1. The combination comprising: a disk drive including a recording disk rotatable about its axis, an electromagnetic read/write head, actuator means for moving said head with respect to the axis of rotation of said disk, servo control means responsive to a set of instructions for controlling the speed and direction of movement of said head by said actuator means, and programmed data processing means including a microprocessor, first memory means for storing microcoded control programs for said disk drive, second memory means for storing some of said set of instructions for said servo control means therein, an address bus for supplying address signals from said microprocessor to said first and second memory means, and a data bus for supplying program instructions from said first memory means to said microprocessor, and for supplying said some instructions of said set of instructions from said second memory means to said servo control apparatus; and an excerciser for said disk drive including means for developing test track address signals, means capable when enabled of supplying said test track address signals onto said data bus for application to said microprocessor, third memory means for storing microcoded diagnostic programs for said excerciser, first decoder means coupled to said address bus and to said third memory means for decoding an address signal on said address bus in order to generate a new address signal for said third memory means, and means for supplying program instructions accessed from said third memory means in response to said new address signal onto said data bus for application to said microprocessor for initiating disk drive diagnostic operations.