Von Neumann Architecture | Computer Science | BCA Course | IGNOU

The Von Neumann architecture was the first major proposed structure for a general purpose computer. However, before describing the main components of von Neumann architecture,
let us first define the term 'computer' as this will help us in discussing about von Neumann architecture in logical detail.
Computer is defined in the Oxford dictionary as "An automatic electronic apparatus for making calculations or controlling operations that are expressible in numerical or logical terms".
I The definition clearly categories computer as an electronic apparatus although the first computers were mechanical and electro-mechanical apparatuses.
The definition also points towards the two major areas of computer applications viz., data processing's and computer assisted controlsloperations. Another important aspect of the definition is the fact that the computer can perform only those operations calculations, which can be expressed in Logical or Numerical terms. 1 Some of the basic questions that arise from above definition are: How like the are computer? the data processing and control operations performed by an electronic device I Well, electronic components are used for creating basic logic circuits that are used to perform calculations.


These components are further discussed in the later units. However, for the present discussion, it would be sufficient to say that there must be a certain unit that will perform the task of data processing and control. What is the basic function performed by a computer? The basic function performed by a computer is the execution of the program. A program is a sequence of instructions, which operates on data, to perform certain tasks such as finding a prime number. The computer controls the execution of the program. What is data in computers? In modem digital computers data is represented in binary form by using two symbols 0 and 1. These are called binary digits or bits. But the data which we deal with consists of numeric data and characters such as decimal digits 0 to 9, alphabets A to Z, arithmetic operators (e.g. +,-, etc.), relations operators (e.g. =. > , etc.), and many other special characters (e.g.;,@,{,], etc.). Therefore, there has to be a mechanism for data representation. Old computers use eight bits to represent a character. This allows up to 7,' = 256 different items to be represented uniquely. This collection of eight bits is called a byte. Thus, one byte is used to represent one character internally. Most computers use two bytes or four bytes to represent numbers (positive and negative) internally. The data also includes the operational data such as integer, decimal number etc. We will discuss more about data representation in the next unit. Thus, the prime task of a computer is to perform instruction execution. The key questions, which can be asked in this respect, are: (a) how are the instructions supplied to the computer? and (b) how are the instructions interpreted and executed? Let us answer the second question first. All computers have a Unit that performs the arithmetic and logical functions. This Unit is referred to as the Arithmetic Logic Unit (ALU). But how will the computer determine what operation is to be performed by ALU or in other words who will interpret the operation that is to be performed by ALU? This interpretation is done by the Control Unit of the computer. The control unit accepts the binary form of instruction and interprets the instruction to generate control signals. 'These control signals then direct the ALU to perform a specified arithmetic or logic function on the data. Therefore, by changing the control signal the desired function can be performed on data. Or conversely, the operations that need to be performed on the data can be obtained by providing a set of control signals. Thus, for a new operation one only needs to change the set of control signals. The unit that interprets a code (a machine instruction) to generate respective control signals is termed as Control Unit (CU). A program now consists of a sequence of codes. Each code is, in effect, an instruction, for the computer.


The hardware interprets each of these instructions and generates respective control signals such that the desired operation is performed on the data. The Arithmetic Logic Unit (ALU) and the Control Unit (CU) together are termed as the Central Processing Unit (CPU). The CPU is the most important component of a computer's hardware.
All these arithmetic and logical Operations are performed in the CPU in special storage areas called registers. The size of the register is one of the important considerations in determining the processing capabilities of the CPU. Register size refers to the amount of information that can be held in a register at a time for processing. The larger the register size, the faster may be the speed of processing. But, how can the instructions and data be put into the computers? The instructions and data to a computer are supplied by external environment; it implies that input devices are needed in the computer. The main responsibility of input devices will be to put the data in the form of signals that can be recognized by the system. Similarly, we need another component, which will report the results in proper format.


This component is called output device. These components together are referred to as input output (110) devices. t In addition, to transfer the information, the computer system internally needs the I system interconnections. At present we will not discuss about Input Output devices and system interconnections in details, except the information that most common input'output devices are keyboard, monitor and printer, and the most common interconnection structure is the Bus structure. These concepts are detailed in the later blocks. Input devices can bring instructions or data only sequentially, however, a program may not be executed sequentially as jump, looping, decision-making instructions are normally encountered in programming. In addition, more than one data element may be required at a time. Therefore, a temporary storage area is needed in a computer to store temporarily the instructions and the data This component is referred to as memory.

The memory unit stores all the information in a group of memory cells such as a group of 8 binary digits (that is a byte) or 16 bits or 32 bits etc. These groups of I memory cells or bits are called memory locations. Each memory location has a unique 1 address and can be addressed independently. The contents of the desired memory I locations are provided to the CPU by referring to the address of the memory location. i The amount of information that can be held in the main memory is known as memory I capacity. The capacity of the main memory is measured in Mega Bytes (MB) or Giga  Bytes (GB). One-kilo byte stands for "2" bytes, which are 1024 bytes (or approximately 1000 bytes). A Mega byte stands for 220 bytes, which is approximately a little over one million bytes, a giga byte is 230 bytes.
Let us now define the key features of

Von Neumann Architecture

The most basic function performed by a computer is the execution of a program, which involves: the execution of an instruction, which supplies the information about an operation, and o the data on which the operation is to be performed. The control unit (CU) interprets each of these instructions and generates respective control signals. The Arithmetic Logic Unit (ALU) performs the arithmetic and logical Operations in special storage areas called registers as per the instructions of control unit. The size of the register is one of the important considerations in determining the processing capabilities of the CPU.  Register size refers to the amount of information that can be held in a register at a time for processing.
The larger the register size, the faster may be the speed of processing. An Input/ Output system involving 1/0 devices allows data input and reporting of the results in proper form and format. For transfer of information a computer system internally needs the system interconnections, One such interconnection structure is BUS interconnection. Main Memory is needed in a computer to store instructions and the data at the time of Program execution. Memory to CPU is an important data transfer path. The amount of information, which can be transferred between CPU and memory, depends on the size of BUS connecting the two. It was pointed out by von-Neumann that the same memory can be used for Storing data and instructions. In such a case the data can be treated as data on which processing can be performed, while instructions can be treated as data, which can be used for the generation of control signals. The von Neumann machine uses stored program concept, i.e., the program and data are stored in the same memory unit for execution. The computers prior to this idea used to store programs and data on separate memories. Entering and modifying these programs was very difficult as they were entered manually by setting switches, plugging, and unplugging. Execution of instructions in von Neumann machine is carried out in a sequential fashion (unless explicitly altered by the program itself) from one instruction to the next