Types of Memories


Types of Memories

Memories used in computers can be classified as primary memory and secondary or auxiliary memory.

1) Primary Memory:

Every computer has a certain amount of primary memory. It is in the form of semiconductor memory ICs fixed on the CPU board itself. The size of this type of memory varies from 640 KB to 64 MB for a particular computer. There is always a limit on the size of primary memory a computer can have as all the ICs are to be fixed on the CPU card which will increase the size of the motherboard.

Primary memories consist mainly of Random Access Memory (RAM) and Read Only Memory (ROM) or Erasable Programmable Read Only Memory (EPROM) in the form of semiconductor ICs, associated with CPU.


It is read & writes type of memory. It is used by the CPU as scratch pad for temporary storage of programs, data, intermediate results during computations, outputs etc. during execution of a program. When power is executed off, the contents of this type of memory are lost. The amount of RAM in a PC varies from 640 KB to 64 MB. The first 640 KB are known as basic block, next up to 1 MB is known as extended memory and remaining is called expanded memory block.


It is a read only type of memory. Information stored can only be read from it. The information in the form of programs or data is stored at the time of fabrication of the IC itself and the contents can not be changed afterword.


It is Erasable Programmable Read Only Memory in the form of semiconductor ICs. The contents are normally read only but can be changed if needed. For this, the existing contents are first erased by exposing the chip to Ultra Violet light and then the required information in the form of program instructions & data are stored into it using special EPROM programming devices. In computers, the BIOS is supplied by the manufacturer in an EPROM chip.

2) Auxiliary Memory:

The secondary or auxiliary type of memory consists mainly of magnetic media used for mass towage of information. It consists of floppy Diskettes, Hard Disks and cassette tapes.

Magnetic floppies are available in two sizes 5.25” with a storage capacity of 1.2 MB and 3.5” with storage capacity of 1.44 MB. These are read/ written by CPU through a read/ write head in the floppy Disk Drive. Hard Disks have comparatively large storage capacity. The storage capacity of Hard disks ranges from 200 MB to 4 Giga bytes. Magnetic cassettes are also used for mass storage of data and the capacity ranges form 150 MB to 2 GB. In recent days, compact disks (CD) are also used for data storage. These are optical media and driven in a CDD. A special device called CD cutter is required for writing data on to a CD.

Sr No





Floppy Diskettes




Hard Disk


1.44 MB,2.88MB


Magnetic Tape


200 MB to 2.1 GB


Compact Disk



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