A. Storage Organization : A magnetic disk's surface has a number of invisible, concentric circle called tracks. Tracks have numbers consecutively from outermost to innermost starting from zero Number of tracks varies greatly, with low-capacity disks having as few as 40 tracks and high-capacity disks having several thousands tracks.
A magnetic disk's surface also has invisible, pie-shaped segments. Hence, if there are eight such segments, each track has eight parts. Each part of a track is called a sector.
Typically, a sector contains 512 bytes. It is the smallest unit of data access by a disk drive. That is, disk drives can access (read/write) a complete sector at a time. Even if a computer needs to change just one byte out of 512 bytes stored on a sector, it rewrites the entire sector.
When the people refer to the number of sectors a disk has, the unit they use is sector per track- not just sectors. Hence, if a disk has 200 tracks and 8 sector per track, it has 1600 (200×8) sector with each sector having a unique number.
To access a piece of data (a record) from a disk, we need to specify its dark address, which represents the physical location of the record on the disk. It is comprised of sector number, track number, and surface number (when double-sided disks are used).
Often, multiple disks are stacked together as a disk pack to create large capacity disk-storage systems. The disk pack is sealed and mounted on a disk drive consisting of a motor to rotate the disk pack about its axis. The disk drive also has an access arms assembly having separate read/write heads for each recordable surface of the disk pack. Normally, a disk pack goes not use the upper surface of the topmost disk and the lower surface of the bottommost disk because these surfaces get scratched easily. However, modern disk drives use miniaturization and precision components to eliminate this drawback.
All acess arms (on which read/write heads are fixed) of an access assembly for all disk surfaces move together. Hence, if the read/write head serving the 0th recording surface is positioned over the 5th track, each of the heads on the arms serving other recording surfaces is also positioned over the 5th track of respective surfaces. For faster access of data, disk packs use a concept called cylinder for data organization. A set of corresponding tracks on all recording surfaces of a disk pack together from a cylinder. For example, the 5th track of all recording surfaces together from the 5th cylinder of the disk pack. Hence, if there are 200 tracks on each disk surfaces, there are 200 tracks on the each disk surface, there are 200 cylinders in the disk pack. Obviously, disk address of a data record on a disk pack consists of a sector number, cylinder number, and surface number (track number is not required because track number and cylinder number are same). This addressing scheme is called CHS (Cylinder-Head-Sector) addressing. It is also known as disk geometry. Cylinder-based organization achieves faster access of data by avoiding movement of access arms when an application has to process large number of records in sequence. It stores related records of a file on the same cylinder of a disk pack so that in one revolution of the disk pack, the disk drive can read/write all records stored on, say cylinder 5 of all surfaces.