Secondary Memory is the external memory, which is located outside of the computer and this memory is not accessible directly by the CPU. Secondary storage is needed to keep programs in data long-term. It has non-volatile nature i.e. it is used to store data or information and programs on a permanent basis.
Generally, secondary Memory’s storage capacity is very high. It is of low cost and operating speed is also very slow.
For example Magnetic Disc, Magnetic Tape, Flash Memory, Optical storage, etc.
Magnetic tape storage offers a low-cost, high-density storage medium. The storage medium, access electronics, and mechanical components make up a tape unit. A tape player works in a sequential manner i.e. the data on a tape can only be accessed in order.
To get to data on a tape, and I/O program will have to tell the tape unit to rewind the tape and then scan it sequentially from the beginning until it found a match. The addressed data can then be accessed.
Magnetic storage devices have been around since the 1950s, though they’ve evolved greatly over time. A magnetic disk is a circular disk that is coated with magnetic material where data can store on both surfaces of the disc.
It uses the magnetization process to write, rewrite and access data. The data is stored in tracks and sectors within magnetic disks.
Its examples are Hard disc, Zip disc, and floppy disks.
The hard disk is one type of Magnetic disk, Where we store several gigabytes of data. A single hard disk usually consists of several platters. Each platter requires two read-write heads, one for each side. Every track has a unique location and is divided into several sections called sectors.
Platters of hard disks rotate thousands of times in one second. The rotation is measured in RPM (Rotation Per Minute). And normally the speed of the hard disk is from 5200 RPM to 10000 RPM or even more.
In each spinning disk, it has trillions of tiny fragments that can be magnetized to represent bits. Read/write head scans the spinning platters and magnetizes fragments to write digital information onto the HDD, or detects magnetic charges to read information from it.
Floppy disks are a magnetic storage medium for computer systems. It is also called flexible disks or diskettes that can store several million characters of information. It is composed of a thin, flexible magnetic disk sealed in a square by plastic.
It is used to distribute software, transfer files and create backup copies of data.
The main purpose of using floppy disks are,
- Share files between different computers.
- Load new program or software on the computer.
- Store back up of data and small programs to future adaption.
It is slower and has less storage capacity, but portable and less expensive. Initially, floppy disks of 8-inch diameter were introduced, which were then reduced to 5.25 inches (Mini floppy, size 1.2MB) and 3.2 inches (micro, size 1.44 MB).
A Zip disk is the advanced and updated version of the floppy disk developed by Iomega. This disk needed a special drive called the Zip drive.
It is similar to floppy disks, but was slightly larger, thicker, stronger plastic casing and making them easier to store and handle than floppy disks. Zip disks were lightweight, portable, and relied on magnetic storage techniques.
Its storage capabilities range from 100 MB to 250 MB.
It is one type of secondary storage device, where reads and writes data with the help of laser light. It is a flat, usually, circular disc that encodes binary data in the form of pits and lands in a special material (often aluminum). (Pits represent binary value 0 and lands represent value 1) due to the reflection when red. It is also one type of secondary memory.
The most commonly used disks are CD-ROM, CD-RW, DVD ROM, and Blue Ray Disk.
It has not as fast of a seek time as hard drives, but it has many other advantages. Because optical discs are not based on magnetic charges.
The discs are also more durable than hard drives and much cheaper to produce, making them great for backups and for transferring small amounts of data.
A compact disk is a portable storage device. It is a molded plastic disk that contains digital data and is scanned by a laser beam to reproduce captured sound and other material.
The storage capacity usually ranges from 650 to 750MB. Compact disks are of the following types:
CD-ROM Stands for “Compact Disc Read-Only Memory“. A CD-ROM is a CD that can be read by a computer with an optical drive.
Here the data on the disc is “read-only“. This means the data burned/stored on a CD cannot be changed.
CD-RW stands for (Compact Disc-Re-Writable) is similar to CD-ROM except that you can erase the previous content and write on it multiple times.
CD-RWs, as opposed to CDs, it is available for specialized readers that have sensitive laser optics.
The DVD stands for “Digital Versatile Disk” It is a so-called Digital Video Disc. DVDs have much more storage capacity. They used a laser beam for a shorter wavelength. The bottom layer DVD ROM is polycarbonate plastic. In this, tracks are placed closer together.
The storage capacity of DVD ROM is 4.7 to 8.5 GB running on both sides. DVD can store any kind of digital data, software, and other computer files, as well as video programs that are watched using DVD players.
Blu-ray Disk is an optical disc format such as CD and DVD. It has high-capacity rewriting and playing HD video and for storing a large amount of data, program. It is the upgrade and replacement to CDs and DVDs.
Blu-ray discs can hold up to 25 GB of data. With double-sided, dual-layer Blu-ray discs, it can store 50 GB of data in a single disc. Heavy games, 4K movies, etc are distributed using Blu-ray discs.
Flash memory is a non-volatile memory chip used for storing and transferring data between a personal computer (PC) and digital devices. It can be electronically reprogrammed and erased.
It is a type of (EEPROM) often found in USB flash drives, MP3 players, digital cameras, smartphones, tablet computers, and solid-state drives. They are highly portable to carry & reliable than other memory devices.
The capacity of Flash memory may range from 1 GB to 1 TB.