Each surface is divided into concentric tracks and each track is divided into sectors. The set of corresponding tracks on all surfaces of a stack of disks form a logical cylinder. Data bits are stored serially on each track.

Data on disks are addressed by specifying the surface number, the track number, and the sector number.

In most disk systems, read and write operations always start at sector boundaries. If the number of words to be written is smaller than that required to fill a sector, the disk controller repeats the last bit of data for the remaining of the sector.

During read and write operation, it is required to specify the starting address of the sector from where the operation will start, that is the read/write head must positioned to the correct track, sector and surface. Therefore the address of the disk contains track no., sector no., and surface no. If more than one drive is present, then drive number must also be specified.

The format of the disk address word is shown in the figure. It contains the drive no, track no., surface no. and sector no.

The read/write head will first positioned to the correct track. In case of fixed head system, the correct head is selected by taking the track no. from the address. In case of movable head system, the head is moved so that it is positioned at the correct track.