PC Hardware

Storing Data

As noted in previous tutorials, data is stored using binary code. Within the computer's memory, ones and zeroes are stored as electrical impulses. On magnetic media, the ones and zeroes can be stored as either magnetic or nonmagnetic areas on the drive surface. Although there are magnetized and nonmagnetized positions on the hard disk drive, the ones and zeroes of the binary code are stored in terms of flux reversals. These flux reversals are actually the transitions between magnetized and nonmagnetized positions on the hard drive surface.

Early hard disk drives used a method of encoding called FM (frequency modulation). FM technology is based on timing. To differentiate a 1 from a 0, it measures the time the drive head spends in a magnetized state. For FM to work, it requires every 1 or 0 to be preceded by a timing bit. The early FM drives worked well, but all the extra bits added to the work and slowed the process of data transfer. In order to improve efficiency and speed of the data transfer, the FM was replaced by an improved version that reduced the number of timing bits required. This new technology was called MFM (modified frequency modulation). MFM uses the preceding data bit to indicate whether the current bit is a 1 or a 0, thus reducing the number of timing bits by more than 50 percent.

Another method used to place data on hard disk drives is run-length limited (RLL) encoding. RLL replaces the timing bits with patterns of 1s and 0s that represent longer patterns of 1s and 0s. Although this looks inefficient, the elimination of the timing bits speeds overall performance.

Unless you're working with hard disk drives manufactured before 1989, it is not necessary to know which type of data encoding is used.