Hard disk stores data changes principle implementation according to report, magnetism. Hard disk by or the material such as the metal that plating of a few surfaces has magnetic material or glass dish piece and dish the magnetic head of an installation of two sides place and corresponding control circuit are comprised (graph 1) , among them dish piece with magnetic head sealed in the metallic carapace that does not have dirt.
When hard disk works, dish piece rotate in order to design rotate speed high speed, the setting is in radial below circuit control shift reachs dish of magnetic head of a surface appoint the position to store or read data next take. When the system keeps data to hard disk, in magnetic head " keep data " voltaic generation magnetic field makes dish condition happening changes an exterior magnetism material, still can maintain after writing voltaic magnetic field to disappear, such data came down with respect to memory; Read data from inside hard disk when the system when, magnetic head is passed dish piece designate area, dish magnetic field makes magnetic head generation faradic or coil impedance produces an appearance change, after circuit is handled related classics reductive into data. Because this wants only can will dish piece finishing gets more flowing, magnetic head to be designed more accurately and rise as far as possible dish a rotate speed coming back degree, can build a capacity bigger, read write the hard disk with data rapidder rate. Because,this is dish smoother, rotate speed jumps over a finishing to be able to make magnetic head leaves quickly more dish a surface is closer, rise read, keep sensitivity and rate; Magnetic head designs Yue Xiaoyue nicety to be able to make magnetic head is in dish piece on take up the space is jumped over small, make magnetic head is in a piece dish piece on build more tracks in order to store more data.

2, the logistic structure of hard disk.
Hard disk by a lot of dishes piece (Platter) composition, every dish piece every face has a read/write head. If have N dish piece. Have 2N face, corresponding 2N magnetic head (Heads) , from 0, 1, 2 begin number. Every dish piece be differentiated a certain number of homocentric round track (on logic, it is sightless. ) every dish piece differentiate regulation is same normally. Such every dish piece the cylinder that the homocentric circle that radius all is worth R to secure formed to be an axis with electric machinery main shaft on logic again (Cylinders) , from outside come li of number for 0, 1, 2... every dish piece every track that go up is differentiated to be area of a few fan again (Sector) , normally the capacity is 512byte, it is according to certain and regular number 1, 2, 3... form Sector of × of Cylinders × Heads fan area. These 3 parameter are the physical parameter of hard disk namely. A lot of practice below us need to understand the significance of these 3 parameter deeply.
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