This invention relates to control of operation of magnetic disk drives of the type in which the disk is divided into "hard" sectors; that is, one in which the format of the disk is permanently defined at the time of manufacture of the disk. In particular the disk is divided into sectors by radial index marks followed by information defining the various tracks extending circularly around the disk. Each track is thus divided by the index mark into a plurality of sectors. Within each sector there are various permanently written data used for identification of the track and of the sector and for, e.g., timing of the data read and write operations as well as areas left for the actual storage of data on the disk. In order that the head can be properly controlled to read and write the data at the appropriate time, a format sequencer is required in order to keep track of the position of the head with respect to the disk. That is, to provide a signal indicating that, for example, the head is currently juxtaposed to the data field. Clearly, it would be desirable that such a format sequencer be capable of performing as many operations with as little hardware as possible. Further, it would be desirable if such a format sequencer could be used as well as for actual format sequencing, for example, for data read and write control.