4th Class Computers Science Characteristics of Memory Devices

Characteristics of Memory Devices

Introduction

You may have used the computer to store games, music, files and pictures in it. These files get stored in the memory devices. A memory is just like a human brain. It is used to store data and instruction. A memory system is a hierarchy of storage devices with different capacities, costs and access times. When a user issue commands to start application software, such as word processing, the operating software, such as word processing, the operating system locates the program in the storage and loads it into memory. In this we will learn about some characteristics of memory device.

Storage Capacity

Like human beings need to memorize things, which can be retained and recalled on need computer also need to store data and instruction for future use. A computer deals with different types of memories. Two major types of computer memories are (1) Primary (main) Memory (2) Secondary (auxiliary) Memory.

Storage capacity term refers to the size of memory.

Primary memory is generally costly and has capacity limitation, further it cannot retain data for longer period of time. However, we need to store data and instructions for long time so that they can be used later. For this purpose, Secondary storage is used. The Secondary storage stores large amount of data, instructions and information permanently. The popular secondary devices are hard disk, compact disk (Cd\s), digital versatile disks (DVDs), and pen drives.

Secondary memory is not directly accessible to processor of a computer but requires use of computer's input/output channels. Such memory is usually slower than primary memory but it always has higher storage capacity. Data remains unchanged even after switching off the computer. Secondary memory/storage is also known as auxiliary memory storage.

Units of measure memory are given in the following table:

The capacity of any memory device is expressed in terms of:

            (i) Word size

            (ii) Number of words

            (i) Word size: The words are expressed in number of bytes (8 bits). For example,

            1 byte = 8 bits

            2 bytes = 16 bits and 5 bytes =40 bits.

          (ii) Number of words: This specifies number of words available in the particular memory device. For example, if a  memory device is given as 4k * 16. The means the device has a word size of 16 bits and a total of 4096 (4k) words  in memory.

Access Modes

Access mode is the sequence or order in which memory can be accessed. The information from the memory can be accessed in the following ways:-

Random Access Memory (RAM):

The Random access memory all the memory operations take the same amount of time independent of the address of the bytes or word at the memory.

Serial Access Memory;

Sequential access means the system must search the storage device from the beginning of the memory address units it. Finds the required piece of data. Memory device which supports such access is called sequential Access Memory Magnetic tape is an example of Serial Access Memory.

Direct Access

Direct access memory or Random access memory, refers to conditions in which a system can go directly to the information that the user wants. Memory device which supports such access is called a direct access memory magnetic disks are example of direct access memory

Auxiliary Memory

Auxiliary memory is also called secondary memory. This memory is much larger in size than the main memory but is slower. It normally stores system programs, instructions and data files. It can also be used as an overflow/virtual memory in case the main memory capacity has been exceeded.

Access Time

Access time is the time required between the requests made for read/write operations and times for the completion of the request. Computer access time is commonly measured in nanoseconds or milliseconds. Memory access time refers to the time it takes to transfer a character from memory to or from the processor. Access time is also frequently used to describe the speed of disk drives. The access time depends upon the physical characteristics and the access modes used for that device. Primary memory has a faster access time as compared to secondary memory. The time taken to access a given byte of information stored on a hard disk is typically a few thousandths of a second or milliseconds. By contrast, the time taken to access a given byte of information stored in random access memory is measured in billionths of a second or nanoseconds.

Permanence of Storage

Some memories retain information for longer duration while others are used to store information for a very short period. There are memories which require constant refreshing to preserve the information; these memories are called dynamic memory. These memories are volatile in nature and lose contents on power failure. Another memory, which retains memory forever, is called static memory. This comes under the non-volatile category. As you have learned earlier, RAM is volatile memory and ROM is non-volatile memory.

Physical Characteristics

The physical characteristics of a memory device can be categorized into four parts, namely electronic, magnetic, mechanical and optical. Hard disks and floppy disks are examples of magnetic media and CD ROMs and DVDs are examples of optical media. One of the important requirements of the storage device is that it should exhibit two physical states, 0 or 1. The access time of the memory depends upon now quickly the state can be recognized and changed. The quicker the device recognizes the stare, the taster will be the access. If a memory device continues hold data even it' power is turned off. The memory device is non-volatile else it is volatile.