There are primarily two types of computer memory. One is main or primary or internal memory and the other is secondary or auxiliary memory. Internal memory is the memory used by the computer's processor to run its applications, programs and operating system. this internal memory is often referred to as RAM, which stands for Random Access Memory.

The other form of memory is the auxiliary memory which is where the computer will store information that is user generated. Examples of auxiliary memory are your hard disk, your CD RAM's and your DVD's.

Primary Memory

Primary memory is the memory that communicates directly with the CPU. Primary memory is made up of silicon chips. Primary memory has following two types:

(a) RAM

It stands for Random Access Memory. This memory is accessed randomly, that is any byte of memory can be accessed without touching the previous sequence of bytes. RAM is the most common type of memory found in computers and other devices, such as printers. This memory generally loses its contents when you switch off the computer. This is why RAM is also known as volatile memory.

(b) ROM

It stands for Read Only Memory. ROM is designed to hold permanently stored programs installed by the computer manufacturer. ROM stores the start-up routine for the booting process and the diagnostic programs that check the computer hardware. The CPU can only read the instruction from ROM. It cannot write, erase or change the contents of ROM. is non-volatile, meaning that when the computer is turned off, none of the information stored on the ROM is lost.

Secondary Memory

When you want to keep data or information permanently, you need to transfer the data from primary memory to a magnetic disk or optical disk. Since primary memory is made up of silicon chips that is why it is expensive than secondary memory and has less capacity than secondary storage. The device used to read (or write) data from the disks is called a disk drive. The main functions of the secondary storage device are storage and retrieval. there are three common types of secondary storage devices - floppy disk, hard disk and CD - ROM.

(a) Floppy Disks


Floppy disk is a soft magnetic disk. It is called floppy. Floppy disks are portable because they can easily be carried from one place to another. Disk drives for floppy disks are called floppy drive. Floppy disks are slower to access than hard disks and have less storage capacity.

Floppy disks are obsolete now.

(b)Hard Disks


Hard disk is a magnetic disk on which you can store computer data. Hard disks hold more data and are faster than floppy disks. Now-a-days, hard disks capacity of 320 GB and 500 GB are very common.


A hard disk usually consists of several hard platters. Each platter requires two read/write head, one for each side. All the read/write heads are attached to single access arm so that they cannot move independently. Each platter has the same number of tracks, and a track location that cuts across all platter is called a cylinder.

In general, hard disks are less portable than floppies, although removable hard disks are also available in the market.

(c) Optical Disks



An optical disc drive (ODD) is a disk drive that uses laser light or electromagnetic waves near the light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can only read from discs, but recent drives are commonly both readers and recorders. Recorders are sometimes called burners or writers. Compact disc (CD), and Digital Versatile Disks (DVD) are common types of optical media which can be read and recorded by such drives.