Chapter Primary memory


Memory unit of a computer holds (store) all data, instructions and processed data.Memory unit of computer is classified in following two categories:
1. Primary Memory (we study it in this chapter)
2. Secondary Memory (we will study in the next chapter)

Primary memory or main memory is known as computer’s own memory or internal memory, bacause CPU can directly use it by giving appropriate memory location.Different types of main memory are available inside the computer unit such as RAM, ROM Cache etc. We mainly concentrated on RAM and ROM.


The memory of a computer is measured in bytes. As there are house numbers in cities to locate particular house, in the same way each data has its address in the memory. This address is called memory address or location address of a data.
The primary memory can be divided in two parts :
1. RAM (Random Access Memory)
2. ROM (Read Only Memory)

RAM (Random Access Memory)

You can store data and instruction in any location of the memory at random. Therefore this memory is called Random Access Memory. You can retrieve data from these memory locations. You can read and write to this memory. It is also called read/write memory. But there is a drawback of this memory.That is, when the power is switched off, all the contents of Random Access Memory get erased i.e. it is volatile in nature.

ROM (Read Only Memory)

This memory allow user to read it only.It is mainly used during the starting the computer. A small program is made permanent in the ROM chip to operate a particular hardware in the same way. This memory is non-volatile in nature. When power goes off, the information stored in a ROM is not lost.

There are many types of ROM as given below:

PROM (Programmable Read Only Memory)

A PROM is a ROM that can be programmed to record information using a special facility. This special facility is known as PROM Programmes. You cannot alter these programmes once programmed.

EPSOM (Erasable Programmable Read Only Memory)

The EPROM is another type of ROM that can be erased and can be reprogrammed to record information in other way. Ultraviolet ray is used to erase the existing programs from EPROM.


Memory is measured in bit. Bit is the smallest unit of measuring memory. It is denoted by 0 and 1.

Units of data

1 Byte = 8 bits
1 Kilobyte (KB)= 1024 Bytes
1 Megabytes (MB)= 1024 Kilobytes
1 Gegabytes (GB)= 1024 Megabytes

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