Introduction:-
                        We have seen the basic hardware of the computer which consists of Central Processing Unit (CPU), the input and output devices and the backing storage devices. We here shall discuss about main storage.
Main storage:- 
                           Devices used for main storage have been improving steadily over the years drastically changing the capacity, size and application area of the computer. The earlier first generation electronic computer used vaccum tubes for memory. After 1960 cores of magntisable material were used in the shape of tiny rings, popularly known as magnetic core memory. This memory was also called as non-volatile memory.
                The development of semi-conductor technology has outdated earlier techniques as new devices have enormous advantages being much smaller, cheaper and faster though they are volatile.
Type of semi-conductor memory:-
                                                             Semi-Conductor memory devices are tiny integrated circuits (IC's) made on silicon wafers and often called as "chips". Generally they are made of using metal oxide semiconductor technology (MOS). Each memory chip contains several hundred storage cells arranged in rows and columns alongwith the control circuiting for addressing any particular cell and reading or writing into it. 
The various semi-conductor storage types are:-
1. Random Access Memory (RAM):- Most of the main memory is made up of chips called as RAM. It is responsible for both read from and write to location within RAM and any location may be accesed at will by specifying its address.
               This is the main read/write memory of the present day computer.
2. Read Only Memory (ROM):- Read only memory chips contain data which can be read randomly as required but cannot be written into data which is hard-wired into them at the time of manufacture. They hold certain essential programs and mocro programs basically to cause the computer to initialise peripherals and take control of them. In addition micro programs are provided as library routines which perform low level machine functions such as subtraction.
3. Programmable Read Only Memory (PROM):- To enter a frequengly required program on to ROM, a user may obtain a Programmable Read Only Memory (PROM). Each bit of this can be individually programmed to a 1 or 0by burning out a fusible link with the selected cells. A fused link cannot be stored.
4. Eraseable Programmable Read Only Memory (EPROM):- The EPROM makes it possible for the user to repeatedly erase and reprogram ROM. Erasing is done by exposing it to Ultra-Violet light of a specific frequency.
5. Electrically Eraseable Programmable Read Only Memory (EEPROM):- This memory comes closer to the ideal of a permanent memory which may be programmed in situ.
6. Cache Memory:- To decrease the processing time, large computers use costlier and high speed memory devices to from a buffer or cache. This is used by certain process as work area, temporarily holding intermediate data and instructions. Restricted use of such memory can increase processing speed without drastically increasing costs. This is used in microprocessor chip around which micro computers are built.

Virtual memory:-
                               With limited memory sizes available in earlier computers larger programs could not be run as they could not fit into available primary memory. To solve this problem, software operating system, which help run computers stored using a technique known as "Virtul storage". The program is divided by the software into pages or segments. At any given time, only the page actually required alongwith 1 or 2 before and after it are read into memory from the backing storage. A computer can run a program which ix larger than its available memory size and therefore, to the programmet, seems to have a memory of practically unlimited size called as "Virtual Memory".

Backing storage:-
                                Like most parts of a computer, the magnetic recording is often taken for granted today for they have not come up overnight but have evolved steadily through trial and error method. Use of this media stored with magnetic drum (cylinders coated with magnetic recording material) and eventually became successful in the shape of magnetic tapes and discs.
          The major difference between disks and tapes is that tape is a direct access medium while the other provides several access. Any one who has used an audio tape and a gramophone record  for playing back recorded songs is aware of the different approaches to access. If one wishes to listen to the fifth song on a tape, one has to run the tape forward until it reaches the required point, passing through the first four songs without processing. With the two dimensional surface of a disk, however, it is possible to instantly move to the required track without serially going through all previous records. A similar principle works in the magnetic disk  used for storage of computer data.
SEQUENTIAL AND DIRECT ACCESS DEVICES:-
1. RAM Disk:- Ram disk or Virtual Disk storage facility is offered by some versions of operating software, mainly with PC's. A section of the main memory is treated as a disk, with data being organised in files and the same commands being used to access or control it as for backing storage devices requiring Input/Output. Instead of writing to backing storage, however the data
a is written into RAM chip in main memory. Data in memory will be lost, if power is switched off. The files are recopied on a backing storage before switching off.
Magnetic bubble memory:- These are semiconductor chips which may be used for backing storage as they do not loose data when the power is switched off. Bubble chips have not gained wide usage as their costs are still high as compared to disks.
MAGNETIC TAPE
                      Although it is a serial medium, it is popular because of the larger volumes of data which may be stored at relatively much lower cost.
Types and characteristics:-
                                                Standard computer tape contain magnetic material located on a half inch (12.7mm) wide plastic tape. The bits in each character of data are stored alongwith the width of the tape, with an entire character (7 to 9bits) being written at one time. A standard tape may store between 800-6250 characters per inch. Rate of transfer of characters between tape and CPU storage are in the range of 20,000 to 2,00,000 characters per second. A tape cartridge simplifies the task of loading the tape and provides a more rugged system. Both the take up and supply reel are packed together in one of these eliminating the need to manually handle and thread the tape.