Computer Storage
Computer Storage
Unit-3
The CPU needs data and instructions to operate the computer. These things must be stored in an area somewhere inside the computer. When data and programs are not in use, they must be kept permanently in another safe area. In computer, all these areas are called memory. Therefore, memory is the area in the computer, which holds data, instructions or programs.
As input unit sends data or instructions, the computer at first keeps them into the memory unit. When the computer completes processing of data, it again keeps them into memory unit. When a user launches a program, it is also loaded into and run from memory unit. Thus memory unit is a space, which is allocated for temporary storage of data and programs.
Memory is a part of computer (device) that is used to store data as well as instructions (programs). Physically, memory consists of chips either on the motherboard or on a small circuit board attached to the motherboard.
The various units used to measure computer memory are:
4 bits…………………………… 1 Nibble
8 bits…………………………… 1 Byte
1024 Bytes…………………… 1Kilobyte (KB)
1024 Kilobytes……………… 1 Megabyte (MB)
1024 Megabytes…………… 1 Gigabyte (GB)
1024 Gigabytes…………….. 1 Terabyte (TB)
There are mainly two types of memory:
Primary Memory and Secondary Memory.
Primary Memory
Primary memory is also called main memory. Primary memory allows the computer to store data for immediate manipulation and to keep track of what is currently being processed. The major limitation of this type of memory is that it is volatile. It means that when the power is turned off, the contents of the primary memory are lost forever.
Primary memory are of two types:
Random Access Memory (RAM)
Read Only Memory (ROM)
Random Access Memory (RAM)
The read and write memory of a computer is known as RAM. The users of the computer can write information into RAM and read information from RAM. The main drawback of RAM memory is that it is a volatile memory, i.e., when the power goes off, the contents of RAM gets erased. RAM is available in the form of a chip with different memory capacity ranging, from 640 Kilobytes to 256 Megabytes. Increasing RAM capacity improves system performance.
There are two types of RAM:
Static RAM
Dynamic RAM
Static RAM (SRAM)
A static RAM retains stored data and programs as long as power supply is on.
Its cost is high. It is made up of flip flops and it stores the bit as a voltage.
Speed is high.
Produce more heat.
Larger than DRAM
Dynamic RAM (DRAM)
It loses the stored information in a very short time even though the power supply is on.
Cost of DRAM is less (low price)
Made up of transistors and logic gates, and stores the bit as a charge.
Speed is lower than SRAM.
Produce less heat.
Smaller than SRAM.
Read Only Memory (ROM)
The read only memory (ROM) is a memory unit that performs the read operation only; it does not have write capabilities. The information stored in a ROM is made permanent during the hardware production and cannot be altered. ROMs are non-volatile memory, i.e. information stored in ROM is not lost even if the power supply goes off. So, ROMs are used for storing the programs to boot the computer handling the operating system and monitor program controlling a machine. They are slower than RAM. Actually, the ROM is built and assembled in the motherboard.
There are various types of ROM:
a. PROM (Programmable Read Only Memory)
b. EPROM (Erasable Programmable Read Only Memory)
c. EEPROM (Electrically Erasable Programmable Read Only Memory)
a. PROM (Programmable ROM)
A PROM is a memory chip on which data can be written only once. Once a program has been written onto a PROM, it remains there forever and cannot be changed. They are manufactured as blank memory. To write data onto a PROM chip, you need a special device called a PROM programmer or PROM burner. The process of programming a PROM is sometimes called burning the PROM.
b. EPROM (Erasable Programmable ROM)
EPROM is a special type of memory that retains its contents until it is exposed to ultraviolet light for 10 to 20 minutes. The ultra-violet light clears its contents, making it possible to reprogram the memory. For erasing purpose, the EPROM chip has to be removed from computer.
c. EEPROM (Electrically Erasable Programmable ROM)
It is a special type of PROM that can be erased by exposing it to an electrical charge. The time required to erase this type of PROM is very short (few seconds). Unlike EPROM chips, EEPROMS do not need to be removed from the computer to be modified.
Secondary Memory:
Secondary memory is also called secondary storage, auxiliary storage, mass storage or, simply storage. It holds large volume of data, programs, information and games for future uses. Secondary memory is non-volatile. It can keep data, programs, and information permanently even when electrical power is removed from the computer.
The secondary memory is used to store data, information and programs permanently. So, they are often referred as “storage memory”. The capacity of secondary memory is larger than the main memory. The two main categories of storage technology used today are magnetic storage and optical storage. The common secondary or auxiliary memories used in computer are floppy disks, hard disks and compact disk.
There are three types of secondary memory:
1.Magnetic Storage Devices
2. Optical Storage Devices
3. Solid-State Devices
1. Magnetic Storage Devices
Computer systems need to store data in digital format. One of the most widely used types of digital data storage is magnetic storage. This refers to any type of data storage using a magnetized medium. Digital data consists of binary information, which is data in the form of zero and ones. There are two types of magnetic polarities, each one used to represent either zero or one.
Several types of magnetized media are used in computer systems, including magnetic tape, floppy disks and hard disk drives. The basic approach to magnetic data storage, however, is very similar for the different types of media. A read-write head moves very close to the magnetic surface - the distance is often no more than tens of nanometers. The head is able to detect and modify the magnetization of the material. The magnetic surface is divided into very small regions, each of which has a mostly uniform magnetization. As the head moves relative to the surface, the changes in magnetization from region to region are detected and recorded as zeros and ones. Different technologies vary in how the head moves relative to the surface of the media and how the regions on the media are organized, but the basic principle is the same.
Types of Magnetic Storage Devices
a. Magnetic Tape Storage
Magnetic tape is one of the older types of magnetic storage media. The magnetic tape recorder was invented in 1928 and was primarily used for analog audio recordings. Before music CDs were introduced in the 1980s, portable music devices used magnetic tape in the form of music cassettes. Early computers adapted this technology to store digital information. One of the major weaknesses is that information on a tape can only be accessed in a very sequential fashion. This is fine if you want to listen to a whole music album in sequence, but computer systems typically need to access data in a non-sequential manner. For magnetic tape, this means you may need to fast forward through a lot of tape to get to a specific piece of data. While magnetic tape is a very cheap way to store data, the very slow access to the data meant that it was primarily used for creating backups of data in case older forms of storage failed. Tape backup systems are still in use today, but their importance has greatly declined with the advance of cheap, large capacity hard-disk drives.
b. Hard Disk
Hard disk is a secondary storage medium that stores large amount of data, is made from metal. It holds more data and is faster than floppy disk (capacity- 120 GB, 500 GB. and speed – 3600 rpm(Revolutions per minute) to 7200 rpm).
A single hard disk may consist of single or multiple platters (disks). Each platter requires two read/write heads, one for each side. All the heads are attached to a single access arm (comb like structure) so that they cannot move independently. Each platter has the same number of tracks and a track location that cuts across all platters is called a cylinder. The motor rotates the platters on the central spindle. The read/write head along with arm does not physically touch the disk surface; it floats above the surface of the disk and detects the data.
If there are 100 tracks per surface in a disk having 8 surfaces, then the number of cylinders is equal to 100 and each cylinder will have 8 tracks. If a certain amount of data is to be stored on the hard disk, the recording always starts from the first sector of the first track of the first cylinder. When the system has filled the track with data, it moves to the next surface on the same track i.e. from surface 1 to surface 2 at the same arm position. Thus no time is wasted because switching from one head to another is done electronically and no seek time is involved.
c. Floppy Disk
They are soft magnetic disks made up of flexible Mylar or a plastic, coated with magnetic substances. A diskette is usually encased in a hard plastic shell with a sliding shutter. A diskette can record data as magnetized spots on tracks on its surface. It is also known as floppies or diskettes. A disk drive for a floppy is called Diskette drive or floppy disk drives.
It is called floppy because it can be blend and “flops” if you wave it. Diskettes are used to transfer files between computers, as a means for distributing software, and as a backup medium. Diskettes come in two sizes: 5.25-inch and 3.5-inch.
Capacity of Floppy Disk
Capacity = No. of tracks per surface x No. of sectors per track x No. of bytes per sector x No. of sides of a disk
For Example:
3.5” floppy disk has following capacity
=80 x 18 x 512 x 2
= 1474560 Bytes
= 1440 KB
= 1.4065 MB (i.e. 1.44 MB)
d. Zip Disk
A Zip disk was an advanced version of the floppy disk developed by Iomega. The disk needed a special drive called the Zip drive in order to be used. Zip disks were available in 100- and 250-MB capacities and were used to store, share and back up large amounts of data, which was not possible with ordinary floppy disks. With the introduction of new and better storage mediums such as memory sticks and DVDRWs, along with higher capacity hard disks, the Zip disk became less favored and eventually disappeared from the market.
2. Optical Storage
Devices Optical disc is a secondary storage medium from which data is read and to which it is written by lasers. These storage devices works on a principle similar to magnetic storage devices, however, they use laser technology to read and write data. A laser beam is projected on the reflecting surface to read data from the disc. By detecting the light intensity reflected from the surface, the information stored on the disc. By detecting the light intensity reflected from the surface, the information stored on the disc can be accessed.
Data is laid out on a CD-ROM disk in a long, continuous spiral that starts at the outer edge and winds inward to the centre. Data is stored in the form of lands, which are flat areas on the metal surface, and pits, which are depressions or hollows. A land reflects the laser light into the sensor indicating 1 and a pit scatters the light indicating 0.
There are six basic types of optical discs.
- CD-ROM
- CD-R
- CD-RW
- DVD-ROM
- DVD-R
- DVD-RW
a. CD-ROM (Compact Disc - Read Only Memory)
CD-ROM is a type of optical disc capable of storing large amount of data. A single CDROM has the storage capacity of 600 floppy disks. It contains pre-recorded data that can be read only (i.e., cannot be erased and modified/changed). A laser beam is used to write into and read data from CDROM.
CD-ROM discs are plastic discs coated with Aluminum on the surface. A layer of thin transparent plastic is further deposited on the disc. Information on a CD-ROM is written as a single continuous spiral, unlike magnetic discs with discrete cylinders and tracks.
The CD-ROMs take longer time to read data as compared to hard disks (slower than hard disk). These discs are specially used for software and multimedia system (to store data, sound and pictures). A CD-ROM is prepared by using a high power laser to burn 1-micron (10 –6 of a meter) holes in a master disc.
How a CD-ROM drive reads data from the surface of a compact disc?
Data is stored in the form of lands, which are flat areas on the metal surface, and pits, which are depressions or hollows. A land reflects the laser light into the sensor indicating a data bit of 1 and a pit scatters the light indicating a data bit of 0.
To store data, the disk's metal surface is covered with tiny dents (pits) and flat spots (lands), which cause light to be reflected differently. When an optical drive shines light into a pit, the light cannot be reflected back. This represents a bit value of 0 (off). A land reflects light back to its source, representing a bit value of 1 (on).
b. CD-R (Compact Disc - Recordable)
It is also known as WORM (Write Once Read Many). With a WORM disc drive, you can write data onto a WORM disc, but only once. The writing process is normally slower than the reading process. After writing onto a WORM disc, it behaves just like a CDROM.
c. CD-RW (Compact Disc- Re-Writable)
CD-R/W disc is a new type of CD disc that enables the users to read and write data many times. With CD-R/W drives and discs, you can treat the optical disc just like magnetic disk, writing data onto it again and again.
d. DVD-ROM (Digital Versatile Disc- Read Only Memory)
DVD-ROM is also called as Digital Video Disc – Read Only Memory. DVD-ROM is a highdensity medium capable of storing a full-length movie on a single disc. It uses special data compression technology to store data on a disc.
e. DVD-R (Digital Versatile Disc- Recordable)
DVD-R lets us record data into a special recordable digital video disc, using DVD-RW drive. Once data are written, they can’t be erased and re-written. f. DVD-RW (Digital Versatile Disc- Re-Writable) DVD-RW lets us record, erase, and re-record data into a special recordable digital video disc, using DVD-RW drive. They can read and write all type of CDs and DVDs.
3. Solid-State Storage Devices
Solid-state storage devices are unique among today’s storage devices because they do not use disks or tapes and have no moving parts. They are neither magnetic nor optical devices. Instead, it relies on ICs to hold data. Some solid-state devices are non-volatile, while others are volatile in nature. Solid-state storage devices store data on memory circuits rather than disks or tapes. They store data electronically, not in a magnetic or optical form. Examples of such devices include flash memory, smart card, and solid-state disks.
a. Flash Memory
A flash memory is a special type of memory that combines special features of RAM and ROM. Like RAM, flash memory lets a user or program access data randomly and allows us to write and read data any no. of times. Like ROM, flash memory is nonvolatile, so data is retained even when power is off. Flash memory is commonly used in digital cameras, mobile phones, and multimedia players such as MP3 player, MP4 player, etc.
b. Smart Cards
A smart card is a device which contains a small chip that stores data. Using a special type of device, called a smart card reader, the user can read data from the card, add new data, or revise existing data. c. Solid-state Disks A solid-state disk (SSD) is not a disk at all. Rather, this device uses very fast memory chips, such as synchronous dynamic RAM (SDRAM), to store data.
Large-scale SSD systems can store a terabyte or more of the data. SSDs are used primarily for enterprise-level network storage, to make data available to a large number of users at one time.
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