Wednesday, April 11, 2012

What is Magnetic disc head ?

Magnetic disc head is the key component for data reading and writing. Its main function is to transfer the magnetic information stored in disk into electrical signals and send them out. One disk usually has 1 or 2 heads. When it is difficult to enlarge capacity of single disk, the number of disks is increased to largen disk capacity, and two or more heads are needed in this operation.
Brief Introduction to Head
The function of head is to transfer magnetic electricity. The cost of head takes up 40% of total disk cost. If capacity of single disk is broken through, technology of head must be improved. Generally, one disk has only one head. But the newest technology can make 2 heads read one disk at the same time. And the maximum heads can be set to 255 in LBA mode. Generally, the value of heads is recorded in BPB of DBR. Its length is 2bit and offset is 1A .As the following picture shows.

Magnetic disc head is the key component for data read and write. Its main function is to transfer the magnetic information stored in disk into electrical signals and send them out. Its working principle is that Resistance of Special materials can be changed along with the changes of magnetic field and these changes are applied to read and write data in disk. The property of head dramatically decides the storage density of disks. Now GMR( Giant Magneto Resisive) is commonly used. Materials with better magneto-resistance effect and multilayer structure are applied in GMR head. So it is more sensitive than traditional head and MR(Magneto Resisive). In relative changes of magnetic field, it can cause bigger changes of resistance and realize higher storage density.

CHS Algorithm
In the old structure of disk storage, CHS(Cylinder/Head/Sector)structure system was used. A long time ago, when the capacity of hard disk storage structure was very small, the structure similar to floppy was turned out to be structure of hard disk storage. Every track in disk contains the same amounts of sectors and this produces 3D parameters (Disk Geometry including Heads, Cylinders and Sectors) and the corresponding accessing mode of 3D.

In CHS structure system, heads indicate the numbers of head( disks) in structure of hard disk storage and the maximum number is 255( stored in 8 binary bit); Cylinders indicate the number of tracks in every disk and the maximum number is 1023(stored in 10 binary bit). Sectors indicate the number of sector in every track and the maximum number is 63 (stored in 6 binary bit). And every sector is 512 bytes normally. Theoretically, users can take any appreciative value, however, no other value has been found.
The maximum capacity of s hard disk is:
Or units commonly used by manufacturer of hard disk storage structure:255×1023×63×512/1000000=8414MB(1M=1000000Bytes)
Of course, Heads, Cylinders, and sectors can easily be viewed through the third-party partition magic---MiniTool Partition Wizard

It is shown clearly in the picture that Cylinders are 9729, heads are 255 and Sectors are 156299375. And the information such as number of tracks and total size of disk can also be known without complicated calculations. 

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