Memory unit
Memory means storage of data and the program. In computer’s memory both data and programs are stored in the binary form. The binary system has only two values 0 and 1. These are called bits. As human beings we all understand decimal system but the computer can only understand binary system. It is because a large number of integrated circuits inside the computer can be considered as switches, which can be made ON, or OFF. If a switch is ON it is considered 1 and if it is OFF it is 0. A number of switches in different states will give you a message like this: 110101......10. So the computer takes input in the form of 0 and 1 and gives output in the same form, i.e., 0 and 1 only. Every number in binary system can be converted to decimal system and vice versa; for example, decimal number 9 means 1001 . Therefore, it is the computer that takes information or data in decimal form from you, converts it into binary form, processes it, producing output in binary form and finally again converts the output to decimal form. A bit is an acronym for binary digit, which stands for one binary piece of information. This can be either 0 or 1. Memory unit is made up of several small storage locations called cells. Each of these cells can store a fixed number of bits called word length. Each cell has a unique number assigned to it called the address of the cell and it is used to identify the cells. The address starts at 0 and goes up to (N-1). You should know that the memory is like a large cabinet containing as many drawers as there are addresses of memory. Each drawer contains a word and the address is written on outside of the drawer. Memory sizes(Capacity): The size of memory is termed in terms KB,MB,GB,etc.
1 Byte = 8 bits
1 Kilo Byte(KB) = 1024 Bytes(210)
1 Mega Byte(MB) = 1024 KB
1 Giga Byte(GB) = 1024 MB
1 Tera Byte(TB) = 1024 GB
1 Peta Byte(PB) = 1024 TB
1 Exa Byte(EB) = 1024 PB
1 Zetta Byte(ZB) = 1024 EB
1 Yotta Byte(YB) = 1024 ZB
Byte is used to store one character such as (alphabets (a-z),digits (0-9) or
special symbols +,-,$,% etc).You know that each cell of memory contains one
character or 1 byte of data. So the capacity is defined in terms of byte or
words. Thus 64 kilobyte (KB) memory is capable of storing 64 x 1024 = 32,768
bytes. (1 kilobyte is equal to 1024 bytes). A memory size ranges from few
kilobytes in small systems to several thousands of Giga bytes in large mainframe
and super computers.
Suppose a memory card size is 2 GB means that it can store
2GB = 2 x 1024 MB
= 2 x 1024 x 1024 KB
= 2 x 1024 x 1024 x 1024 Bytes
= 2147483648 bytes
There are two kinds of computer memory: primary and secondary.
Primary Memory
Primary Memory is also known as internal memory or main memory.
As soon as the computer is switched off, the contents of the primary memory
is lost. You can store and retrieve data much faster with primary memory
compared to secondary memory. Primary memory is more expensive than
secondary memory. When the computer is doing any job, the data that have to
be processed are stored in the primary memory. This data may come from an
input device like keyboard or from a secondary storage device like a floppy
disk, memory card or pen drive etc.,.
The primary memory itself is implemented by two types of memory
technologies. The first is called Random Access Memory (RAM) and the other
is read only memory (ROM). A more appropriate name for RAM is RWM
(Read Write Memory), the CPU can write and read information from any primary
memory location implemented using RAM. The other part of primary memory
is implemented using ROM which stands for Read Only Memory.
Secondary Storage(Auxiliary Memory)
You are now clear that the operating speed of primary memory or main
memory should be as fast as possible to cope up with the CPU speed. These
high-speed storage devices are very expensive and hence the cost per bit of
storage is also very high. Again, the storage capacity of the main memory is also
very limited.
Often it is necessary to store hundreds of millions of bytes of data for the
CPU to process. Therefore, additional memory is required in all the computer
systems. This memory is called auxiliary memory or secondary storage.
In this type of memory the cost per bit of storage is low. However, the
operating speed is slower than that of the primary memory. Huge volume of
data are stored here on permanent basis and transferred to the primary storage
as and when required. Most widely used secondary storage devices are magnetic
tapes, magnetic disks and floppy disks, Compact disks and Pen drives etc.,
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