1. An optical disk apparatus comprising: an optical pickup having a photo-detector; means for generating a servo error signal in response to an output signal from the photo-detector; a digital signal processor for executing processing of signals including the servo error signal; first and second registers within the digital signal processor, the first registers exclusively allocated to storage of data utilized in execution of servo error signal processing, the second registers allocated to storage of data utilized in execution of processing of signals other than the servo error signal, at least a portion of the data utilized in the execution of the servo error signal processing being stored only in the first registers; means for generating an interrupt signal and for supplying said interrupt signal to the digital signal processor, the digital signal processor executing servo error signal processing in response to the interrupt signal; and servo means for controlling the optical pickup in response to the processed servo error signal.