Functional Units of Digital Computer

UNIT: III

Chapter 6: Computer Fundamentals

Syllabus

Functional Units of a Digital Computer: Von Neumann Architecture - Operation and Operands of Computer Hardware Instruction - Instruction Set Architecture (ISA): Memory Location, Address and Operation - Instruction and Instruction Sequencing - Addressing Modes, Encoding of Machine Instruction - Interaction between Assembly and High Level Language.

Functional Units of Digital Computer

• The digital computer or simply computer in its simplest form is a fast electronic calculating machine that accepts digitized information from the user, processes it according to a sequence of instructions stored in the internal storage, and provides the processed information to the user.

• The computer consists of five functionally independent units :

• Input

• Memory

• Arithmetic and logic

• Output and

• Control units.

• The Fig. 6.1.1 (a) and (b) show these five functional units of a computer and its physical locations in the computer.

• The input unit accepts the digital information from user with the help of input devices such as keyboard,mouse, microphone etc.

• The information received from the input unit is either stored in the memory for later use or immediately used by the arithmetic and logic unit to perform the desired operations.

• The program stored in the memory decides the processing steps and the processed output is sent to the user with the help of output devices or it is stored in the memory for later reference.

• All the above mentioned activities are co-ordinated and controlled by the control unit.

• The arithmetic and logic unit in conjunction with control unit is commonly called Central Processing Unit (CPU).

Input Unit

• A computer accepts a digitally coded information through input unit using input devices.

• The most commonly used input devices are keyboard and mouse.

• The keyboard is used for entering text and numeric information.

• Mouse is used to position the screen cursor and thereby enter the information by selecting option.

• Apart from keyboard and mouse there are many other input devices are available, which include joysticks, trackball, spaceball, digitizers and scanners.

Memory Unit

• The memory unit is used to store programs and data.

• Usually, two types of memory devices are used to form a memory unit: primary storage memory device and secondary storage memory device.

• The primary memory, commonly called main memory is a fast memory used for the storage of programs and active data (the data currently in process).

• The main memory is a semiconductor memory.

• It consists of a large number of semiconductor storage cells, each capable of storing one bit of information.

• These cells are read or written by the central processing unit in a group of fixed size called word.

• The main memory is organized such that the contents of one word, containing n bits, can be stored or retrieved in one write or read operation, respectively.

• To access data from a particular word from main memory each word in the main memory has a distinct address. This allows to access any word from the main memory by specifying corresponding address.

• The number of bits in each word is referred to as the word length of the computer. Typically, the word length varies from 8 to 64 bits.

• The number of such words in the main memory decides the size of memory or capacity of the memory.

• The size of computer main memory varies from few million words to tens of million words.

• An important characteristics of a memory is an access time (the time required to access one word). The access time for main memory should be as small as possible. Typically, it is of the order of 10 to 100 nanoseconds. The access time depends on the type of memory. In randomly accessed memories (RAMs), fixed time is required to access any word in the memory. In sequential access memories this time is not fixed.

• The main memory consists of only randomly accessed memories. These memories are fast but they are small in capacities and expensive. Therefore, the computer uses the secondary storage memories such as magnetic tapes, magnetic disks for the storage of large amount of data.

Stored program concept

• Today's computer are built on two key principles

1.Instructions are represented as numbers.

2. Programs can be stored in memory to be read or written just like numbers. These principles lead to the stored-program concept.

• According to stored-program concept, memory can contain the program (source code), the corresponding compiled machine code, editor program and even the compiler that generated the machine code.

Arithmetic and Logic Unit

• The arithmetic and logic unit (ALU) is responsible for performing arithmetic operations such as add, subtract, division and multiplication and logical operations such as ANDing, ORing, Inverting etc.

• To perform these operations, operands from the main memory are brought into the high speed storage elements called registers of the processor.

• Each register can store one word of data and they are used to store frequently used operands.

• After performing operation, the result is either stored in the register or memory location.

Output Unit

• The output unit sends the processed results to the user using output devices such as video monitor, printer, plotter, etc.

• The video monitors display the output on the CRT screen whereas printers and plotters give the hard-copy output.

• Printers are classified according to their printing methodology: Impact printers and non-impact printers.

Control Unit

• The control unit co-ordinates and controls the activities amongst the functional units.

• Control unit fetches the instructions stored in the main memory, identify the operations and the devices involved in it and accordingly generate control signals to execute the desired operations.

• It uses control signals or timing signals to determine when a given action is to take place.

• It controls input and output operations, data transfers between the processor, memory and input/output devices using timing signals.

• The control and the arithmetic and logic units of a computer are usually many times faster than other devices connected to a computer system. This enables them to control a number of external input/output devices.

Review Questions

1. Explain different functional units of a digital computer.

2. What is a stored program concept? Explain the functional units of a stored program digital computer, along with a block diagram.