Secondary Memory Types

 SECONDARY MEMORY

In the previous section, we saw that RAM is expensive and has a limited storage capacity. Since it is a volatile memory, it cannot retain information after the computer is powered off. Thus, in addition to primary memory, an auxiliary or secondary memory is required by a computer. The secondary memory is also called the storage device of computer. In this chapter, the terms secondary memory and storage device are used interchangeably. In comparison to the primary memory, the secondary memory stores much larger amounts of data and information (for example, an entire software program) for extended periods of time. The data and instructions stored in secondary memory must be fetched into RAM before processing is done by CPU.

Magnetic tape drives, magnetic disk drives, optical disk drives and magneto-optical disk drives are the different types of storage devices.

  ACCESS TYPES OF STORAGE DEVICES

The information stored in storage devices can be accessed in two ways—

1.        Sequential access

2.        Direct access

Sequential Access Devices

Sequential access means that computer must run through the data in sequence, starting from the beginning, in order to locate a particular piece of data. Magnetic tape is an example of sequential access device. Let us suppose that magnetic tape consists of 80 records. To access the 25th record, the computer starts from first record, then reaches second, third etc. until it reaches the 25th record. Sequential access devices are generally slow devices.

Direct Access Devices

Direct access devices are the ones in which any piece of data can be retrieved in a non-sequential manner by locating it using the data’s address. It accesses the data directly, from a desired location. Magnetic disks and optical disks are examples of direct access devices. There is no predefined order in which one can read and write data from a direct access device. In a magnetic disk consisting of 80 records, to access the 25th record, the computer can directly access the 25th record, without going past the first 24 records. Based on access, magnetic tapes are sequential access devices, and, magnetic disks, optical disk and magneto-optical disks are direct access devices.

MAGNETIC TAPE

Magnetic tape is a plastic tape with magnetic coating . It is a storage medium on a large open reel or in a smaller cartridge or cassette (like a music cassette). Magnetic tapes are cheaper storage media. They are durable, can be written, erased, and re-written. Magnetic tapes are sequential access devices, which mean that the tape needs to rewind or move forward to the location where the requested data is positioned in the magnetic tape. Due to their sequential nature, magnetic tapes are not suitable for data files that need to be revised or updated often.

They are generally to store back-up data that is not frequently used or to transfer data from one system to other.

The features of magnetic tape are:

·         Inexpensive storage device

·         Can store a large amount of data

·         Easy to carry or transport

·         Not suitable for random access data

·         Slow access device

·         Needs dust prevention, as dust can harm the tape

·         Suitable for back-up storage or archiving

MAGNETIC DISK:

 Magnetic disk is a direct access secondary storage device. It is a thin plastic or metallic circular plate coated with magnetic oxide and encased in a protective cover. Data is stored on magnetic disks as magnetized spots. The presence of a magnetic spot represents the bit 1 and its absence represents the bit 0.

The features of magnetic disk are—

·         Cheap storage device

·         Can store a large amount of data

·         Easy to carry or transport

·         Suitable for frequently read/write data

·         Fast access device

·         More reliable storage device

·         To be prevented from dust, as the read/write head flies over the disk. Any dust                 particle in between can corrupt the disk.

  Floppy Disk

·         Floppy disk (FD) is a flat, round, single disk made of Mylar plastic and                   enclosed in square plastic jacket.

·         Floppy Disk Drive (FDD) is the disk drive for floppy disk.

·         The floppy disk is inserted into the floppy disk drive to read or write data to it.

·         Floppy disk has a write-protect slide tab that prevents a user from writing to it.

·         A floppy disk may be single-sided or double-sided disk, i.e., data can be read and                           written on one and both sides of floppy disk, respectively.

·         They are portable. They can be removed from the disk drive, carried or stored separately.

·         They are small and inexpensive.

·         Floppy disks are slower to access than hard disk. They have less storage capacity             and are less expensive than hard disk.

·         They come in two basic sizes—5-¼ inch and 3-½ inch.

·         The 5-¼ inch disk came around 1987. It can store 360 KB to 1.2 MB of data.

·         The 3-½ inch disk has capacity of 400 KB to 1.44 MB. It usually contains 40                 tracks and 18 sectors per track and can store 512 bytes per sector.

  

  Hard Disk

·         A hard disk (HD) consists of one or more platters divided into concentric tracks and sectors. It is mounted on a central spindle, like a stack. It can be read by a read/write head that pivots across the rotating disks. The data is stored on the platters covered with magnetic coating.

·         Hard disk is a fixed disk. The disk is not removable from the drive, unlike floppy disk.

·         The hard disk and Hard Disk Drive (HDD) is a single unit.

·     Hard disk can store much more data than floppy disk. The data in hard disk are packed more closely (because fast spinning uses smaller magnetic charges) and they have multiple platters, with data being stored on both sides of each platter. Large capacity hard disks may have 12 or more platters

·     Unlike floppy disk, the read/write head of hard disk does not touch the disk during accessing.

·         Hard disk can spin at the speed of up to 10,000 revolutions per minute and have an access time of 9—14 ms. It stores 512 bytes per sector but the number of sectors are more per track (54 or more) than floppy disk.

·         Nowadays, hard disks are available that can store up to 500 GB of data. Generally, PCs come with 160 GB hard disk.

·         Hard disk is the key secondary storage device of computer. The operating system is stored on the hard disk. The performance of computer like speed of computer boot up, loading of programs to primary memory, loading of large files like images, video, audio etc., is also dependent on the hard disk.

·         Nowadays, portable external hard disk drive is available which can be attached to the USB drive of the computer. They come in the storage capacities of 80 GB to 500 GB.

   Zip Disk

·         They are high-capacity removable disk and drive.

·         They have the speed and capacity of hard disk and portability of floppy disk.

·         Zip disk are of the same size as floppy disk, i.e., 3–½ inch but have a much                       higher capacity than the floppy disk.

·        Zip disk and drive were made by Iomega Corp. It comes as a complete unit—disk, drive, connection cable, power cord and operating system. It can be connected to the computer system externally using a parallel chord or SCSI cable.

                 ·         Their capacity ranges from 100 MB to 750 MB. They can be used to                           store large files, audio and video data.

 OPTICAL DISK

Optical disk  is a flat and circular disk which is coated with reflective plastic material that can be altered by laser light. Optical disk does not use magnetism. The bits 1 and 0 are stored as spots that are relatively bright and light, respectively.

·         An optical disk consists of a single spiral track that starts from the edge to the centre of disk. Due to its spiral shape, it can access large amount of data sequentially, for example music and video. The random access on optical disk is slower than that of magnetic disk, due to its spiral shape.

·         The tracks on optical disk are further divided into sectors which are of same length. Thus, the sectors near the centre of disk wrap around the disk longer than the sectors on the edges of disk. Reading the disk thus requires spinning the disk faster when reading near the centre and slower when reading near the edge of disk. Optical disks are generally slower than hard disks.

·         Optical disks can store large amount of data, up to 6 GB, in a small space. Commonly used optical disks store 600–700 MB of data.

·         The access time for an optical disk ranges from 100 to 200 ms.

·         There are two most common categories of optical disks—read-only optical disks and recordable optical disks.

    CD-ROM

·         Originally, Compact Disk (CD) was a popular medium for storing music. Now, it is used in computers to store data and is called Compact Disk-Read Only Memory (CD-ROM).

·         As the name suggests, CD-ROM (Figure 3.16) is an optical disk that can only be read and not written on. CD-ROM is written on by the manufacturer of the CD-ROM using the laser light.

·         A CD-ROM drive reads data from the compact disk. Data is stored as pits (depressions) and lands (flat area) on CD-ROM disk. When the laser light is focused on the disk, the pits scatter the light (interpreted as 0) and the lands reflect the light to a sensor (interpreted as 1).

·         As CD-ROM is read only, no changes can be made into the data contained in it.

·         Since there is no head touching the disk, but a laser light, CD-ROM does not get worn out easily.

·         The storage density of CD-ROM is very high and cost is low as compared to floppy disk and hard disk.

·         Access time of CD-ROM is less. CD-ROM drives can read data at 150Kbps. They come in multiples of this speed like—2x, 4x, 52x, 75x, etc.

·         It is a commonly used medium for distributing software and large data.

    DVD-ROM

 ·         Digital Video Disk-Read Only Memory (DVD-ROM) is an optical storage device used to store digital video or computer data .

·         DVDs look like CDs, in shape and physical size.

·         It improves on CD technology.

·         It is a high-density medium with increased track and bit density.

·         DVD-ROM uses both sides of the disk and special data compression

       technologies. The tracks for storing data are extremely small.

·         A full-length movie can be stored on a single disk.

·         Each side of DVD-ROM can store 4.7 GB of data, so a single DVD can store 9.4 GB of data.

·         New DVD-ROMs use layers of data track, to double its capacity. Such dual layer disks can store 17 GB of data.

    Recordable Optical Disk

 In addition to the read only CDs and DVDs, recordable optical disks (Figure 3.18) are also available. Users can record music, video, audio and data on it. The recordable optical disks are—

·         Compact Disk-Recordable (CD-R) is a Write Once-Read Many (WORM) disk. A CD-R disk allows the user to write data permanently on to the disk. Once the data is written, it cannot be erased. CD-R disk uses a laser that burns pits into the disk surface. It looks like a CD disk externally. To write to a CD-R disk, a device named CD-Writer or CD–burner is required. A CD-R disk can store 700 MB of data that can run for 80 minutes. CD-R is used to create music CDs in home computers, back up data from other storage devices, archives of large data, etc.

·         Compact Disk-ReWritable (CD-RW) allows data to be written, erased and re-written on. The capacity of CD-RW is same as a CD. They generally do not play on all CD-ROM drives.

·         Digital Video Disk-Recordable (DVD-R) allows recording of data on a DVD. A DVD writer device is required to write the data to DVD. The data once written on a DVD cannot be erased or changed.\

       MAGNETO-OPTICAL DISK

·   Magneto-optical disks use laser beam to read data and magnetic field to write             data to disk.

• These are optical disks where data can be written, erased and re-written.

· They are expensive and outdated. They were used during the mid             1990s. They have now been replaced by CD-RW and DVD-R.

       USING THE COMPUTER MEMORY

The computer starts using the memory from the moment the computer is switched on, till the time it is switched off. The list of steps that the computer performs from the time it is switched on are—

· Turn the computer on.

· The computer loads data from ROM. It makes sure that all the major components of the computer are functioning properly.

·  The computer loads the BIOS from ROM. The BIOS provides the most basic information about storage devices, boot sequence, security, plug and play capability and other items.

· The computer loads the OS from the hard drive into the system’s RAM. CPU has immediate access to the OS as the critical parts of the OS are maintained in RAM as long as the computer is on. This enhances the performance and functionality of the overall system.

· Now the system is ready for use.

·When you load or open an application it is loaded in the RAM. Since the CPU looks for information in the RAM, any data and instructions that are required for processing (read, write or update) is brought into RAM. To conserve RAM usage, many applications load only the essential parts of the program initially and then load other pieces as needed. Any files that are opened for use in that application are also loaded into RAM.

·   The CPU requests the data it needs from RAM, processes it and writes new data back to RAM in a continuous cycle. The shuffling of data between the CPU and RAM happens millions of times every second.
·  When you save a file and close the application, the file is written to the secondary memory as specified by you. The application and any accompanying files usually get deleted from RAM to make space for new data.

· If the files are not saved to a storage device before being closed, they are lost.

Sometimes, when you write a program and the power goes off, your program is lost if you have not saved it. This is because your program was in the RAM and was not saved on the secondary memory; the content of the RAM gets erased when the power is switched off.