The evolution of the disk controllers that interface disk drive storage devices and subsystems with their hosts is reviewed. With magnetic disk storage density increasing by over five orders of magnitude in 37 years, the nature of the controller and its functions has undergone a significant change. In the 1960s, introduction of the IBM 360 and 370 systems led to architectures oriented to mainframe systems. In the 1980s, the personal computer brought into being the single-board controller most frequently attached to a single hard disk drive. In the 1990s, the functions that are being included within the drive are radically changing the attachment of a drive to a system.>

This review article attempts to give a perspective on the evolution of magnetic recording storage devices, highlight the frontiers of current activity, and address the future of this storage technology. Tape, disk (both rigid and flexible), and mass storage systems are covered. It is of interest to note that digital magnetic recording spans not only the twenty-five year history of the IEEE Computer Society which this issue is recognizing but has during this whole period undergone continual technical advances and widespread growth in usage. It is fair to say that magnetic recording is among the most important technologies in the history of the computer age to date.

The merit of any high density detection method is ultimately dependent on the ``resolution'' characteristic of the magnetic recording components. Justification of readback waveform synthesis through ``single pulse'' superposition is given. A comprehensive, yet general readback simulation program is described which will automatically, for any characteristic pulse, simulate all possible readback signal patterns and test them for specified reading logic as a function of bit density. Amplitude, phase, peak, etc., sensing are compared and the influence of parameter variation on performance indicated. Good correlation with experiment has been realized and has greatly reduced time at the bench. The significance of pulse waveform is clearly revealed and this study has provided a guide to head design (ring and probe), permitting the optimization of a total recording system for high-density storage.

The nonreturn-to-zero method for the magnetic recording of binary information saturates the surface in a continuous fashion, one sense of saturation being used for a ``1'' and the opposite sense of saturation for a ``O.'' A system is described for reading NRZ digital data which utilizes the inherent alternating characteristic of the readback signal waveform to effect a logical correction. The reading system corrects the ``normal'' sources of error which limit the usable bit storage density and set the degree of reliability. The system employs only amplitude sensitive waveform operators and samples the readback signal only at the regular clock times.