Topic 4: Storage Devices 4.1 Introduction A storage device is a computer peripheral which is used to store data and programs for a specific interval of time. A computer system usually contains several kinds of storage, each with an individual purpose. Some examples of storage devices are the hard disk, CD, DVD and USB Flash Drive (amongst others). 4.2 Memory Size Bit: The smallest measure of memory is the bit. A bit can either have the value of 1 or the value of 0. Byte: 8 bits, are equivalent to 1 byte. 1 byte is needed to store a single character. For example: A = 0100 0001 Unit Equivalent to 1 byte 8 bits Equivalent to (in bytes) 1 Kb (Kilobyte) 1024 bytes 1024 1 Mb (Megabyte) 1024 Kb 1024 2 1 Gb (Gigabyte) 1024 Mb 1024 3 1 Tb (Terabyte) 1024 Gb 1024 4 Ms. Scicluna P a g e 1
4.3 Modes of Access Storage devices are described by the way data is read or written from/to it. Data on storage device can be accessed in two different ways. There are commonly referred to as the modes (or methods) of access: - Serial (Sequential) Access - Random (Direct) Access Serial (Sequential) Access With Serial Access, reading or writing of data is done in a sequential order. Records are read or written one after the other, starting from the beginning of the file. When searching for a particular record, all the record must be read until you find your record. For example, if we want to read the file on the 150 th record, we must read from record 1 to record 149, and then we will find the required record. An ideal use for this type of mode of access is when data is always accessed in the same order such as in a payroll system. An example of a storage device which uses serial access is the Magnetic Tape. Random (Direct) Access With Random Access, reading or writing of data records is done at random (anywhere in a file). Records can be read or written by going directly to the particular record we require and not accessed one after the other. Therefore, with random access, if we want to access record 150, we go directly to it. An ideal use for this type of mode of access is for an airline booking reservations system. Some examples of storage devices which uses serial access are the Hard Disk, CD,USB Flash Drive. Ms. Scicluna P a g e 2
Note: Disks are capable of supporting both serial and direct access modes. 4.4 Storage Devices Storage devices may be divided into two: - Primary (Main) Storage - Secondary (Backing or Auxiliary) Storage The following is a diagram which shows how computer memory may be classified: Storage Primary Storage Secondary Storage RAM ROM Magnetic Media Optical Media Electronic Media Magnetic Tape CD USB Flash Drive Floppy Disk DVD Flash Memory Card Hard Disk Blu Ray Disk Ms. Scicluna P a g e 3
4.5 Primary Storage There are mainly two Primary Storage Devices: Note: - Random Access Memory (RAM) - Read Only Memory (ROM) - Primary Storage devices are physically outside the CPU but they can be directly accessed by the CPU at a very fast speed. - Thus, it takes longer to access data on Secondary Storage than in Main Memory. Random Access Memory (RAM) The RAM is the working memory. RAM holds programs and data which are currently being used by the computer, for example, a word-processed document that is being edited will be loaded into main memory. The computer can read data (retrieve) from and write data (store) to RAM. RAM is used to store: - the programs that are running and the data that it is used by the CPU - data which is produced as a result of running a program - data that is being transferred to and from the peripherals - the contents of the screen (desktop). Adding more RAM will help make the computer run faster (however, it s not the only factor which determines computer s performance). Since anything that is being run by the computer must be loaded onto the RAM, the more RAM available, the more programs can be loaded at the same time without the computer slowing down. Obviously, there is a limit. RAM is like a whiteboard, on which you can write information which you are currently using and then rub it off when you are not using it anymore. Just the same, information may be retrieved or stored in the RAM. RAM will hold this information while the computer is using it. But when the computer is switched off, the RAM is completely cleared from information. Thus, we say that the RAM is volatile, since it will lose the information when the computer is switched off. Ms. Scicluna P a g e 4
Why is this? We said that RAM holds information which is currently being used by the computer! If the computer is switched off, does the computer need to use any information? No! Thus, the RAM will hold no information within it when the computer is switched off! Therefore, the RAM is a temporary store for data and instructions! The RAM is connected directly to the CPU through an address and data bus. Through the address bus, the CPU sends the memory address of the information which it wants to access. Then, through the data bus, the CPU passes the information which it wants to read or write. Nowadays, the size of the RAM is measured in Gigabytes. Types of RAM: Dynamic RAM (DRAM) which needs to be refreshed thousands of times per second. DRAM is the most common type of RAM. Static RAM (SRAM) which does not need to be refreshed and so it is faster but it is also more expensive than DRAM. Synchronous DRAM (SDRAM): fast access time Double Data Rate SDRAM (DDR): consume less power: suitable for notebook computers. Read Only Memory (ROM) ROM is like a textbook, on which the author writes information once, but then the reader can only read and cannot write onto it, thus, it cannot be changed! Just the same, ROM holds information that was built into it by its manufacturer. The computer can read it, but it cannot write onto it. In other words, ROM is good at remembering, but cannot change its mind. Once data has been written onto a ROM chip it cannot be removed and can only be read by the computer. The ROM is non-volatile, which means that, its contents are not lost when the power is switched off. If the ROM was volatile, it would lose its data and it would not be possible to write anything back (since it is read only). Thus, since ROM is used to hold information which is vital for the running of the computer, its contents will not be removed when the computer is switched off. Ms. Scicluna P a g e 5
ROM is the computer s permanent store of very important information or programs that must be readily available. This way, such information (including both instructions and data) will be protected against intentional or even un-intentional change! One such program is the bootstrap loader, which runs when the computer is switched on. The bootstrap loader is a program which tells the computer how to start up. It runs when the computer is switched on to get the computer started. Types of ROM: - Programmable Read-Only Memory (PROM): PROM are manufactured as a blank chip on which data can be written by the user using a special device called a PROM programmer. Once data has been written, it cannot be erased. - Erasable Programmable ROM (EPROM): Data can be erased by exposing it to ultraviolet radiation, so that new data can be written to it as if it was a new PROM. - Electrically Erasable Programmable ROM (EEPROM): Data can be erased by exposing it to an electrical charge. PROM, EPROM and EEPROM are still described as "read-only memory" (ROM) even though data can be written on them. This is due to the fact that the reprogramming process is generally infrequent, it is comparatively slow and random access writes to individual memory locations are not permitted. Note: Both RAM and ROM are randomly accessed. RAM AND ROM in Printers RAM is used to store the next set of data to be printed. Some ROM is used to store programs that direct the printer how to print the data and to keep record of the different printing fonts. RAM AND ROM in Washing Machine In a modern washing machine, the processor needs ROM to permanently store the control program. RAM is used to store temporary data created by the program. Ms. Scicluna P a g e 6
4.6 Secondary Storage Secondary Storage (Backing storage or auxiliary storage) is used to store data and instructions for a long term. Any programs or data that are not currently being used by the computer will be kept on backing store. Since such information is not currently being used by any program, it can stay in the Secondary Storage so that more essential information is placed in the RAM. When programs or data need to be used they are copied (loaded) into RAM for faster access. Note: It takes much longer to access data which is stored on backing store than data which is stored in RAM. Typically, data on backing storage takes 100 to 1000 times as long. Apart from storing data, secondary storage is also used to keep backups just in case the first medium fails. Like the main memory, secondary storage devices are also located outside the CPU. Backing storage media are non-volatile. This data will not be removed when the computer is switched off. There are basically 3 types of Storage Media: - Magnetic Media - Optical Media - Electronic Media Magnetic Media Magnetic Tape A magnetic tape is a narrow plastic ribbon. This ribbon is coated with magnetisable material and data is written by magnetizing different areas of the tape. The tape is similar to the tape used in domestic video systems but it is of a higher quality since it needs to be accelerated and decelerated very rapidly. Though the magnetic tape was very popular as a storage medium in the 1950s and 1960s, nowadays, it is mostly replaced by Magnetic disks and other type of media. It is only used to make backup copies of important data. It is common for 20,000 bits of information to be recorded on one centimetre of tape. Ms. Scicluna P a g e 7
- Mode of Access: Serial access - Drive: Tape drive or Tape streamer - Storage Capacity: 120Mb 5Tb - Application: To backup programs & data - Advantages: o It is very fast. o Millions of characters can be recorded on magnetic tapes. o It is cheap and convenient. - Disadvantages: o Data access is slow and sequential. o A record on a magnetic tape cannot be updated in place. Records which need to be updated have to be read, updated and then written on a second tape. o Must be stored in a suitable environment where humidity, temperature and dust are tightly controlled. o Not suitable for long-term storage of history files. Magnetic Disks A Magnetic Disks is coated with a magnetisable material. The head of the drive is positioned close to the disk to write or read data on it. The disk, head and all electronics are built together into a sealed unit. But how does it work? The surface of the disk is magnetisable and therefore, when the electromagnetic head of the disk drive is passed on the disk, it creates a pattern of magnetized and non-magnetized areas on disk's surface. This is the way in which data is written. Ms. Scicluna P a g e 8
Magnetic Disks are divided into tracks and sectors - Tracks are circular ring on one side of the disk. Each track is numbered. - Sectors are a wedge-shape piece of the disk. Each sector is numbered. The track and sector number are used as an address to find where data is stored on the disk. When a disk is formatted, addresses are being assigned to the different areas of the disk. Apart from this, an area is reserved for keeping the list of such addresses, so that the location of the data can be monitored. The removal of all data is the consequence resulting from formatting! Hard Disk A hard disk consists of a number of inflexible, shining silvery disks (platters) which are coated with a magnetic material. These magnetic disks are sealed in a protective case which protects the disks from dust and moisture. Data is recorded on these platters. Platters are coated with a magnetic material. The platters spin at a high speed and information is written on them as soon as they pass under the head. The head moves across the disc via an arm. Hard Disks write data by magnetizing the magnetic material at certain points to represent 0s and 1s. Hard Disks read data by seeing if the material at a certain point is magnetized. Ms. Scicluna P a g e 9
Hard Disks may be both internal and external and are used to store operating systems, application software and users files. Internal hard disks, are fixed inside the computer, but nowadays there are also removable hard disks, known as external hard disks. External hard disks are portable but more expensive than internal ones. The storage capacity of hard disks is always increasing and hard disks are becoming cheaper to buy. Hard disks can store more data than floppy disks. Also, data stored on a hard disk can be accessed much faster than data stored on a floppy disk. - Access: Direct access - Storage Capacity: 160Gb 4TB - Application: To store the operating system, application programs and user files - Advantages: o The protective case makes a hard disk more reliable than a floppy disk there is better protection against dirt. o Data access is fast and direct. - Disadvantage: o It is expensive. (compared to magnetic tape) Zip Disk A zip disk is a high-capacity floppy disk. The surface of the disks is more tightly packed with magnetic particles and thus it can store high density of data. Though zip disks can store much more data than floppy disks, they are only slightly larger in size than the floppy disks and about twice as thick. - Access: Direct access - Storage Capacity: 100Mb 750Mb - Application: To backup hard disks and transport large files - Advantages: o Zip disks are relatively cheap and durable. o They are of medium capacity. o Data is accessed directly. - Disadvantage: o Data access is slow. Ms. Scicluna P a g e 10
Floppy Disk Floppy disks are also called diskette or disk. Floppy Disks consist of a flexible magnetic disk held in a protective plastic jacket/case. The disk is coated with microscopic magnetic iron particles. On one side of the floppy disk, there is a silver circle with two holes in it. These holes are used so that when the disk is inserted into the disk drive, the drive hooks into holes to spin the disk inside. The disk spins at about 360 rpm (rotations per minute). Then, the silver metal cover is pushed aside so that the read/write heads in the disk drive can read from and write to the disk. Data on such disks may be protected by sliding the small write-protect tab, thus not allowing any writes to the disk. A floppy disk accesses data much slower than other magnetic media and it holds a very small amount of data. - Access: Direct access - Storage Capacity: 1.44Mb - Application: To transfer data files (such as word processing and spreadsheet files) between computers - Advantages: o Floppy disks are cheap. o They are lightweight and portable. o Data is accessed directly. - Disadvantages: o Floppy disks can hold a small amount of data (1.44Mb). o Data access is slow. Optical Media Optical media are storage media from which data is read from and written to by lasers. Compact Disk (CD) A CD is an exchangeable and portable medium, with the capacity of 650 MB. CDs are mostly used for large amounts of text and graphics, for example in encyclopaedias, catalogues and technical manuals. Information on a CD is accessed directly since the head in the CD drive moves directly to that required file. Ms. Scicluna P a g e 11
On a CD, information is stored in terms of pits and lands. - Pits are microscopic indentations and - Lands are flat surfaces. In order to write data on a CD, a powerful laser beam is used to burn a pattern of lands and pits on the CD corresponding to 1s and 0s. In order to read data from a CD, a low power laser throws a beam of light on the surface of the CD. When the light hits a pit, it is shone into an electric eye; but, when the light hits a land, it is not shone into that an electric eye. Thus, data is read as a series of reflections. View the animation in the site (http://electronics.howstuffworks.com/cd5.htm) to understand what happens better. Accessing data from an optical medium is faster than accessing a floppy disk but slower than accessing a hard disk. Types of CDs: - CD-ROM (Read Only Memory) Data is written once (before it is sold) using a powerful laser beam to burn patterns (land or pit) on its surface. Once written, data cannot be erased replaced - data can only be read. This is why they are often described as Write Once Read Many times (WORM) disks. Access: Direct access Drive: CD-ROM player Storage Capacity: 650Mb - 700Mb Application: To store information that requires large storage capacity such as catalogs, technical manuals and multimedia (graphics, sound and video) Advantages: o CD-ROMs can hold large amounts of data. o They are portable, inexpensive and durable. o Data access is direct and faster than access to data on a floppy disk. Disadvantage: o CD-ROMs are not as fast as hard disks. Ms. Scicluna P a g e 12
- CD-R (Recordable) CD-Rs are also WORM disks. They are blank when bought and the user of a CD-R can write data to the disk in multiple sessions until the disk is full. Once all the tracks of a CD-R have been filled, no other data can be recorded on the disk. Access: Direct access Drive: CD-R drive (to write to the disk) Storage Capacity: 650Mb - 700Mb Application: To store information and to make permanent backups of data Advantage: o Data can be written to the disk until the disk is full. Disadvantages: o Disk can only be written once. Data written to the disk cannot be changed. - CD-RW (Rewritable) CD-RW disk are also bought blank. Data can be written on the disk in multiple sessions using a CD-RW drive. CD-RW disks are erasable optical disks. This means that data on a CD-RW can be changed and erased just like the data stored on a floppy or hard disk. Access: Direct access Drive: CD-RW drive Storage Capacity: 650Mb - 700Mb Application: To backup fixed hard disks Advantage: o Disk can be erased and written to over and over again. Disadvantage: o CDs can store less data than hard disks Ms. Scicluna P a g e 13
Digital Video Disk (DVD) DVDs use the same technology as CDs, yet they are able to store more data than a CD-ROM. This is because tracks on a DVD are more closely packed, data is stored on both sides of the disk and there is more than one layer of data on each side of the disk. They are often used for movies, which may also be interactive (unlike tape). Ex. Special features etc. DVD drives are backward-compatible with CD-ROMs. This means that DVD drives can play old CD-ROMs as well as new DVD-ROMs. Newer DVD players can also read CD-R disks. - Access: Direct access - Drive: DVD drive - Storage Capacity: 4.7Gb - Application: To store movies and other multimedia presentations - Advantages: o High quality digital sound and video. o Sometimes provided with different language tracks. - Disadvantages: o Not capable of storing High Definition movies BluRay Disk The BluRay Disc is a recent optical storage medium designed to outdo the standard DVD format. Its main uses are for storing high-definition (HD) video and for Playstation 3 Video Games. - Access: Direct access - Drive: Blu-Ray drive Blu-Ray player - Storage Capacity: 25Gb 50Gb - Application: o HD video, Playstation 3 video games - Advantage: o Can store better quality videos o Can store more data than DVDs - Disadvantage: o More expensive than other optical media. Ms. Scicluna P a g e 14
Electronic Media Electronic media are storage media from which data is read from and written to electronically. Hard Discs and CDs are mechanical. This is because they have moving parts like the head, the motor etc. However, Electronic Media have no moving parts! USB Flash Drives USB Flash Drives are small, lightweight, removable & rewritable storage devices with a USB interface. They consist of a small circuit board enclosed in a plastic or metal casing. In order to access data stored in a flash drive, the flash drive must be connected to a compute via the computer s USB port. USB flash drives have several advantages over other portable storage devices, particularly the floppy disk. USB flash drives are faster, they hold more data and are more reliable (due to their lack of moving parts). - Access: Direct access - Drive: USB port - Storage Capacity: 1Gb 32Gb - Application: To store information that needs to be transferred from one computer to another. - Advantages: o USB flash drives are easy-to-use. o Are very small and thus portable. o Are reliable since they do not contain any internal moving parts. o Can be plugged into any computer with a USB drive. - Disadvantage: o Less storage capacity than both internal and external hard disks. Flash Memory Cards Flash memory cards are small, lightweight, removable and rewritable storage devices. Such memory cards are used with digital cameras, handheld and laptop computers, cell phones, music players, video game consoles and other electronics. In order to access data stored in a flash memory card, the card must be connected to a computer, via a memory card reader. Ms. Scicluna P a g e 15
Nowadays, some new computers have built-in slots for a variety of memory cards. - Access: Direct access - Drive: Memory card reader - Storage Capacity: 16Mb - 4Gb - Application: To store information that needs to be transferred from one computer to another - Advantage: o Flash memory cards are light. o They are very small and thus portable. o Flash memory cards are reliable since they do not contain any internal moving parts. - Disadvantage: o Less storage capacity than both internal and external hard disks. Battery-Backed Memory Some data, such as the date and time, need to be updated even when the computer is switched off. This is why computers have a small battery, which gives power to a chip which keeps track of date and time, even when the computer is switched off. Ms. Scicluna P a g e 16