1. Time slicing apparatus for a disk drive comprising: first memory means for storing microcoded program routines and subroutines for carrying out predetermined disk drive operations, one of said routines being a main program routine that is comprised of a plurality of service routines, at least one of said service routines including a plurality of service subroutines; second memory means including at least one register respectively associated with said at least one service routine for storing the addresses of locations in said first memory means at which the program instructions for said at least one service routine are stored; a data processor coupled to said first and second memory means and including means for executing said plurality of service routines in sequence during a respective plurality of time slots of the main program routine processing cycle, means for executing said plurality of service subroutines in sequence with only one being executed each processing cycle during the time slot of said at least one service routine, and means including a program counter for supplying first address signals to said first memory means and second address signals to said second memory means; means responsive to said first address signals for accessing program instructions in said first memory means and forwarding said accessed program instructions to said data processor for execution; means responsive to said second address signals for loading the contents of said at least one register into said program counter during the time slot of said at least one service routine; and means for changing the contents of said at least one register each processing cycle to the address of the first program instruction in said first memory means for the service subroutine next in sequence to be executed.