What is Magnetic Memory? How Magnetic Storage Mechanism Works?
Magnetic Memory:
The limitations of primary storage devices like limited data storage capacity, high cost and volatile nature, some inexpensive data storage devices are needed to be used as additional memory, that has larger data storage capacity, low cost and is, non-volatile in nature. All these features are found in magnetic memory. The basic types of magnetic memory used are:
- Magnetic Disks: Hard Disks and Floppy Disks.
- Magnetic Tapes: Magnetic tapes are cheaper and provide a good choice of secondary storage.
Storage Mechanism:
Magnetic disk drives contain both side coated metal disks. These disks are rotated at a high speed to read or write data. The tracks are circular and electromagnetic read or write heads are positioned by access arms between the slightly separated disks. Data is saved on these tracks in binary form. The storage capacity is really high which can store every type of data be it text, sound, picture, etc.
Magnetic disks can be used as fixed unit or removable unit. Normally removable are used for keeping backup of the data to be on the safer side.
Data Organization:
The surface of a magnetic disk is divided into concentric tracks. These tracks are used to record data on. Each track is further divided into sectors. Each sector normally holds same capacity.
Hard Disk Drives (HDD):
Hard disk is a non-volatile, random access storage device. It is capable of storing variety of data or programmes. These are available permanently to the computer and thus hard disks are also called on line storage devices. Another name for hard disks is Winchester Disk. It is also called platter. Multi platter configuration is used to increase the data storage capacity.
Storage Organization in HDD:
There are rotating platters on a motor-driven spindle. The read and write heads that float on a film or air above the platter read and write data on the disk. A disk is rotated by a motor. It is mounted on the disk drive. The set of tracks which are there on the same diameter is called a cylinder.
The disk can be divided into many drives, specifically called the disk drives. These are controlled by the disk controller. The function of the disk controller is to receive commands from the computer and positions the read/write head of the specified disk for reading or writing. The drive number, cylinder number, surface number, and sector number must be specified for writing or retrieval.
Access time on a magnetic disk:
The process of reading data from the magnetic disks but writing on it is done sequentially. The desired disk address has to be specified by specifying track number, surface number and the sector number to fetch the data. The process of writing starts from the beginning of a sector and reading is done from the back. Random access makes the speed fast. The role of the disk controller is. to accept command read or write, then the read or write heads are positioned onto the specified address.
Seek times:
The time needed to position these heads on proper track is known as seek time. This name is taken by the process of seeking the data or information. The seek time Ts depends on its position at the time of receiving the command of read or write.
If it is on a track which is to be read or written onto then the time will be minimum and if the far off track is to be reached then it will take more time. This happens in movable head systems where the heads can move but in a fixed head system the time is always-negligible because there is a head for each track.
Latency time or Search time:
Only positioning of the read write heads on the desired’ track is not enough, in order to follow command of read or write it has to fetch the desired data. The heads even after coming to the desired track, has to wait to come the specified data address under its head as the disk is rotating continuously.
This rotational waiting time called the latency time or search time. It depends on the initial positioning of the head and the rotational speed of the disk. If the speed is slow then the time will be higher. The above discussion tells that the total access time is a sum of seek time and latency time.
Pen drive:
Pen drive is a removable storage device which can be easily carried and is very reasonable in terms of price. Its data storage-capacity is really high. It comes in various storage capacities ranging from mb to 256 gb. It resembles a pen, hence it is called a pen drive. It has a USB ( Universal Serial Bus) interface which connects to the computer in seconds and allows data transfer.
Magnetic tapes:
Magnetic tapes are secondary storage devices. These are portable, removable and easy to carry. These can be used to save a large amount of data on a single disk and the cost is also reasonable. These do not have a very high speed but the benefits are many hence this disadvantage can be overlooked. Basically these are used to keep backups and to share the data or software. The storage capacity may vary. This can be calculated as:
Data recording density X length:
Here density means the amount of data which can be stored on a given length of the tape.
