressing.
Self-modifying code has largely fallen out of favor, since it is
usually hard to understand and debug, as well as being inefficient
under modern processor pipelining and caching schemes.

Capabilities
======================================================================
On a large scale, the ability to treat instructions as data is what
makes assemblers, compilers, linkers, loaders, and other automated
programming tools possible. It makes "programs that write programs"
possible. This has made a sophisticated self-hosting computing
ecosystem flourish around von Neumann architecture machines.

Some high level languages leverage the von Neumann architecture by
providing an abstract, machine-independent way to manipulate
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 BITBLT or 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'. 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.

Independently, J. Presper Eckert and John Mauchly, who were developing
the ENIAC at the Moore School of Electrical Engineering, at the
University of Pennsylvania, wrote about the stored-program concept in
December 1943.
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, which required huge amounts of calculation. This
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' based on the work of
Eckert and Mauchly. It was unfinished when his colleague Herman
Goldstine circulated it with only von Neumann's name on it, to the
consternation of Eckert and Mauchly. The paper was read by dozens of
von Neumann's colleagues in America and Europe, and influenced 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'". His Los Alamos colleague Stan Frankel said of von
Neumann's regard for Turing's ideas:

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 (ACE)'. He presented this to
the Executive Committee of the British 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, 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. V.
Bowden), a section in the chapter on 'Computers in America' reads as
follows:

The Machine of the Institute For Advanced Studies, 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 page 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"�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 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."

In the same book, the first two paragraphs of a chapter on ACE read as
follows:

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. A. M. Turing, F.R.S., in a paper1. 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. J. 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. F. M. Colebrook.

Early von Neumann-architecture computers
======================================================================
The 'First Draft' described a design that was used by many
universities and corporations to construct their computers. Among
these various computers, only ILLIAC and ORDVAC had compatible
instruction sets.
* ARC2 (Birkbeck, University of London) officially came online on May
12, 1948.
* 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 (Council for Scientific and Industrial Research) Australia
(November 1949)
* EDVAC (Ballistic Research Laboratory, Computing Laboratory at
Aberdeen Proving Ground 1951)
* ORDVAC (U-Illinois) at Aberdeen Proving Ground, Maryland (completed
November 1951)
* IAS machine at Princeton University (January 1952)
* MANIAC I at Los Alamos Scientific Laboratory (March 1952)
* ILLIAC at the University of Illinois, (September 1952)
* BESM-1 in Moscow (1952)
* AVIDAC at Argonne National Laboratory (1953)
* 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 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 US patent. However it was partially electromechanical, not fully
electronic. In practice, instructions were read from paper tape due to
its limited memory.
* The ARC2 developed by Andrew Booth and Kathleen Booth at Birkbeck,
University of London officially came online on May 12, 1948. It
featured the first rotating drum storage device.
* 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 relatively prime.
* The ENIAC was modified to run as a primitive read-only
stored-program computer (using the Function Tables for program 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 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 CSIR Mk I ran its first program in November 1949.
* The 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 was completed in July 1950.
* The Whirlwind was completed in December 1950 and was in actual use
in April 1951.
* The first ERA Atlas (later the commercial ERA 1101/UNIVAC 1101) was
installed in December 1950.

Evolution
======================================================================
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
devices be treated the same as memory. A single system bus could be
used to provide a modular system with lower cost. This is sometimes
called a "streamlining" of the architecture.
In subsequent decades, simple microcontrollers would sometimes omit
features of the model to lower cost and size.
Larger computers added features for higher performance.

Von Neumann bottleneck
========================
The shared bus between the program memory and data memory 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 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:

Surely there must be a less primitive way of making big changes in
the store than by pushing vast numbers of 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.

Mitigations
=============
There are several known methods for mitigating the Von Neumann
performance bottleneck. For example, the following all can improve
performance:
*Providing a 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 chip,
providing greater locality of reference and thus reducing latency and
increasing throughput between processor registers 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 supercomputers. 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. Modern functional programming and
object-oriented programming are much less geared towards "pushing vast
numbers of words back and forth" than earlier languages like FORTRAN
were, but internally, that is still what computers spend much of their
time doing, even highly parallel supercomputers.

As of 1996, a database benchmark study found that three out of four
CPU cycles were spent waiting for memory. Researchers expect that
increasing the number of simultaneous instruction streams with
multithreading or single-chip multiprocessing will make this
bottleneck even worse. In the context of multi-core processors,
additional overhead is required to maintain cache coherence between
processors and threads.

Self-modifying code
=====================
Aside from the von Neumann bottleneck, program modifications can be
quite harmful, either by accident or design. In some simple
stored-program computer designs, a malfunctioning program can damage
itself, other programs, or the operating system, possibly leading to a
computer crash. Memory protection and other forms of access control
can usually protect against both accidental and malicious program
modification.

See also
======================================================================
* CARDboard Illustrative Aid to Computation
* Interconnect bottleneck
* Little man computer
* Random-access machine
* Turing machine
* Neuromorphic engineering
* Eckert architecture

Further reading
======================================================================
*
*
* republished as:
* 'Can Programming be Liberated from the von Neumann Style?'. 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]
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)
*
*
*
*
*

External links
======================================================================
*
[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]

License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL: http://creativecommons.org/licenses/by-sa/3.0/
Original Article: http://en.wikipedia.org/wiki/Von_Neumann_architecture