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Computer Memory or Storage Devices

Memory is the most essential part of the computer. We need memory to store the data. This storage enables us to use the stored data to in future. Computer memory is mainly divided as primary and secondary memory. The memory and storage devices are measured in terms of bits and bytes.

In last post we have already seen the information about the primary memory and the difference between static RAM and dynamic RAM
.
Secondary memory is also called Auxiliary memory, backup storage, bulk storage or mass storage of programs, data and other information.  Secondary device facilitate storing of data and instructions permanently. The size of the secondary memory is normally large and the cost is also reasonable. Some of the secondary storage devices are magnetic tape, floppy disk, hard disk, CD, DVD, etc
o   Magnetic tape:
Magnetic tapes appear similar to the tape used in music cassettes. It is a plastic tape with magnetic coating on it. The magnetized portion of the surface refers to the bit value”1” where as the demagnetized portion refers to the bit value “0”.
Magnetic tapes are very durable and can be erased as well as reused. They are inexpensive and reliable storage medium. They are not suitable for data files. Tapes are also slow.
o   Magnetic Disks: Magnetic disks are thin circular plastic plate on which some magnetic material is coated. It come in various sizes and materials. They use the properties of magnetism to store the data on a magnetic surface. A disk pack consists of a number of such disks mounted on a central shaft. The central shaft rotates at speeds of about 7200 revolutions per minute (rpm). In a disk plate information is stored on both surfaces. The surface is further divided into a number of individual concentric circles called as tracks. The tracks are further divided into various sections called as sectors.
The main advantage of such devices are high storage capacity, reliable in use and allow direct access to data. There are two types of magnetic disks
                         I.          Floppy disk:
A floppy disk is a round, flat piece of Mylar plastic coated with ferric oxide and encased in a protective plastic cover. It is a removable disk and is read  and written by a floppy disk drive(FDD).
When a user inserts a disk into the FDD, the drive grabs the disk and spins it inside its plastic jacket. Also the drive has multiple levers that get attached to the disk.
                      II.          Hard disk:
The hard disk is also called the drive or fixed disk, is the secondary storage unit of the computer. A hard disk can be external or internal and can hold a large amount of data. It consists of a stack of disk platters that are made up of aluminum alloy or glass substrate coated with a magnetic material and protective layers. The surface  of a disk is divided into imaginary tracks and sectors. Tracks refers to the concentric circles where the data are stored .sectors  refers to the number of fixed sized areas that can be accessed by one of the disk drives read/write heads in one rotation of the disk. The storage capacity of the disk is determined as (number of tracks *number of sector * byte per sector* number of read/write heads)
o   Optical Disk:
Optical disk is a random accessed, removable disk on which data is written and read through the use of laser beam. There are different types of optical laser disks.
·       Compact Disk(CD):
CD is the most popular and the least expensive data storage medium. CD are the storage medium from which data is read and to which it is written by laser. The information stored on CD is arranged according to certain rules, and is shaped like an "Optical track", called "light rail" in spiral shape. The depression part on the Optical track is called information pit and the flat part is called land. Pit and land are used to record information.
·       Digital Versatile Disk(DVD):
Also called as "digital video disc." With the advances in optical disc technology, It cannot only store video program, but also store music, data, along with increased use, it will be this type of CD-ROM referred to as "Digital Versatile Disc".
·       Blue ray Disk(BD):
Blue-ray Disc, referred as BD.BD disk features are: It consists of a thickness of 1.1mm of the recording layer and a thickness of only O.lmm composite made of a transparent protective layer.

1.    Cache memory:
The cache is a small amount of high-speed memory, present between the primary memory (RAM) and CPU (processor). Its access time is much less compared to that of the main memory. It is an intermediate memory and is not accessible to users. It stores instructions and data, which are to be immediately executed. It is used to reduce the average access time reading data, which are stored in main memory. Thus the cash memory increases the operating speed of the system. But it is mush costlier than main memory. There are two levels of cache memory
 L1 and L2, L1 cache memory will present inside CPU, where as the L2 cache will be present on the mother board. L1 cache is very fast since it runs at a speed of processor. L2 cache is larger but slower in speed than L1 cache.

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