Secondary Storage and its Types

It is also known as backup memory, auxiliary memory, external memory, and storage memory. The CPU cannot directly access secondary memory. They are of the non-volatile variety. Secondary memory, as contrast to primary memory, has a much larger storage capacity. The primary memory is essentially supported by this memory. Secondary storage is any form of storage that is non-volatile (data is not lost after a power outage), able to store a significant amount of data for an extended period of time, and independent of the CPU and primary storage. It is utilized to permanently store data and instructions. Additionally, it is employed for data transfer between computers. For read/write, it is typically slower and less expensive.

It is divided into the following categories based on data accessed by secondary storage devices:

• Sequential Access Media:
  • The information contained on the medium can only be read sequentially, and in order to access one point on the medium, all preceding points must first be read.
  • Example: Magnetic Tape
• Random Access Media:
  • It is possible to obtain the data at random without going through the previous points. A disk drive can randomly visit any spot while traversing intermediate points.
  • Example: Optical disk, pen drive, etc.

Secondary storage comes in a variety of forms, and daily progress is being made in this area of technology. For instance, a hard drive, CD, DVD, or pen drive. Magnetic memory and optical memory are the two types of secondary memory.

Magnetic storage

To enable data storage, the surfaces of magnetic memory are covered in many microscopic iron particles. It has been in use since the very first computer generations. Further categories for magnetic memory include:

• Magnetic Drum:
  • It is a drum that is used to create magnetic oxide coating. In essence, it is employed to store a big amount of binary data. It has a rotating motion. Binary data can be read from the track or stored along the drum's tracks and read when the drum turns. One track in the magnetic drum regulates the reading and writing associated with the other tracks. It was utilized in the first and second computer generations.
• Magnetic tape:
  • It resembles audio cassette tapes in many ways. It is constructed of plastic ribbon that has an iron oxide-based coating on one side that can be magnetized. On the magnetized side, there are tiny visible spots where data are stored. Only serial access is possible. A sequential access memory is what it is. Therefore, the read/write data speed is slower. It is mostly used to store backup data as well as music and video. It is very trustworthy. For reading and writing data, a magnetic tape drive is needed. It can hold 100MB to 200GB of data. Data storage on tapes is far less expensive than data storage on disks. However, accessing data from tapes takes longer than from a disk. Because tapes are too slow, they are generally used for long-term storage and backup. Magnetic tapes are available in ½ inch, ¼ inch, 8mm and 3mm sizes. Magnetic tapes are used for transporting huge volumes of data from one place to another.
• Magnetic disk:
  • ​​​​​​​Magnetic disk is currently the secondary storage type that is used the most. Both small and large computer systems use magnetic disks, which have several benefits. Direct access to individual records is made possible. This implies that in order to locate record disk storage, the computer does not need to search through the entire file as it would with sequential file tape. A metal or plastic circular disc can be found inside the magnetic disk. The disc's two sides are typically utilized to store data. Magnetic oxide is applied to the disc surface. A track is a collection of concentric circles on the disc. Sectors are a smaller section of the track that are further split. Sectors are used to store data. Different types of magnetic disk exist. They are:
  1. Hard disk: It serves as the computer's primary storage component. It makes use of 2-4 thin metallic disks (platter) that are coated on both sides in iron oxide. Aluminum is typically used to make the disk. Except for the upper side of the highest disk and the lower side of the lowermost disk, both sides of the disk are used to store data. Each read/write head on a data storage surface is independent. During data read/write operations, the platter rotates between 3600 and 15000 rpm. Since the data holding disk and the data read/write components are merged, hard disk is also known as hard drive. It can store anywhere between a few megabytes to a terabyte. Several hard drives are put together on a vertical shaft in larger systems. Read/write heads, which practically fly over the spinning disks, are used to write information to or read it from drives. Instead of touching the disk directly, as they would in a stereo, the heads hover a few thousandths of an inch above it instead. It only takes a smoke particle or human hair to turn the head into a disk.
  2. Floppy disk: The floppy disk was a common component of personal computers. It has a magnetic covering and is a flat, 3.5-inch disk made of polyester film. This disk has a storage capacity between 720KB and 2.88MB. The lone plastic disk is inside. It provides access at a slower rate than a hard drive. Sector technique is used to store the data. Pie-shaped segments sever the disk's surface. A distinct number is given to every section. A sector number and individual data record number address can be used to find data. It is extremely portable, light, and the data can be moved around with ease. It was initially employed as the primary storage device, but it is currently used for data transmission of data from one computer to another. It requires a floppy drive for its operation. It is not reliable as hard disk because the data storing surface is exposed. So, it may be affected by dust particles and magnetic field.

Optical Memory:

The best substitute for magnetic storage is the optical disk. The light beam is used by optical memory to function. It was created during the fourth computer generation. It is mostly used to carry data as well as store and backup audio and video. For it to function, an optical drive is necessary. Compared to hard drives and flash memory, it reads and writes data more slowly.

Advantages:

• Optical disks cost less.
• Data can be retained for a longer time.
• Because of their compact size and light weight, they are manageable and portable.

There are following types of the optical memory:

• CD-ROM: Compact disk read-only memory is what it is called. It can hold 700 MB of data, or roughly 90 minutes of normal music. A rigid round plastic with an aluminum alloy coating is contained on one side of a CD. This metal preserves data. An additional thin plastic coating serves as protection. CD needed a CD drive to function. Large amounts of static data, such as those found in encyclopedias, directories, online databases, and multimedia program archives, are best stored on CD-ROM.
• WORM: The phrase means "Write Once, Read Many." It has a similar appearance to a CD-ROM, except frequently they are gold rather than silver in color. These disks can be used by the end-user company to write its own content, often for storing static graphic or photographic pictures. A magnetic substance that can switch magnetic polarity once when heated coats the disk surface. A strong laser beam warms tiny areas in the magnetic medium that enable it to accept magnetic patterns for data storage. By illuminating the magnetic layer with a weaker laser beam and observing the reflected light, data can be read. In place of microfilm, WORM is utilized to archive digitized document images. A disk access time is 150 to 250 milliseconds and has a data transfer rate of 262 Kilobytes per second. A WORM disk may have a capacity of 2 Gigabytes.
• VCD: This signifies a video CD. It is a compact disk format with a 74-minute video limit. Additionally, VCD stores the audio data related to the video pictures.
• CDRW: For Compact Disk Rewritable, it is an acronym. The user may read and write data on this drive numerous times. A CDRW drive can play a standard CDROM disk.
• DVD: It can store anything from 4.7GB to 17GB. Although it has a similar size and structure to a CD, its storage capacity is different because of different chemical components, and data is compressed before being stored. For it to function, a DVD drive is needed. DVD read/write speeds are slower than CD read/write speeds. Movies and multimedia programs with a lot of video and graphics are stored on DVDs.
• BD: Its acronym is Blu-ray disk. It offers a 25GB to 50GB storage capacity. For it to function, a BD drive is necessary. Its dimensions resemble those of CDs and DVDs.
• Erasable Optical Disks: The erasable optical disks (EOD), which are essentially magneto-optical disks, can be written onto several times. When compared to a transmission rate of 1 Megabyte per second, the access time is slower, at 48 milliseconds. Large read/write heads with a laser and a magnetic coil are used. They have large capacities for PC configurations; each cartridge has many Gigabytes of removable storage as opposed to permanent magnetic disks.

Virtual Memory

The computer has many kinds of secondary disk storage and primary memory. While main memory is expensive and has a small amount of storage, secondary disk memory is inexpensive and has a lot of storage space. Therefore, we must employ some secondary disk memory as a main memory in order to process the data. It is known as virtual memory.

There are two primary benefits to using virtual memory:

• To give users access to a lot of addressable memory without being constrained by the capacity of the physical main memory.
• To make it possible for multiple users to efficiently share memory space in a multi-programming system.

Reference:

Ghishing, Er.Ashim. Computer Science. Kathmandu, Nepal: Benchmark , 2008.