Example: Disk Device Driver

• only part of operating system that knows about disk controller:
  • how many registers, sectors, tracks, cylinders
  • arm motion, motor drive speed, head setting time
• operating system has to maintain separate data for each instance of a device
• sector id: unique identifier for a sector of data on the disk pack
  
• a sector is identified by a cylinder number c, a track number t, and a sector number s

• Rather than using the three numbers (c, t, s) to identify a sector of data, they are combined to form a sector id (also called a block number)

SectorId = (c TracksPerCylinder + t) SectorsPerTrack + s where:
c = cylinder number
t = track number within the cylinder
= platter surface number
s = sector number within a track