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Memory and Storage Devices - Notes | Study Cyber Olympiad for Class 9 - Class 9

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Storage Devices
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Storage Devices

Storage Devices are the data storage devices that are used to store data, instructions, and information for future use in computers. A computer has many types of data storage devices. Some of them can be classified as removable data storage devices and others as non-removable data storage devices.
Classification of Storage according to volatility

According to volatility, memory can be classified into two categories: 

  • Non-volatile Memory: It will retain the stored information even if it is not constantly supplied with electric power. It is suitable for the long-term storage of information. 
  • Volatile Memory: It requires constant power to maintain the stored information. Since primary storage is required to be very fast, it predominantly uses volatile memory.

Classification of storage according to accessibility

According to accessibility, memory can be classified into two categories: 

  • Random access: Any location in storage can be accessed at any moment in approximately the same amount of time. Such characteristic is well suited for primary and secondary storage. Most semiconductor memories and disk drives provide random access. 
  • Sequential access: The access of pieces of information will be in serial order, one after the other; therefore the time to access a particular piece of information depends upon which piece of information was last accessed. Such characteristic is typical of off-line storage.

Classification of storage according to the place of memory

According to the place of memory, memory is classified into two categories:
The primary and secondary memory unit is the amount of data that can be stored in the storage unit. its capacity is expressed in terms of bytes. 

  • Primary Memory: A memory, also known as primary storage holds data and program instructions for processing the data. It also holds the processed information before it is given as output. Memory is sometimes referred to as temporary storage because its contents will be lost if the electrical power to the computer is disrupted. 
  • Secondary Memory: Unlike primary memory, secondary storage devices hold data and programs even after electrical power to the computer system has been switched off. The most important kinds of secondary media are floppy disk hard disks and optical disks.

Accessing Stored Data

We can access  our data stored in memory in the following two ways:

  • Serial/Sequential Access: A serial (sequential) access storage device stores files one by one in a sequence. Thes systems that store things on tape (video, music, computer data, etc.) are always in serial access. 
  • Direct/Random Access: A direct (or ‘random’) access storage device is one that stores files so that they can instantly be accessed, there is no need to search through other files to get to the one you want. An example of a direct access device would be a DVD movie, you can jump to any scene on a DVD. All parts of the DVD are directly accessible. This type of file storage is called direct access.

Criteria of storage device

The storage units of a computer system are ranked according to the following criteria:
(i) Access time: This is the time required to locate and retrieve stored data from the storage unit in response to program instructions. A fast access time is always preferred.
(ii) Storage capacity: It is the amount of data that can be stored in the storage unit. A  memory with a large capacity is preferred because it provides more space for storage.
Some storage media can only store a very limited amount of data, whilst others can store vast amounts. Data storage capacity is measured in bytes (B): A thousand bytes is known as a kilobyte (kB) i.e. 1,000 B = 1 kB A million bytes is known as a megabyte (MB) i.e. 1,000,000 B = 1 MB A thousand million bytes is called a gigabyte (GB) 1,000,000,000 B = 1 GB A million million bytes is called a terabyte (TB) 1,000,000,000,000 B = 1 TB
(iii) Cost per bit of storage: It shows the cost of storage per bit. The final goal is to minimize this cost.

Few higher storage units are following

Memory and Storage Devices - Notes | Study Cyber Olympiad for Class 9 - Class 9

Primary Memory
Computer memories are internal storage areas in the computer used to either temporarily or permanently store data or instructions to be processed.
Types of primary memories are:
Classification of Primary MemoryClassification of Primary Memory

Random Access Memory (RAM)

It allows the computer to store data for immediate manipulation and to keep track of what is currently being processed. It is the place in a computer where the operating system, application programs, and data in current use are kept so that they can be accessed quickly by the computer’s processor. RAM is much faster to read from and write to than other kinds of storage in a computer (like hard disk or floppy disk). However, the data in RAM stays there only as long as the computer is running. When the computer is turned off, RAM loses all its contents. Hence a backup uninterruptible power system in often used with computers. When the computer is turned on again, the operating system and other files are once again loaded into RAM. When an application program is started, the computer loads it into RAM and does all the processing there. This allows the computer to run the application faster. Any new information that is created is kept in RAM and since RAM is volatile in nature, one needs to continuously save the new information to the hard disk.
There are two types of random access memory, which are as follows:
(i) Static RAM (SRAM): This RAM retains the data as long as power is provided to the memory chip i.e. RAM is volatile memory. It needs not to be refreshed periodically. It is very fast but much more expensive than DRAM. SRAM is often used as cache memory due to its high speed.SRAM Clip uses a matrix of 6-transistors since it uses transistors, it need not be refreshed. SRAM has longer life.
(ii) Dynamic RAM (DRAM): This type of RAM.  needs to be refreshed continuously to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small. DRAMs are made up of memory cells that use one capacitor and one transistor.

Read-Only Memory (ROM)

Just like a human being needs instructions from the brain to perform actions in certain events a computer also needs special instructions every time it is started. This is required because during the start-up operation, the main memory of the computer is empty due to its volatile maturity, so some instructions need to be (special boot programs) stored in a special chip that enables the computer system to perform start-up operations and transfer the control to the operating system. This special chip, where the start-up instructions are stored, is called read-only memory (ROM). It is non-volatile in nature, that is, its contents are not lost when the power is switched off. The data and instructions stored in ROM can only be read and used, but cannot be altered thereby making ROM much safer and more secure than RAM. ROM chips are used not only in the computer but also in other electronic items like washing machines and microwave ovens.
Different types of ROMs are as follows:

  • Masked ROM: The earliest ROMs, known as masked ROMs, were hard-wired devices that contained a pre-programmed set of data or instructions. The contents of such ROMs had to be specified before chip production so that the actual data could be used to arrange the transistors inside the chip.
  • Programmable ROM (PROM): Creating a ROM chip from a scratch is a time-consuming and expensive process. For this reason, developers created a type of ROM known as programmable read-only memory (PROM), which can be programmed. Blank PROM chips can be bought economically and coded by the users with the help of a special device known as a PROM programmer. However, once a PROM has been programmed, its contents can never be changed. As a result, PROM is also known as a one-time programmable (OTP) device.
  • Erasable ProgrammablE ROM (EPROM): An EPROM is programmed in exactly the same manner as a PROM. However, unlike PROMs, an EPROM can be erased and reprogrammed repeatedly. It can be erased simply by exposing it to a strong source of ultraviolet (UV) light for up to 40 minutes. An EPROM eraser is not selective. It will erase the entire EPROM. EPROM is however more expensive than PROM.
  • Electrically Erasable Programmable ROM (EEPROM): This type of ROM can be erased by an electrical charge and then be written to by using slightly higher-than-normal voltage. Both erasing and programming take about 4 to 10 milliseconds EEPROM can be erased one byte at a time, rather than erasing the entire chip with UV light. Hence, the process of re-programming is flexible but slow. Because these chips can be changed without opening a casing, they are often used to store programmable instructions in devices like printers.
  • Flash ROM: Flash ROM, also called flash BIOS or flash memory, is a type of constantly powered nonvolatile memory that can be erased and reprogrammed in blocks. It is a variation of EEPROM, which, unlike flash memory, is erased and rewritten at the byte level. Flash memory is often used to hold the control code such as the BIOS in a personal computer. When BIOS needs to be changed or rewritten, the flash memory can be written in block sizes, thus making it easier to update. Flash memory gets its name because the microchip is organized in such a way that a section of memory cells is erased in a single action or ‘flash’. Flash memory is used in digital cellular phones, digital cameras, LAN switches, PC cards for notebook computers, digital set-up boxes, embedded controllers, and other devices.

Memory and Storage Devices - Notes | Study Cyber Olympiad for Class 9 - Class 9

Virtual Memory 

Virtual memory typically comes into the picture when applications are too large for the RAM to handle. The operating system uses the hard drive to temporarily store information and take it back when needed. This is normally much slower than actual RAM and can possibly degrade performance if used too heavily.

Cache Memory 

The speed of the CPU is extremely high compared to the access time of the main memory. Therefore the performance of the CPU decreases due to the slow speed of the main memory. To decrease the mismatch in operating speed, a small memory chip is attached between the CPU and the main memory whose access time is very close to the processing speed of the CPU. It is called cache memory. Cache memories are accessed much faster than conventional RAM. It is used to store programs or data currently being executed or temporary data frequently used by the CPU. So each memory makes the main memory to be faster and larger than it really is. It is also very expensive to have a bigger size cache memory so its size is normally kept small.

Registers 

The CPU processes data and instructions at high speed, there is also the movement of data between various units of the computer. It is necessary to transfer the processed data at high speed. So the computer uses a number of special memory units called registers. They are not part of the main memory but they store data or information temporarily and pass it on as directed by the control unit.

Secondary Memory 
The secondary memory is used to store data permanently in the computer. Secondary memory (secondary storage) is the slowest and cheapest form of memory. It cannot be processed directly by the CPU. It must first be copied into primary storage (also known as RAM ). Secondary memory devices include magnetic disks like hard drives and floppy disks; optical disks such as CDs and CD ROMs; and magnetic tapes, which were the first forms of secondary memory.

Characteristics of Secondary Memory

  • It is non-volatile memory. 
  • Computers may run without secondary memory. 
  • These are magnetic and optical memories. 
  • It is known as backup memory. 
  • Data is permanently stored even if the power supply is switched off. 
  • It is used for the storage of data on a computer.
  • It is slower than primary memory. 
  • Its classification is shown in the following figure:

Memory and Storage Devices - Notes | Study Cyber Olympiad for Class 9 - Class 9

Sequential Access Memory

When the access of pieces of information is in serial order, one after the other; the time to access a particular piece of information depends upon when the piece of information was last accessed. Such characteristic is typical of off-line storage.
Magnetic Tape: Magnetic tape has been used to store data for over 50 years. To store large amounts of data, the tape can be substantially less expensive than disk or other data storage options. Tape storage has always been used with large computer systems. Modern usage of the tape is primarily a high-capacity medium for backups and archives. Magnetic tape is an older secondary storage medium that uses a strip of thin plastic, coated with a magnet-sensitive recording medium. Writing and retrieving data is slow and it uses serial access for reading and writing. Magnetic tapes are used for an application that requires an extremely large storage capacity where the speed of access is not an issue. It is commonly used for backups of file servers for computer networks, in a variety of batch processing applications such as the reading of bank cheques, payroll processing, and general stock control.

Random Access Memory A Random (or direct) Access Memory is one that stores data in such a way that it can instantly be accessed. There is no need to search through other files to get to the one you want. For example, a DVD movie is a direct access device. Unlike the Video Tape movie, you can jump to any scene on a DVD. All parts of the DVD are directly accessible. This type of file storage is called direct access.

Magnetic Disc Memory: Magnetic medium is a storage medium that uses magnetic techniques to store and retrieve data on disks or tapes coated with magnet-sensitive materials. Like iron filings on a sheet of waxed paper, these materials are reoriented when a magnetic field passes over them.

Floppy disk drive: Floppy disks are made of round plastic disks located with magnetic oxide particles. The disks are enclosed in a plastic jacket which protects the magnetic recording surface from damage. Floppy disks come in 3 sizes: 8 inch, 5¼ inch and 3/2 inch. Single-sided disks store data on only one side of  the disk and double-sides disks store on both sides. Floppy disks can also be classified on the basis of data they can. This is called disk density. There are 3 levels of floppy disk density: single density, double-density and high-density.
Construction of 5.25 inch Floppy Disk

Memory and Storage Devices - Notes | Study Cyber Olympiad for Class 9 - Class 9

  • Disk Jacket: The disk is enclosed in a plastic jacket to protect the disk surface from contamination caused by dust, dirt, and smoke. Two notches on the bottom of the disk jacket are called stress relief notches. They help to relieve stress on the disk. Some drives also use these notches to keep the disk in the proper position in the drive.
  • Media Access Hole: There is a media access hole on each side of the disk. When you insert the disk into a drive, the heads are positioned over these holes to read or write on the disk.
  • Index Hole: The index hole indicates the start of the sector. Soft–sectored disks have only 1 index hole and hard sectored disks have 8 or 9 index holes and each hole represents the start of a sector.
  • Write Enable Notch: In-order to write on a disk, this notch must be present. If you want to protect the disk, from accidental loss of data, then the notch should be covered with a strip of tape.
  • Central Hub: In the center of the disk, there is a big hole known as the central hub access hole. When you insert the disk into the drive, a cone-shaped clamp enters the disk and clamps it to the spindle motor.

Hard Disk 

A hard disk drive is a device used to store large amounts of digital information in computers and related equipment like iPods and game consoles such as the Xbox 360 and PS3. Hard disk drives are used to store operating systems, software, and working data. These are suitable for any application which requires very fast access to data for both reading and writing. However, hard disk drives may not be suitable for applications that need portability. Almost all computers use a fixed hard disc. It is used for online and real-time processes that require direct access. It is also used in file servers for computer networks to store large amounts of data. The components of the hard disk are shown in the figure given below.
Memory and Storage Devices - Notes | Study Cyber Olympiad for Class 9 - Class 9

Optical Disc Memory

An optical medium is a secondary storage medium for computers on which information is stored at an extremely high density in the form of tiny pits. The presence or absence of pits is read by a precisely focused laser beam. Optical medium types include:

Read-Only Optical Discs 
Read-only optical discs have data written on them when they are manufactured. This data cannot be changed.
CD-ROM: Compact Disc - Read-Only Memory (CD-ROM) discs can hold around 800 MB of data. The data cannot be altered (non-volatile) and hence,  it cannot be accidentally deleted. CD-ROMs are random-access devices. There are used to dis- tribute all sorts of data: software, games, music, e-books.
DVD-ROM: Digital Versatile Disc - Read-Only Memory (DVD-ROM) discs can hold around 4.7 GB of data (a dual-layer DVD can hold twice that). DVD-ROMs are random-access devices. and are used in the same way as CD-ROMs but, since they can hold more data, they are also used to store high-quality video.
Blu-Ray: Blu-Ray is a high-capacity optical disc. These disks are a recent replacement for DVDs. A Blu-Ray disc can hold 25 – 50 GB of data (a dual-layer Blu-Ray disc can hold twice to that). BluRay discs are random-access devices. These discs are used in the same way as DVD-ROMs but, since they can hold more data, they are also used to store very high-quality, high-definition (HD) video. The ‘Blu’ part of Blu-Ray refers to the fact that the laser used to read the disc uses blue light instead of red light. Blue light has a shorter wavelength than red light (used with CDs and DVDs). Using a blue laser allows more data to be placed closer together on a Blu-Ray disc, than on a DVD or CD, that is why Blu-Ray has a much higher storage capacity than these older discs.
HD DVD: High-density DVD (HD-DVD) discs can hold around 15 GB of data (a dual-layer HDDVD can hold twice that). HD-DVDs are random-access devices. HD-DVD discs are used in the same way as DVD-ROMs but, since they can hold more data, they are also used to store very high quality, high-definition (HD) video. The HD-DVD format was launched at the same time as Blu-Ray. For about a year, they competed to be the ‘next DVD’. For various reasons, BluRay won the fight, and the HD-DVD format has been abandoned.
Recordable Optical Discs: Recordable optical discs can have data written on to them (‘burnt’) by a computer user using a special disc drive (a disc ‘burner’).
CD-R and DVD-R: CD-Recordable (CD-R) and DVD-recordable (DVD-R) discs can have data burnt onto them, but not erased. You can keep adding data until the disc is full, but you cannot remove any data or re-use a full disc.
CD-RW and DVD-RW: CD-ReWritable (CDRW) and DVD-ReWritable (DVD-RW) discs, unlike CD-Rs and DVD-Rs, can have data burnt onto them and can also be erased so that the discs can be re-used. When CD-Rs and DVD-Rs are burnt, the laser makes permanent marks on the silver-colored metal layer. This is why these discs cannot be erased. When CD-RWs and DVD-RWs are burnt the laser makes marks on the metal layer, but in a way that can be undone. Hence these discs can be erased.
DVD-RAM: DVD-Random Access Memory (DVD-RAM) discs are a type of re-writable DVD. They often come in a floppy-disc style case (to protect the disc). DVD-RAM discs have a similar capacity to a normal DVD, holding 4.7 GB of data. DVD-RAM discs are random-access devices. DVD-RAM discs are used in many camcorders (video recording cameras). The discs are of a much higher quality than normal DVDRWs and can reliably store data for up to 30 years. This means that they are often used for video and data back-up and archiving.
Solid-State Storage Devices: The term ‘solid-state’ essentially means ‘no moving parts’. Solid-state storage devices are based on electronic circuits with no moving parts (no reels of tape, no spinning discs, no laser beams, etc.). Solid-state storage devices store data using a special type of memory called flash memory.
Flash Memory: Flash memory is a type of Electronically-Erasable Programmable ReadOnly Memory (EEPROM). Flash memory is nonvolatile (like ROM) but the data stored in it can also be erased or changed (like RAM). Flash memory can be found in many data storage devices. You might wonder, why, since flash memory is nonvolatile, normal computers don’t use it instead of RAM. If they did, we would have computers that you could turn off, turn back on again and no data would be lost – it would be great! The reason is speed – saving data to flash memory is very slow compared to saving it to RAM. If a computer used flash memory as a replacement for RAM, it would run very slowly. However, some portable computers have stated to use flash memory (in the form of solid-state ‘discs’ as a replacement for hard drives) No moving parts mean fewer chances of going wrong and longer battery life.
USB Memory Sticks: Memory sticks (or ‘thumb-drives’) have made many other forms of portable storage almost obsolete (why burn a CD or DVD when you can more easily copy your files onto a memory stick?). Memory sticks are non-volatile, random-access storage devices. Each of these small devices has some flash memory connected to a USB interface. Plug it into your computer and it appears as a drive. You can then add files, erase files, etc. You can use it to move any type of file between computers. Flash memory used to be very expensive, but in recent years it has become much cheaper and you can now buy a 16 GB memory stick for just a few dollars.
Memory Cards: Many of our digital devices (cameras, mobile phones, MP3 players, etc.) require compact, non-volatile data storage. Flash memory cards provide this facility and come in a variety of shapes and sizes. One of the most common formats used by digital cameras is the SD card. The cards store the digital images taken by the camera. Mobile phones contain a subscriber identity module (SIM) card that contains the phone’s number, phonebook numbers, text messages, etc. Many phones also have extra memory cards to store music, video, photos, etc. (e.g Tiny Micro-SD cards).
Smart Cards: Many credit cards (‘chip-and pin’ cards), door entry cards, satellite TV cards, etc. have replaced the very limited storage of the magnetic strip (the dark strip on the back of older cards) with flash memory. This is more reliable and has a much larger storage capacity. Cards with flash memory are called smart cards.

Difference between Primary and Secondary Memory

The following table shows key differences between primary and secondary memory:
Memory and Storage Devices - Notes | Study Cyber Olympiad for Class 9 - Class 9

Memory Hierarchy 

Memory in a computer system is arranged in a hierarchy. At the top, we have primary storage, which consists of cache and main memory, and provides very fast access to data. Then comes secondary storage, which consists of slower devices such as magnetic disks. Tertiary storage is the slowest class of storage devices; for example, optical disks and magnetic tapes. Currently, the cost of a given amount of main memory is about 100 times the cost of the same amount of disk space, and tapes are even less expensive than disks. Slower storage devices such as tapes and disks play an important role in database systems because the amount of data is typically very large. Since buying enough main memory to store all data is expensive, we must store data on tapes and disks and must build database systems that can retrieve data from lower levels of the memory hierarchy into main memory as needed for processing. There are reasons other than the cost for storing data on secondary and tertiary storage. On systems with 32-bit addressing, only 232 bytes can be directly referenced in main memory; the number of data objects may exceed this number.
Further, data must be maintained across program executions. This requires storage devices that retain information when the computer is restarted (after a shutdown or a crash); we call such storage non-volatile. Primary storage is usually volatile (although it is possible to make it non-volatile by adding a battery backup feature), whereas secondary and tertiary storage is non-volatile. Tapes are relatively inexpensive and can store very large amounts of data. They are a good choice for archival storage, that is, when we need to maintain data for a long period but do not expect to access it very often. A Quantum DLT 4000 drive is a typical tape device; it stores 20 GB of data and can store about twice as much by compressing the data. It records data on 128 tape tracks, which can be thought of as a linear sequence of adjacent bytes, and supports a sustained transfer rate of 1.5 MB/sec with uncompressed data (typically 3.0 MB/sec with compressed data). A single DLT 4000 tape drive can be used to access up to seven tapes in a stacked configuration, for a maximum compressed data capacity of about 280 GB. The main drawback of tapes is that they are sequential access devices. We must pass through all the data in order and cannot directly access a given location on the tape. For example, to access the last byte on a tape, we would have to wind through the entire tape. This makes tapes unsuitable for storing operational data, or data that is frequently accessed. Tapes are mostly used to back up operational data periodically.

Memory and Storage Devices - Notes | Study Cyber Olympiad for Class 9 - Class 9

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