Memory and Storage Devices:
Memory refers to the internal storage areas in a computer. Eight bits make up a byte. One byte is required to store a character or a symbol. Memory capacities are measured in terms of Kilobyte (K or KB), Megabyte (M or MB), and Gigabyte (G or GB). A Kilobyte is equal to 1024 bytes or characters. A megabyte is equal to one million bytes and a gigabyte is equal to one billion bytes.
The programs and data which are stored in files are held by the storage devices. To work with the contents of a file - the computer (CPU) transfers the file to its memory, which is a temporary workplace. Although using a large storage area is technically possible, it is not economically feasible.
Some storage devices can retrieve (read) recorded data as well as record (write) new data while others cannot. Such devices which perform both the functions are called read/write devices. For example, a CD-ROM drive is a read-only device as it can only read recorded data while a CD Read/Write drive can write as well as read from the disc.
Storage devices can also be categorized by the manner in which they access the requested data. They can be classified as sequential storage devices or random access storage devices. In case of a sequential storage device, such as a magnetic tape, the computer follows a fixed sequence in order to access the required file. These devices are very slow and inexpensive. On the other hand, a random access storage device can directly access the required file. These devices are faster. Hard disks, floppy disks, and CD-ROMs are random access storage devices.
There are two types of storage devices in computers. These are primary memory and secondary storage devices.
Primary memory:
Primary memory is used for storing data which could probably be used for current purposes. This memory is accessible to the CPU of a computer system and does not require any input or output channels. Primary memory is also called the internal memory of the computer. Access to the contents of the primary memory is faster than the access to data stored in secondary storage media. Also, many types of primary memory are volatile in nature, i.e., the entire data may be lost during power failure. The various types of primary memory media are:
Random Access Memory (RAM): The main memory or the primary memory of the computer is referred to as the Random Access Memory (RAM). It is possible to randomly select a location where data and instructions can be directly stored and retrieved from. Compared to Read-Only Memory, RAM is much faster and can read as well as modify data or information. This is the reason why RAM is also referred to as read/write memory ¾ the information can be read from a RAM chip and can also be written onto it.
Each memory location has an address called the memory address. Using this address, the processor can store and retrieve data by directly going to a single location in the memory. Most of the personal computer memory chips are Dynamic RAM (DRAM). They are volatile in nature and data is lost if there is a power failure. Most printers also have a memory which is used to temporarily store any information related to printing.
Cache memory: Cache is a high-speed memory. This memory can be a part of the main memory or a completely independent high-speed storage medium. It increases the speed of processing by making current programs and data available to the CPU at a rapid rate. Although cache memory is much faster than RAM, it is much more expensive. Generally, the capacity of cache memory does not exceed 512 KB. But it greatly improves the computer system's overall performance. Cache memory can be accessed more easily and quickly by a microprocessor than the RAM. A microprocessor uses the cache memory to store frequently accessed program instructions and data.
There are two types of cache memory: primary cache and secondary cache. Primary cache or level 1 (L1) cache is included in the microprocessor chip. Secondary cache or level 2 (L2) cache is included on a separate printed circuit board. To improve the secondary cache performance, microprocessors are provided in plastic modules that contain a special type of secondary cache called the backside cache. Performance can be improved by keeping the secondary cache as close as possible to the processor.
Read-Only Memory (ROM): In ROM, the information is permanently stored. Compared with RAM which is read/write memory, ROM can be used only to read. This is because ROM is mainly used for storing system level programs which need to be there in the computer system throughout. The instructions to start the computer are stored in ROM chips which are non-volatile. These chips are manufactured with permanently stored instructions. For instance, the instructions to start the computer are placed on a special chip known as ROM BIOS (Basic Input/Output System) chip. To allow ROM upgrades, most computers use flash memory, also called flash BIOS.
The most important features of ROM are permanence and security. The data stored in ROM is permanent in nature and therefore ROM is called a non-volatile storage medium. For instance, ROM can be taken out from a computer system, stored for an uncertain time period, and then replaced. Even then the data in the ROM will still exist. Modifications cannot be made in ROM. This is the reason why ROM is considered to be a highly secure storage medium.
Registers: As the CPU interprets and executes instructions, there is movement of information between the various units of a computer system. In order to handle this process smoothly and speed up the rate of transferring information, computers use a number of special memory units called registers. Registers are temporary memory units that are used for storing words. The registers are part of the processor which enables faster access and storage. These registers are not considered as part of the main memory and are used to retain information on a temporary basis. There are several types of registers, each designed to perform a specific function.
Secondary storage media:
Secondary storage media is used for storing data meant for use in the future. It is not directly accessible to the CPU of a computer system and requires the input and output channels. Secondary storage media is also known as external memory. This medium is non-volatile in nature and is slower than the primary storage media. It is best used for storing information during a power loss. The various secondary storage media are discussed here.
Disk drives: Disk drives are the most widely used medium for storage for all types of computers. These disks are coated with magnetically sensitive materials. As these devices are random access devices, they are reasonably fast. They enable the computer to both read as well as write data onto them. The hard disk drives and the floppy disk drives are the most familiar type of disk drives.
A magnetic disk should be formatted before being used for storage. During the process of formatting, the ‘read/write head' lays down a magnetic pattern on the surface of the disk. This pattern enables the disk drive to store data in an organized manner. Formatting is one of the basic tasks of the operating system of a computer and is accomplished by a formatting program.
Data is recorded on the disk in concentric circular bands called tracks. Each track is divided into pie-shaped wedges called sectors. Two or more such sectors combine to form clusters. Once formatting is complete, a file allocation table is created. When the disk is later used to store program and data files, this table keeps vital records that show exactly where a given file is stored.
RAID: RAID is an acronym for Redundant Array of Inexpensive Disks. It is a data storage medium which makes use of multiple hard drives to share data among the various drives. Several drives are grouped together for a variety of purposes. The conglomeration of drives can be used to provide redundancy if one of the drives fails. Several drives can be grouped together and configured to the computer system as one large drive. The grouping of drives together with various RAID levels can yield better performance features.
The individual drives can contribute more as a member of a RAID set. Each drive contributes to the total throughput of information as data being read from or written to the drives is placed across all the drives in the set. This allows a more efficient utilization of the available bandwidth and a higher aggregate throughput. RAID provides enhanced data integrity, throughput, capacity, and also has increased fault-tolerance capabilities compared to the single drives.
Floppy disks: Floppy disks are circular plastic disks that are available in diameters of 3½ inches, 5¼ inches, and 8 inches. The most common type is a floppy disk with a diameter of 3½ inches. These disks are coated with a magnetically sensitive film. A sliding metal shutter protects the disc from fingerprints, dust, and dirt. Floppy disks contain a write-protection head that can be opened to protect data from being overwritten or deleted. This also enables the floppy to switch to the read-only mode. Compared to the hard disks, floppy disks are portable and less expensive. However, they have slower access rates and less storage capacity.
Hard disks: Hard disks are magnetic disks which store the computer operating system and applications that provide space for the user's data. Hard disks work like floppy disks but store comparatively more data and work faster. The magnetic read/write heads move across the surface of the disk which is coated with a magnetically sensitive material. It contains two or more vertically stacked platters[5], each with its own read/write head. All the read/write heads are connected together in such a way that they cannot move independently. Each platter consists of an equal number of tracks. The location at which all the tracks are situated across the platters is called a cylinder.The storage capacity of the hard disk is measured either in mega bytes or giga bytes. A disk can store 8.4 GB, i.e., 8,400,000,000 bytes or more than two million pages of printed text. In most hard disks, the rapidly spinning platters are enclosed in the drive case and cannot be removed. Though hard disks that use removable disk cartridges are available, they are not as fast and cannot store as much data as one-piece hard disks. Such hard disks can be used for backup procedures to archive less frequently used data files and exchange large data files with other computer users.
Magnetic tapes: Magnetic tapes were once the most commonly used medium for storage. These tapes store data in a sequential manner. They are used for storing huge amounts of data which is not accessed frequently.
Optical discs: Optical discs are created by using the laser to burn tiny holes on the surface of the disc to represent digital data. An optical drive reads this data by focusing a laser-generated beam of light on the disc's surface. The drive detects the light fluctuations caused by the holes and translates them into data that the computer can use. The different types of optical discs are CD-ROM, Recordable CD (CD-R), CD-RW (Rewritable), DVD-ROM, and Magneto-Optic (MO) disks. CD-ROM discs are pre-recorded discs and cannot be altered. They are read-only and can be used to store large amounts of data at low cost. They can store up to 650 MB of data. Among all the commonly various types of optical discs, the DVD-ROMs are capable of storing huge amounts of data. However, Blu-ray disks and HD-DVD disks are now available with greater storage capacity.
USB pen drives: A USB pen drive (pen drive) is a portable device used for transferring files between USB (Universal Serial Bus)[6] compatible systems. It takes the advantage of a USB connection which enables a large number of different devices to be attached to a single machine without any additional requirement for drivers or software. The pen drive also has the ability to operate at an efficient speed.
A pen drive is a convenient option for transferring files and storage. Pen drives are shock-proof, dust-proof, have no moving parts, and need no batteries. The pen drive is available in capacities that range from 32 MB to 1 GB or more. Pen drives can be used with standard operating systems such as Mac OS, Windows Me/2000 and Linux; there is no requirement of any additional software for ensuring compatibility for these operating systems.
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