Data storage device

Data is processed in memory. Data has to be moved from and to a direct access device e.g. hard disk.

The hard disk will consist of a number of platters mounted on a rotating spindle. Each platter will be separated from each other sufficient to allow the read/write heads of the access mechanism to move between them. This is required to allow the heads to access the tracks and sectors of the cylinders. All the read/write heads move in unison.

Each platter will have a number of concentric tracks - e.g. 2000 tracks.

Each track will be subdivided into a number of sectors - e.g. 8

The data on a sector is the minimum amount of data transferred from disk to memory.

If sectors are grouped together for a larger data transfer, they form a cluster.

The corresponding tracks from all platters accessed without moving the access mechanism form a cylinder. If all the platters each have 2000 tracks, the hard disk will have 2000 cylinders regardless of how many platters form the hard disk.

All surfaces of a cylinder can be accessed simultaneously.

If we know the number of recording surfaces, cylinders, sectors per track and size of the sectors we can work out the overall size of the hard disk.

10 surfaces
2000 cylinders
8 sectors per track
4K per sector

Works out to:

(10 x 2000 x 8 x 4)K = 640000K = 640Mb

Accessing data on a storage device

Data access time is a combination of seek time, rotational delay and data transfer time.

• Seek time - The time taken to move the read/write heads to the correct cylinder containing the required sector/cluster. The further away from the sector the longer the time.
• Rotational delay - The time taken for the correct sector/cluster to rotate around to the access mechanism. The closer the sector the shorter the time.
• Data transfer time - The time taken to transfer the data from the sector to memory. Remember a sector is the smallest amount of data that can be transferred.

It is desirable to store related data as close as possible i.e. in the same cylinder or adjacent cylinders. This will assist in reducing data access time. However, what works for one application may not work for another.

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Stuart Banner