Mechanically Addressed Systems
Mechanically addressed systems utilize a contact structure ('head') to read and write on a designated storage medium. Since circuitry layout is not a key factor for data density, the amount of storage is typically much larger than for electrically addressed systems.
Since the access time depends on the physical location of the data on the device, mechanically addressed systems may not be "random access" as are electrically addressed semiconductor NVRAM. For example, magnetic tape stores data as a sequence of bits on a long tape; transporting the tape past the read/write head is required to access any part of the storage. Tape media can be removed from the drive and stored, giving indefinite capacity at the cost of the time required to retrieve a dismounted tape.
Hard disk drives use a rotating magnetic disk to store data; access time is longer than for semiconductor memory, but cost per stored data bit is very low. Formerly, removable disk packs were common, allowing storage capacity to be expanded. Optical discs store data by altering a pigment layer on a plastic disk. Read-only and read-write versions are available; removable media again allows indefinite expansion, and some automated systems were used to retrieve and mount disks under direct program control.
Read more about this topic: Non-volatile Memory
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