CHAPTER-5 Computer Memory and Storage Devices

CHAPTER-5 Memory and Storage Devices

Memory and Storage Devices

  • Memory

The memory of a microcomputer is where programs and data are stored when they are currently in active use. We will cover different kinds of memory your microcomputer contains:

  1. RAM
  2. Virtual Memory
  3. ROM
  4. CMOS


RAM is the main memory space of your computer. The term RAM means Random Access Memory, and it comes from the early days of computers when mainframes had two types of memory: Random access, in which any bit of memory could be addressed at any moment; and Sequential memory (such as data stored on tape) where bits could only be accessed in a certain order. All of the memory in your computer is random access, so don’t worry about sequential memory.

The RAM is the workspace of your computer. If your computer has more RAM, it can open more and larger programs and documents simultaneously. It’s like you having a large worktable to spread work papers out on instead of a tiny desk. The documents you are currently editing (and the programs your computer is using to let you do it) are stored in the RAM.

RAM consists of banks of microchip transistors that are either on or off (representing a 1 or a zero). RAM chips need constant power to remember what is stored in them; a power interruption of even a fraction of a second (perhaps caused by nearby lightning) can cause the RAM to lose its contents. For this reason, RAM is said to be volatile (from “easily evaporated”) and this is why it is important to save your work often to a more permanent storage such as a hard disk.

The specific type of RAM used by your computer could be SDRAM (synchronous dynamic RAM), or RDRAM (Rambus dynamic RAM), or DDR SDRAM (Double-data-rate SDRAM)—but don’t worry about the details. Just be sure to get the proper kind for your system when you purchase more RAM.

RAM is usually installed into sockets on the motherboard as DIMMs (Dual Inline Memory Module), small circuit boards that hold the RAM chips. You can even install more RAM into your computer, but you must take precautions not to allow static electricity to damage the RAM or motherboard. The architecture of a given motherboard will limit the amount of extra RAM you can add.

RAM size is measured in bytes, kilobytes, megabytes, etc., as discussed in the section on binary numbers. A typical computer in 2004 might have 512 MB of RAM installed.


Virtual Memory

The amount of RAM limits how large and how many programs and data files you can have open at once. You normally couldn’t simultaneously open a two programs that each require 70 MB of RAM on a computer that has only 128 MB of RAM.

However, modern operating systems use virtual memory to get around this roadblock. If everything won’t fit into RAM at once, the OS can automatically swap out currently unused data to the hard disk, and swap in whatever data are needed. But this comes at a price! Hard disks (mechanical devices) are much slower than RAM (electronic storage), so your computer will run much slower if using virtual memory. If you run into this, it’s best to just buy more RAM.


Microcomputers also have some ROM (Read Only Memory) on the motherboard. ROM does not need power to remember its contents, so this is where a computer stores the programs that are needed to start up (boot up) the computer system. (The instructions can’t be stored in RAM, since RAM loses its contents when the computer is off; and they can’t be kept on the hard disk, since just reading data from a hard disk requires programs.)

When the computer is first turned on, the program stored in the ROM is feed to the processor. This initial program checks to see that everything is in order and looks for storage devices on which it can locate a copy of the operating system; it then loads the first part of the OS into RAM, then hands control over to that program to finish the boot process.

The startup instructions stored on ROM in a PC are part of the BIOS (Basic Input Output System). The BIOS also contains the low-level interface code needed to access the drives, keyboard, and produce simple display output.

Note that what we call “ROM” is in most cases nowadays stored on an EEPROM chip (Electrically Erasable Programmable ROM). The motherboard includes special circuitry that allows the “permanent” contents of the chip to be updated if needed, but this is rarely done (EEPROM can only be re-written a limited number of times—but that limit may be 10,000 times).


The CMOS memory (called PRAM, or “Parameter RAM” on the Macintosh) can store such information such as what hard drive or copy of the OS you want to boot from, what are your default monitor settings, etc. The BIOS picks up this information and uses it during boot up. The CMOS memory can also hold the time and date so that your computer remembers this even when power its has been off. If your computer can’t remember the proper time, or can’t remember system settings when it’s off, the small battery may need to be replaced.

Note that I used the term “CMOS memory” in quotes. CMOS (pronounced ‘See-Moss’) is a type of transistor memory that requires very little power to store data, so this type of chip was used in the early days for storing semi-permanent data. However, almost ALL of the chips in your computer nowadays (such as the RAM) uses CMOS technology, and (irony) the “CMOS semi-permanent memory” in many modern PCs is NOT stored on CMOS chips, but may use flash memory (a kind of EEPROM). But the terms “CMOS memory” and “the CMOS” are still used to refer to the semi-permanent memory.

Storage Devices

Storage devices are used to store data and instructions permanently.  Storage devices are also called secondary storage devices / backing storage devices / external storage devices or auxiliary storage devices.   Examples of storage devices include Hard Disk, CD and DVD etc.

Alternatively referred to as storage, storage media, or storage medium, a storage device is any hardware capable of holding information.

There are two types of storage devices used in computers; a primary storage device, such as RAM, and a secondary storage device, like a hard drive. Secondary storage can be a removable, internal, or external storage. 

Types of computer storage

The following are some additional examples of types of storage devices used with computers.

 Magnetic storage devices

Today, magnetic storage is one of the most common types of storage used with computers today and is the technology computer hard drives use.

  1. Magnetic Tape
  2. Magnetic Disk
  1. Hard Disk
  2. Floppy Disk
  3. Zip Disk

1. Magnetic Tape

Magnetic tape is the oldest storage device. It is made of plastic coated with magnetic material. Data is stored on magnetic tape in the form of magnetic spots. Tape drive is used to read and write data on tape. Magnetic disk is sequential access device. It is slower device than magnetic disk or optical disk. Magnetic tapes are used in reels of different lengths, for example 200, 600, 1200 feet etc.

2. Magnetic Disk 

A magnetic disk is a thin circular metal or plastic disk coated with magnetic material. Data is stored in the form of magnetic spots. Magnetic disk is random access device. It is faster than magnetic tape. There are three main types of magnetic disk:
a. Hard Disk b. Floppy Disk c. Zip Disk

a. Hard Disk


  2. The hard disk is also called fixed disk. It consists of one or more metal plates. The plates are fixed in the drive. Metal plates are coated with magnetic material to store data. A motor rotates disks. There is also an access arm and read / write heads to read and write data on disks.
    Following are major characteristics of hard disk:
    – The storage capacity of hard disk is much greater than floppy disk.
    – Now a days hard disks of size 40 GB, 80 GB or more are available.
    – Hard disk is covered in a hard jacket and there are less chances of disk damage due to dust and mishandling.
    – Now days Operating system and application programs are installed on hard disk.
    – Before using a magnetic disk we use operating system to format the disk. Operating system divides the disk into circles called tracks. Each track is divided into pie-shaped areas called sectors. Data is stored in sectors. Each sector can store 512 bytes.






b. Floppy Disk


  2. Floppy disk is a small plastic plate coated with magnetic material. Data is stored in magnetic spots. Main advantage of floppy disk is that it is used to transfer data from one computer to another. Floppy disk drive is used to read and write data on floppy disk.
    Floppy disk is available in two sizes:
    – 3 ½ INCH Floppy Disk
    It is also called micro floppy disk. Its diameter is 3½ inch. It is covered in a hard plastic cover. Read/Write window is covered with a metal shutter. When disk is inserted into disk drive then shutter is automatically opened. It can be write protected by a notch. Storage capacity of micro floppy is 1.44 MB.
    – 5¼ Inch floppy disk.
    It is a 5¼ inch diameter plastic plate coated with magnetic material. Its storage capacity is 1.2 MB. It is not used in modern computers.

    c. Zip Disk

  4. Zip disk is similar to 3 ½ inch floppy disk. But it can store 100 MB or more data. Zip disk drive is used to read and write data on a zip disk.


Optical storage devices

Another common storage is optical storage, which uses lasers and lights as its method of reading and writing data.

LASER technology is used to read and write data on Optical disk. LASER stands for Light Amplification through Emission of Radiation. Laser beam writes on the surface of optical disk by creating very small holes. The presence of hole represents a One and absence of the hole represents a Zero. There are following main types of optical disk.

  1. CD-ROM disc
  2. CD-R  or WORM 
  3.  CD-RW disc
  4. DVD-ROM
  5. DVD-R, DVD+R, 
  6. DVD-RW, and DVD+RW disc
  7. Blu-Ray disc

 1. CD-ROM

CD-ROM stands for Compact Disk-Read Only Memory. The data stored on Cd-Rom can only be read. It cannot be deleted or changed. It is a portable storage device. Data can be easily transferred by CD Rom from one computer to another. Commonly it can store 650 MB / 700 MB/ 750 MB data. It is used to store large amounts of data like Software or audio/video data.We purchase Software, movies, dramas, lectures, books, wallpapers and other data on Cd-Rom.

2. CD-R (CD Recordable) Or WORM (Write Once Read Many)

CD-R stands for Compact Disk -Recordable. Initially, it is a blank disk. User can store data on it only once. This is why it is also called WORM(Write Once Read Many) disk too. Because we can write data on it only once and later we can read many times. The devices to store data on CD-R are called CD Writers or CD Burners. We can write data on CD-R only once, but can read data many times. Because of this write once read many quality, CD-R is often known as WORM(write once read many).

3. CD-RW ( CD Rewritable )
CD-RW stands for Compact Disk Rewritable. This is a special type of CD. User can write data on it, if there is an error in writing data, he may erase the contents and rewrite again. CD-Writers are used to store data on CD-RW.

4. DVD – Digital Video Disk (DVD-ROM)

 DVD stands for Digital Video Disk. A laser beam with a short wave length is used to create very very small holes on the surface of disk. So there are much more tiny holes on the surface of DVD as compared to that of CD. Therefore, storage capacity of DVD is much larger than CD. It can store up to 17 GB or more data. DVD-ROM is pre recorded and it can contain data like movies, videos and software etc. User cannot change or delete data from DVD-ROM. It is read only. 



5. DVD-R (DVD-Recordable)

DVD-R stands for Digital Video Disk Recordable. Initially it is a blank disk. User can store data on disk only once. Then it becomes read only. It is normally used to store high definition video movies. DVD-Writers are used to store data on DVD-R.

6. DVD-RW ( DVD Rewritable)

 DVD-RW stands for Digital Video Disk Rewritable. User can write data on disk many times by erasing the old data. 

7. Blu-Ray Disc

blu ray disc के लिए चित्र परिणाम

  Blu-ray DiscBD or BD-ROM, is an optical disc jointly developed by thirteen consumer electronics and PC companies such as DellHitachiHewlett PackardLGMitsubishiPanasonicSony, and TDK. The Blu-ray was first announced and introduced at the 2006 CES on January 4, 2006 and is capable of storing up to 25 GB on a single-layer disc and 50 GB on a dual-layer disc, each disc being the same size as a standard CD. The picture shows a blue laser in a Blu-ray disc player.


Flash memory devices

Solid State Backing storage

  1. These are the smallest form of memory available in the market today.
  2. Widely used as removable storage.
  3. They are more robust than other forms of storage.
  4. Though expensive than other forms they can be easily written to and updated.

Memory sticks/Pen drives

USB flash drives are typically removable and rewritable, much smaller than a floppy disk. Storage capacities typically range from 64 MB to 64 GB. USB flash drives offer potential advantages over other portable storage devices, particularly the floppy disk.

They have a more compact shape, operate faster, hold much more data, have a more durable design, and operate more reliably due to their lack of moving parts. Flash drives are widely used to transport files and backup data from computer to computer.

Flash memory cards

A memory card or flash memory card is a solid-state electronic flash memory data storage device used with digital cameras, handheld and Mobile computers, telephones, music players, video game consoles, and other electronics.

Online and cloud

Another growing method of storing data online and cloud storage, which is becoming popular as people need to access their data from more than one device.

  1. Cloud storage
  2. Network media

When saving anything on a computer, it may ask you for a storage location, which is the area in which you would like to save the information. By default, most information is saved to your computer hard drive. If you want to move the information to another computer, save it to a removable storage device such as a flash drive.

Why Secondary Storage Devices are Used? 

 Secondary storage devices are used because:

  1. Primary memory (RAM) is volatile and temporary.  When computer is switched off, all data in ram is erased.
  2. Storage devices can store large amounts of data and instructions permanently whereas Primary memory has less capacity of storing data.