Read and translate the text. "Seeking" for Tracks

"Seeking" for Tracks

Before data blocks can be read or written, the read/write head mechanism must be moved to the correct track. The read/write head contains the coil that detects or induces magnetism. It is moved by a stepping motor that can align it accurately over a specific track. Movements of the read/write heads are, in computer terms, relatively slow – it can take a hundredth of a second to adjust the position of the read/write heads. (The operation of moving the heads to the required track is called "seeking"; details of disk performance commonly include information on "average seek times".) Once the head is aligned above the required track, it is still necessary for the spinning disk to bring the required block under the read/write head (the disk controller reads its control information from the blocks as they pass under the head and so "knows" when the required block is arriving). When the block arrives under the read/write head, the recorded 0/1 bit values can be read and copied to wherever else they are needed.

The read circuitry in the disk reassembles the bits into bytes. These then get transferred over the bus to main memory (or, sometimes, into a CPU register). Disks may have their own private cache memories. Again, these are "hidden" stores where commonly accessed data can be kept for faster access. A disk may have cache storage sufficient to hold the contents of a few disk blocks (i.e. several thousand bytes). As well as being sent across the bus to memory, all the bytes of a block being read can be stored in the local disk cache. If a program asks the disk to read a block of data that is in the cache, the disk unit doesn't need to seek for the data. The required bytes can be read from the cache and sent to main memory. Commonly, hard disks have several disk platters mounted on a single central spindle. There are read/write heads for each disk platter. Data can be recorded on both sides of the disk platters (though often the topmost and bottommost surfaces are unused). The read/write heads are all mounted on the same stepping motor mechanism and move together between the disk platters.