3.0 INTRODUCTION

To understand how data is processed, by a computer, we can draw a simple analogy between computers and humans.

Suppose a student asks a teacher “what happens when 15 is multiplied by 8”. S/He receives the answer 120 from teacher. In the case of a computer, this process can be described as follows:

The teacher‟s brain receives the question through his/her ears (analogous to a computer‟s input device), processes the question with the help of his brain‟s information processing and analytical ability (analogous to computer‟s CPU) and gives the answer through the mouth (analogous to a computer‟s output device). Further, just as the teacher can write down the answer on a sheet of paper or blackboard, the computer can also print the answer on a paper through a printer attached to it.

Every modern computer system consists of three basic sections: 

1. Input device (i.e. Keyboard, mouse or scanner etc.) 

2. Processor (or CPU):

  • Control unit (CU)
  • Arithmetic and Logic Unit (ALU) 
  • Memory unit
3. Output device (Visual Display Unit (Monitor/screen) or printer etc.) 
    The basic parts of a computers are shown in Figure 3.1 

  • Input devices such as a keyboard, mouse or scanner are used to enter input (data and/or instructions), directly into the computer. 
  • The CPU is like the human brain; it has a memory and just like there is a faculty in the brain that regulates the functioning of all parts of the body, a computer has a control-unit (CU), which controls its entire operation including its input and output devices.
  • Processing of data is done in the arithmetic and logic unit (ALU). It performs Arithmetic and logic operations such as addition, subtraction, multiplication and division. Here the word “Logic” is used because a computer (unlike a calculator) has the capacity to do logical operations also, such as compare two numbers, and find out which of the two numbers is greater
The Memory unit is an important component of a computer where all the data and information are stored in the form of binary digits 

The Memory unit is an important component of a computer where all the data and information are stored in the form of binary digits (combination of 0‟s and 1‟s) and retrieved whenever necessary. Computer systems use a variety of devices for storing instructions and data. The computer memory is the place where the computer holds data and programs that are in use. Computer memory refers to the physical devices in a computer. If our computers CPU had to constantly access the hard drive to retrieve every piece of data it requires, the operation will be very slow. On the other hand, when the data or information is kept in memory the CPU can access it much more quickly. From the time the computer is turned on until the time it is shut down, the CPU is constantly using the memory system. The act of entering data into a storage location is called a memory write operation, and the act of retrieving data from a storage location is called a memory read operation. Data and instructions are moved, to and from memory, in bunches of word length. These memory devices are categorised according to access time, storage capacity and cost-per-bit of storage. 

Based on these criteria memory is broadly categorised into two types:
  • Primary or main memory (also called semiconductor memory). 
  • Secondary or auxiliary memory (magnetic memory/Optical memory).  
The Table 3.1 summarizes the difference between Primary (or main) memory and Memory System secondary (or auxiliary) memory.

Table: 3.1 : Difference between Memories

That is, Primary memory (i.e. RAM, ROM etc.) have faster access time, smaller storage capacity, and higher cost per bit of storage, as compared to secondary memory.

Based on access time, storage capacity and cost/bit storage, the memory devices (such as RAM, ROM, Hard-disk, Floppy disk, Magnetic disk, Magnetic Tape, CD-ROM, and DVD etc.) can be categorized into three kinds of memory systems:
  • Semiconductor memory such as RAM, ROM etc
  • Magnetic memory such as Hard-disk, Floppy disk, and Magnetic tapes
  • Optical memory such as CD-ROM, DVD etc 
A Central Processing Unit (CPU) in a computer system is an extremely fast device as compared to the main memory, but it can not work on its own. It depends on the main memory that sends data and instruction when required for processing.

A Central Processing Unit (CPU) in a computer system is an extremely fast device as compared to the main memory, but it can not work on its own. It depends on the main memory that sends data and instruction when required for processing. 

(To reduce the cost of a large sized memory, a special type of high speed memory, known as cache memory can be used in between the CPU and the main memory  

Computer memory can also be categorized on the basis of Volatile and Non-Volatile characteristics. Non-volatile Memory is a type of computer memory which can retain the stored information even if the power is shut down. Examples of Non-Volatile Memory are Read-only memory (ROM), flash memory, optical discs etc. On the other hand, a Volatile memory loses its content when the power goes off. Random Access Memory (RAM), which is the most common form of Primary Storage, is a type of volatile memory. When the computer is shut down, everything contained in RAM is lost.

To reduce the idle time of the CPU and increase the speed of processing, a fast memory can be used. This can be done by using a large size of main memory. But the cost of main memories of relatively large size is still very high. To reduce the cost of a large sized memory, a special type of high speed memory, known as cache memory can be used in between the CPU and the main memory (as shown in Figure 3.2). Thus the cost versus access time leads to a hierarchy of computer memory, where the memory is organized into a hierarchy, known as the memory hierarchy. This includes CPU registers, Cache memory, main memory and various secondary storage devices. Although the memory hierarchy is organized in such a way to minimize the cost, without compromising the overall speed of access. 


 In this unit we will mainly focus on storage organization and storage devices such as disk/tape, CD/DVD and ROM/PROM etc. We also discuss the memory hierarchy which includes main memory and high speed memory such as cache memory.

3.1 OBJECTIVES

After going through this unit, you should be able to:
  • differentiate between types of memory and needs of the memory system; 
  • differentiate between various types of memories such as semiconductor, magnetic and Optical memory; describe the various secondary storage devices such as Hard-disk, floppy disk, CD-ROM, DVD-ROM etc; and 
  • describe the importance of the memory hierarchy.