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Von Neumann Architecture
Lecture by Chamodi Adikaram
for Northshore College of Business & Technology

Learning Objectives
? Understand the origin of von Neumann Architecture
? Understand von Neumann Architecture
? Understand its components and functionalities
? To be aware of other architectures

What is an architecture?
? ��The complex or carefully designed structure of
something.��

Fixed Programs
? They were designed to do very speci?c operations
? A calculator is a ?xed program computer.
? It can do basic mathematics, but it cannot be used as
a word processor or a gaming console.
? Changing the program of a ?xed-program machine
requires rewiring, restructuring, or redesigning the
machine.
? It could take three weeks to set up a program on
ENIAC and get it working.

Stored Programs
? A stored-program computer is one that
stores program instructions in electronic
memory
? Therefore easily reprogrammable
? Von Neumann is a stored program architecture
(also known as Princeton architecture)
introduced by John von Neumann, a
Hungarian-American mathematician, physicist,
and computer scientist.

Von Neumann Architecture
cntd��
? The basic concept behind the von Neumann architecture is the ability to store program
instructions in memory along with the data on which those instructions operate.
? The von Neumann architecture describes a general framework, or structure, that a
computer's hardware, programming, and data should follow.
? Evan all the most modern computers and mobile devices are designed based on this
fundamental concept.
? It had 3 basic components:
1. I/O Interfaces
2. CPU
3. Memory
? Buses (denoted by the arrows) carries the data around

I/O Interfaces
? The I/O interfaces allow the computer's memory to
receive information and send data to output
devices.
? Allow the computer to communicate to the user and
to secondary storage devices like disk and tape
drives

Central Processing Unit
? Can be considered the heart of the computing
system
? Includes three main components:
1. Control Unit (CU)
2. Arithmetic Logic Unit (ALU)
3. Registers

CPU: Control Unit
? Responsible for decoding the instructions and
controlling how data moves around the computer
system
? The execution of each instruction is determined by
a sequence of control signals produced by the
control unit.

CPU: Arithmetic Logic Unit
? Carries out the calculations and logical decisions
required by the program instructions.
? The inputs to an ALU are the data to be operated
on, called operands, and a code indicating the
operation to be performed; the ALU's output is the
result of the performed operation.

CPU: Registers
? Registers are memory locations with speci?c purpose
? Accumulator (AC)- Stores the results of calculations made by the ALU
(temporary)
? Program counter (PC)- Keeps track of the location for the next instruction to
be dealt with. The program counter then passes this next address to the
memory address register (MAR)
? Memory Address Register (MAR)- Stores memory location for data or
instructions that needs to be fetched from memory or stored into memory
? Memory Data Register (MDR)- Stores data or instructions fetched from
memory or any data that is to be transferred and stored in memory
? Current Instruction Register (CIR)- Stores the most recently fetched
instruction while it is waiting to be decoded and executed

Memory
? The computer's memory is used to store program
instructions and data.
? Consists of many memory cells (storage units) of a
?xed size. Each cell has an address associated
with it.
? Two of the commonly used type of memories are
RAM (random-access memory) and Secondary
memory

Memory cntd��
? Memory width (W)- How many
bits is each memory cell,
(typically one byte =8 bits)
? Address width (N)- How many
bits used to represent each
address
? Address space- the number of
uniquely identi?able memory
locations (2N)

Advantages of von
Neumann Architecture
? Control Unit gets data and instruction in the same
way from one memory. It simpli?es design and
development of the Control Unit.
? Data from memory and from devices are accessed
in the same way.
? Memory organization is in the hands of
programmers.

Disadvantages of von
Neumann Architecture
? Serial instruction processing does not allow parallel
execution of program. Parallel executions are
simulated later by the Operating system.
? One bus is a bottleneck. Only one information can
be accessed at the same time.
? Instruction stored in the same memory as the data
can be accidentally rewritten by an error in a
program.

Alternative Architectures
Harvard Architecture
? Memory for data was separated from the memory for instruction.
? Two memories with two Buses allow parallel access to data and instructions.
Execution can be 2x faster.
? But control unit for two Buses is more complicated and more expensive.

Summary
? Von Neumann architecture is based on stored
programs concept.
? Has 3 main components.
? All modern computers still use this 70 year old
fundamental concept
? Has its own advantages and disadvantages
? Availability of alternative architectures

References
? University of Oxford?
https://www.robots.ox.ac.uk/~dwm/Courses/2CO_2014/2CO-N2.pdf
? University of Texas?
http://www.cs.utexas.edu/users/fussell/cs310h/lectures/Lecture_9-310h.pdf
? University of Victoria?
http://www.ece.uvic.ca/~wlu/Teaching/Winter/lecture%20301.pdf
? Southern Illinois University?
http://www2.cs.siu.edu/~cs401/Textbook/ch2.pdf
? Technical University of Ostrava?
http://poli.cs.vsb.cz/edu/apps/eng/p01-archcomp.pdf
? Youtube tutorial?
https://www.youtube.com/watch?v=ckDb_W72__c

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Von Neumann Architecture

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