Memory is basically a storage place. In a computer the memory is used for the storage and subsequent retrieval of data and instructions.
The memory in computer is of two types:
1) INTERNAL / PRIMARY / MAIN MEMORY
2) EXTERNAL / SECONDARY / MASS MEMORY
How Information is Stored in Memory ?
Information is stored electronically in a computer in the form of binary digits (Ones or Zeros) called bits. 8 bits is called a byte and has 256 possible combinations of Ones and Zeros. A Kilobyte stores 1000 characters. A Megabyte stores one Million characters and A Gigabyte can store one Billion characters.
Tiny transistors, circuits that act as switches within the computer's circuitry can turned on of off to represent a One or Zero in a binary digit and thereby stores the data.
Sometimes memory be expressed in word length. In PC world, for 80286 processor the word = 2bytes = 16bits and for 80386 and 80486 processor the word = 4bytes =32bits.
For external memory data are often transferred in much large units than a word (8, 16, 32bits) and these are referred as a block.
|Three Structure for Memory|
1) PRIMARY MEMORY :
This memory is built inside the chips. It stores data for sometime. It is volatile in nature, if the computer is switched off or any problem in computer the data should erased. The data or program may be kept in off-line secondary storage and bring into the main memory when needed. This memory can be accessed much faster than the secondary memory. Main memory is formed by a large number of basic units referred to as "Memory Cells" and each cell has an unique address. Each cell or units or circuit is capable of storing a binary digit. Main memory is of two types.
a) ROM (Read Only Memory)
b) RAM (Random Access Memory)
ROM (Read Only Memory)
The ROM chips are pre-programmed information is 'burnt' into ROM chip at manufacturing time. In this memory we can read only the information. We can not write into the chip, ROM is basically non-volatile, i.e. the contents of ROM are not lost where power goes off suddenly. The ROM chips contain instructions for controlling the operation of a computer. These generally are much slower than a RAM.
There are four variations of ROM chips:
i) The pure ROM : It is programmed by the manufacturer.
ii) PROM : This is a programmable ROM that can be directly programmed by using a facility known as a PROM programmer.
iii) EPROM : This is Erasable Programmable Read Only Memory. The contents of this chips can be erased by exposing the chip to ultraviolet light.
iv) EEROM : A newly developed type of memory can be programmed and erased by electrical signals. It is know as Electrically Erasable Read Only Memory. It does not need exposure to ultraviolet light to erase it's contents. EEROM is similar to EPROM, information loaded in this memory can be retained for many years without any power supplied. Thus when power comes the microcomputer can continue working just as nothing had happened.
RAM (Random Access Memory)
This is actually primary memory. The term Random Access Memory means that any word in the memory may be accessed without having to go through the all other words to get into. It also as Read / Write memory. RAM is volatile i.e. it's contents are lost as soon as the power supply is switched off. RAM is power dependent. Data goes to the main processor after being loaded in the RAM. There are two types of RAM 1) DRAM - Dynamic RAM and 2) SRAM - Static RAM.
UNITS OF MEMORY :
Bit : The computer works on the basis of making "ON or OFF" of electronic pulses passing of a pulse represents '1' and blocking of a pulse represents '0'. Bit is the smaller unit of information any one of the binary digits "0 or 1". It is a short term for the word binary digit. A group of 4 bits is called Nibble.
Byte : Byte is made up of 8 bits. A group of bits representing data or instructions that form the basic information unit of computer is called a word. A word may be formed using two or more bytes, the most common being the four bytes combination.
1 Nibble = 4 Bits
1 Byte = 8 Bits
1 KB ( Kilobyte) = 1024 Bytes
1 MB (Megabyte) = 1024 Kilobyte
1 GB (Gigabyte) = 1024 Megabyte