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Von Neumann Architecture

Von Neumann Architecture also known as the Von Neumann model or Princeton architecture, is a computer architecture described by the mathematician and physicist John von Neumann and others in the First Draft of a Report on the EDVAC. The Von Neumann architecture is an abstract model for computer architecture. The Von Neumann architecture describes a general framework, or structure, that a computer’s hardware, programming, and data should follow. Although other structures for computing have been devised and implemented, the vast majority of computers in use today operate according to the Von Neumann architecture.
Von Neumann architecture is followed by vast majority of electronic, digital computers. Von Neumann envisioned the structure of a computer system as being composed of following components:
1. The central arithmetic unit, which today is called Arithmetic Logic Unit (ALU). This unit performs the computer’s computational and logical functions.
2. Memory; more specifically, the computer’s main, or fast, memory such as random access memory(RAM).
3. A control unit, that directs others components of the computer to perform certain actions such as directing the fetching of data or instructions from memory to be processed by the ALU.
4. Man-machine interfaces; i.e. Input and Output devices, such as a keyboard for input and display monitor for output.
The computer consisted of a CPU, memory and I/O devices. The program is stored in the memory. The CPU fetches an instruction from the memory at a time and executes it. Thus, the instructions are executed sequentially which is a slow process. Neumann m/c are called control flow computer because instruction are executed sequentially as controlled by a program counter. To increase the speed, parallel processing of computer have been developed in which serial CPU’s are connected in parallel to solve a problem. Even in parallel computers, the basic building blocks are Neumann processors.
The von Neumann architecture is a design model for a stored-program digital computer that uses a processing unit and a single separate storage structure to hold both instructions and data.
The Von Neumann architecture is a design model for a stored-program digital computer and terms “Von Neumann architecture” and “stored-program computer” are generally used interchangeably. Block diagram of such a computer is given below.

The design of a von Neumann architecture machine is simpler than that of a Harvard architecture machine, which is also a stored-program system but has one dedicated set of address and data buses for reading data from and writing data to memory, and another set of address and data buses for fetching instructions. The most noteworthy concept contained in Von Neumann’s architecture was that of stored-program principle.
Stored Program Computer
According to Von Neumann, a general purpose device should be able to store not only its data and intermediate results of computation, but should also store the instructions, that brought about the computation. In a special purpose machine, the computational procedure could be the part of the hardware. In a general purpose one, the instructions must be as changeable as they can be used for number of different purposes.
The earliest computing machines had fixed program. Changing the program of a fixed-program machine required restructuring of the machine. Then came the idea of the stored program computer. Such a computer by design includes instruction set architecture and can store in memory, a set of instructions that details the computation. It also lets program modify themselves while running. The Von-Neumann architecture is a design model for a stored program digital computer.
Characteristics of the Von Neumann architecture
According to myres, To summarize the main characteristics of the Von Neumann architecture are given below.
• First of all, such a computer is composed of distinct components, which are the ALU, control unit, input/output devices and a single memory unit for sharing both data and instructions.
• Secondly, instructions are carried out sequentially, one instruction at a time. As Von Neumann himself recognized, the sequential execution of programming imposes a sort of “speed limit” on program execution since only one instruction at a time can be handled by the computer’s processor.
• Instruction and data are distinguished only through usage and there is no distinction between the two in memory. The higher level languages currently used for programming make a clear distinction between the instructions and the data and have no provisions for executing data or instructions as data.
• The memory is a single memory, which is sequentially addressed. The memory is one-dimensional. All of the data and instructions are stored into that same memory.
• Finally, the meaning of data is not stored with it. In other words, it is not possible to tell by looking at a set of bits whether that set of bits represents an integer, a floating point number or character string.
Disadvantage of Von Neumann Architecture
• One shared memory for instructions (program) and data with one data bus and one address bus between processor and memory. Instructions and data have to be fetched in sequential order (known as the Von Neumann Bottleneck), limiting the operation bandwidth.
• 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. A buffer overflow is one very common example of such a malfunction.

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