[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Construction and Operation of the Hard Disk | Hard Disk Logic Board ]

Control Circuitry

The drive's internal logic board contains a microprocessor and internal memory, and other structures and circuits that control what happens inside the drive. In many ways, this is like a small embedded PC within the hard disk itself. The control circuitry of the drive performs the following functions (among others):

• Controlling the spindle motor, including making sure the spindle runs at the correct speed.
• Controlling the actuator's movement to various tracks.
• Managing all read/write operations.
• Implementing power management features.
• Handling geometry translation.
• Managing the internal cache and optimization features such as pre-fetch.
• Coordinating and integrating the other functions mentioned in this section, such as the flow of information over the hard disk interface, optimizing multiple requests, converting data to and from the form the read/write heads require it, etc.
• Implementing all advanced performance and reliability features.

The control circuitry of the drive is underrated and misunderstood, even by those interested in hard disk performance issues. The reason is that the quality or optimization level of the control circuitry doesn't manifest itself as a single, simple specification. You can't easily compare the circuitry of five different drive families. Most hard disk manufacturers provide very little information about the "guts" of the logic board, and even if they did, most people wouldn't know what to do with the information (including myself).

In fact, differences in control circuitry account for part of the differences in some specifications. This is probably most true of seek performance, as discussed here. Beyond this, you can't really tell much about what's inside the circuitry. However, if you use two different drives that have very similar specifications and run on the same interface on the same PC, but one just "feels faster" than the other, differences in their internal circuitry may be part of the answer.

Next: Sense, Amplification and Conversion Circuits

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