CBSE Notes for Class 9 Foundation of Information Technology – Concept of Memory
Computers are capable of storing data and instructions and retrieving them. Data is stored in the computer’s memory which consists of many locations. These locations have unique addresses. The capacity of each location to store data is same. Computer understands the language of 0’s and l’s, i.e., binary language. Each character fed to the computer is converted into a string of binary numbers (0’s and l’s). Each of these binary numbers is called a bit, a string of 4 bits is called nibble and a string of 8 bits is called a byte. Memory is usually measured in KB (Kilobytes), MB (Megabytes) or GB (Gigabytes). Nowadays another unit Terabytes is also being used.
Suppose there are n memory locations and each location can store k bits then the word length will be k, i.e., number of bits that can be stored in one memory word.
In a computer system, we have two types of memories — primary memory and secondary memory.
PRIMARY MEMORY
It can be divided into two parts — RAM and ROM.
1. RAM—Random Access Memory: As the name suggests, any data from any location in RAM can be accessed at any time. The time taken to access any location is the same. RAM is read/write memory. We can read data from RAM and we can write data on RAM. Read process is non-destructive i.e., if we read data from a location, it still remains in the location. Write process is destructive i.e., if we write data in a memory location which already has some data stored in it, the previous data disappears and is replaced by the new data. RAM is volatile. Whenever the power is switched off, anything and everything stored in RAM vanishes. This is the reason why we call RAM a temporary memory. While buying a computer, the capacity of its RAM is important to be kept in mind. Nowadays personal computers upto 16 GB RAM are available.
2. ROM—Read Only Memory: As the name suggests, this memory allows only read operations and does not let us write anything on it. ROM is used to store low level machine functions — Microprograms, which increase the efficiency of processor. Unlike RAM, ROM retains its stored data even if the power goes off so it is non-volatile memory. ROM chip is supplied alongwith the computer system. It cannot be changed by the user. There are various types of ROMs which are available nowadays such as:
(i) PROM — Programmable Read Only Memory: This type of ROM chip can be programmed according to the user by using a specialized software. Once it has been programmed, it cannot be altered further.
(ii) EPROM — Erasable Programmable Read Only Memory: This type of ROM can be erased and re-programmed again and again by using specialized software. The process of erasing is done either by using ultraviolet light (UPROM — Ultraviolet PROM) or electricity (EAPROM — Electrically Alterable PROM).
(iii) EEPROM — Electrically Erasable PROM: This type of ROM can be re-programmed with special electric pulses.
SECONDARY MEMORY DEVICES
Since the storage capacity of primary memory is limited and primary memory cannot store the data permanently, so we use various secondary storage devices. In secondary memory devices, the data storage capacity is huge but the retrieval time is more than that of primary memory devices.
Some of the secondary storage devices are explained below:
1. Magnetic Disks: These are generally made of flexible plastic. Magnetic disks are round and coated with a magnetizable recording material. Each disk has a number of concentric circles called tracks. The tracks on the disk surface are divided into sectors. Generally there are 8 sectors per track and each sector can hold upto 512 bytes. If you look at the Fig. 3.2 of the disk carefully you will notice that the innermost sector is smaller
than the outermost sector. But the fact is that the storage capacity of all the sectors is the same. A disk pack has several disks stacked one over the other e.g., in hard disk or Winchester disk. Tracks with same number on all the disk surfaces form a cylinder.
The disk is rotated by a motor at a very high speed. Data is written as tiny magnetic spots on the disk surface. Generally data is written on both the surfaces of the disk. The storage capacity of a disk depends upon the number of bits per track on a surface and the number of Tracks Per Inch (TPI) on the surface.
The different types of magnetic disks are:
(i) Winchester Disk (Hard Disk): This is a set of 14 inches metal disks. The set is sealed permanently in a container alongwith the read/write mechanism.
(ii) Floppy Disk Floppy disk is a single disk and comes in various sizes like 5¼ inches and 3½ inches. A 3½ inches floppy disk can store 1.44 MB of data.
The floppy disk appears to be square in shape but that is actually the shape of the protective plastic jacket inside which the diskette is enclosed. A floppy disk is used to store as well as retrieve data. On the plastic jacket is a write-protect notch which can be adjusted so that the floppy gets write-protected i.e., no data can be written on the floppy but can only be read. Floppy disks are no longer used as better storage devices are now available.
(iii) CD-ROM (Compact Disk Read Only Memory): On a recordable CD (CD-R) or rewritable CD (CD-RW), the data is recorded using a laser beam which changes the properties of a dye and the process is called burning.
CD-ROMS are available with different data storage capacities like 700 MB, 900 MB etc. CD-ROMS are read using CD-ROM drives.
Disks are made from 1.2 mm thick disk of polycarbonate plastic with a thin layer of aluminium to make a reflective surface. The common size is 120 mm in diameter. Data is stored in the form of pits and lands. A laser beam is used to read the disk. The beam is reflected with varied intensity depending on the pattern of pits and lands and is converted into binary data.
(iv) DVD (Digitial Video Disk): DVDs are mainly used to store video and data. DVDs look exactly as CDs but their storage capacity is more than six times as compared to CDs. A CD uses 780 nm wavelength of laser diode light whereas a DVD uses 650 nm wavelength. This is the reason behind DVD’s enormous storage capacity. A DVD storing data on both surfaces has a capacity of approximately 8 GB per side per disk.
(v) Blu Ray: As a successor to DVDs, Toshiba introduced HD DVD which was soon superseded by Blu Ray disk by Sony. It is an optical storage medium having same dimensions as DVDs but enormous storage capacity of about 50 GB. As a blue laser beam of wavelength 450 nm is used to read the disk thus the disk gets its name as Blu Ray.
2. Pen/Flash Drive: Pen/Flash drive is a compact storage device which can hold data upto 125 GB and is upto 4 inches in length.
It is lightweight (28 g) and easy to carry. It is integrated with a USB (Universal serial bus) interface. This storage device has become so common that nowadays computers do not have floppy disk drives but have USB ports for pen drives.
Advantages of Pen/Flash Drive
- Scratch-proof and dust-proof.
- Easy to carry.
- Needs USB interface and most computers as well as other electronic devices like television, music system, cameras have USB ports.
- More storage capacity.
- Uses little power and does not have any fragile parts.
- All operating system support it.
Disadvantages of Pen/Flash Drive
- Can sustain only a limited number of write and erase cycles.
- Does not include write protect mechanism.
- Due to small size gets easily misplaced.
3. Memory Stick/Card: It is a removable flash memory card format.
It is generally seen in cellphones, digital cameras and portable playstations. It is used to store data and also transfer it to the computer if required. Memory cards of different capacities upto 16 GB are available in the market.
