CS101 Computer Basic
Topic 4: Secondary Memory or Auxiliary Memory
CS101 Computer Basic
A computer has two types of memory – Primary memory and Secondary memory. Primary memory is used to store data and instructions that are currently in use. They are expensive, limited in size, and also volatile in nature. Therefore, it is not suitable for storing a large amount of data over a long period. To overcome all these problems, we need secondary memory.
Secondary memories are often called Auxiliary Memory or Storage. They are used to store enormous data for a long time, even when the computer is turned off. The data stored in secondary memory remain intact until it is overwritten or deleted. Unlike primary memory, secondary memories are permanent and non-volatile in nature. Also, it cannot be directly accessed by the CPU like primary memory. Instead, it is accessed through input-output routines. The data is loaded from secondary memory to primary memory before it is sent to the CPU for further processing. Although secondary memories are inexpensive and slow, they provide a permanent larger storage capacity than primary memory.
For example, in a typical computer system, the average size of RAM is between 8 and 16 gigabytes, but at the same time, the average size of the hard disk is between 1 and 2 terabytes. It clearly shows that the ratio of primary memory and secondary memory is about 1:128.
Secondary Memory is a type of computer memory that is permanent and able to store enormous data for a long period.
Secondary memories are classified as below:
Difference between removable and non-removable storage devices
Removable storage devices are portable plug-and-play devices that can be connected or disconnected to the computer easily. For this, we don’t need to turn off or restart the computer. Mostly, these devices are used to transfer data from one computer to another and for archival purposes. They are also called offline storage or tertiary storage.
Instead, non-removable devices are immobile and placed inside a secured computer cabinet. These devices are used to store data and programs inside a computer. We need to unpowered the computer to connect or disconnect these devices.
Hard Disk Drives (HDD)
Hard disk drives or HDDs are electro-mechanical storage devices usually installed inside the computer system. They have one or more circular disks made of aluminum alloy called platters. Platters are coated with a thin layer of magnetic material. All these platters are stacked on a spindle one over another and housed inside an air-sealed aluminum casing.
The spindle rotates the platters at a very high speed. A magnetic head moves over the spinning platters to read and write data. It magnetizes or demagnetizes the magnetic layer to write the data on it. Modern hard disks are available up to 20 terabytes of capacity.
The hard disk is a mass storage device that usually stores the operating system, software programs, files, etc. While the computer starts, the BIOS looks for an operating system and load it from secondary memory to primary memory.
Solid State Drive (SSD)
Solid state drives or SSDs are alternatives to hard disks. They are also installed inside the computer system. Unlike hard disks, they have no mechanical parts, consume less power, and are lightweight. They are made of electronic components. SSDs have lower latency than hard disks. Therefore the data stored in it can be accessed more quickly.
However, SSDs are more expensive, but a combination of SSDs and HDDs reduce cost while providing better performance.
Optical Disk Drives (ODD)
An optical disk drive or ODD is installed inside the desktop computer cabinet or laptop. Its front side has an ejectable tray or slot to load the optical disks. It uses a laser beam to read and write on circular optical disks. These disks are usually 12 cm in diameter, including CDs, DVDs, and Blu-Rays. Inside the ODD, an electric motor rotates the disk at a speed of 200 to 4000 RPM and, the laser illuminates a spiral data path to read or write data on it. These disks store binary information in the form of pits and lands formed by laser diodes.
Optical disks are commonly used for data backup, audio-video, and installing operating systems. Nowadays, ODD drives are not so popular. It is due to its low storage capacity, slow speed, inconvenient size, and read-only properties. Although it is still in use, most people prefer pen drives, memory cards, and portable hard disks.
Floppy Disk Drives (FDD)
A Floppy disk drive or FDD was also installed inside the desktop computer cabinet. It was used to read and write floppy disks. These disks are obsolete now due to their small storage capacity ranging from 100 KB to 1.44 MB. Floppy disks were a key component of most computers for nearly 30 years. They were discontinued after the optical disk drives became popular. The floppy disk consisted of a thin and flexible magnetic disk that stored the data. To protect the disk, it was kept inside a hard plastic case. It was available in three sizes 8 inches, 5.25 inches, and 3.5 inches.
Tape drives are best suitable for backup or archive a large amount of data for a long time. They used magnetic tapes to read and write data. Magnetic tapes are exceptionally durable and less expensive. An ordinary magnetic tape can hold terabytes of data for up to 30 years. Due to their large storage capacity, they are also very cost-efficient. Their per-gigabyte price is lower than any other storage media. Despite all these advantages, tapes are sequential access storage, so they are slower than other storage devices.
USB Storage Devices
USB drives are portable storage devices that can be connected to the computer through a USB port. They include pen drives, card readers, hard disks, optical disk drives, floppy drives, tape drives, etc. These devices are very convenient for transferring data from one computer to another. Pendrives are the most commonly used USB storage. Due to their small size, they can be easily kept inside a pocket. They are available in various designs and capacities.
Memory cards are typically used with mobile phones, tablets, digital cameras, printers, scanners, and other portable devices. They have a very tiny size and can be connected to the computer using the device itself or a USB card reader. They are also available in a variety of shapes, sizes, and capacities.