MAGNETIC DISKS

 

magnetic disk

A disk read and written by electromagnetic means

hard disk

A magnetic disk with a rigid substrate

floppy disk

A magnetic disk with a flexible substrate

fixed disk

A disk drive with non-removable media. 

cylinder

The set of tracks of a disk drive which can be accessed without changing the position of the access arm. 

track

The (circular) area on a disk platter which can be accessed without moving the access arm of the drive. 

sector

A fixed size physical data block on a disk drive. 

seek

To move to a specified location in a file.

block

A physical data record, separated on the medium from other blocks by inter-block gaps.

interblock gap

An area between data blocks which contains no data and which separates the blocks. 

access time

The total time required to store or retrieve data.

seek time

The time required for the head of a disk drive to be positioned to a designated cylinder. 

rotational delay

The time required for a designated sector to rotate to the head of a disk drive. 

transfer time

The time required to transfer the data from a sector, once the transfer has begun.  

 

 

 

 

Magnetic Technology

 

• Data is stored on a magnetic disk by controlling the direction in which small areas of the disk surface are magnetized.
• Data is stored on a disk serially, that is, one bit at a time. 
• Data is recorded on magnetic disks as a series of magnitized areas on the surface of the disk:
  
• The data is read by a head with a coil which is sensitive to changes in magnetism on the data track as the disk rotates. 
  
• The heads are attached to a common shuttle, which moves them in and out, to different redial positions, together:
  
• The data is blocked into physical sectors, which are arranged in many concentric circles called tracks.  For hard drives, the sectors are all the same physical size, and the number of sectors varies with the circumference of the track
  
• For floppy disks, there are the same number of sectors in each track, and the physical size of a sectors varies with the circumference of the track:
  
• Track and cylinder locations are determined by the physical geometry of the drive. 
  
• Track and cylinder numbers begin with 0. 
• Tracks are often referred to as heads. 
  
• The sectors on each track are numbered from 1 up:
  
• The block size of magnetic disks is almost always 512 bytes.:
• The blocks, or sectors, are separated by interblock gaps. 
• Fixed disks are always hard disks. 
• Removable disks are usually floppy disks. 
• Accessing a sector on the drive requires three stems:
  1. Seeking: Moving the head to the right cylinder.
  2. Rotation: WAaiting for the right sector to reach the head.
  3. Transfer: WAaiting for the sector to pass under the head, reading or writing the data.
• Seek time is affected by the size of the drive, the number of cylinders in the drive, and the mechanical responsiveness of the access arm.
• Average seek time is approximately the time to move across 1/3 of the cylinders.
• Rotational delay is also referred to as latency.
• Rotational delay is inversely proportional to the rotational speed of the drive.
• Average rotational delay is the time for the disk to rotate 180°.
• Actual transfer time may be limited by the disk interface.
• Transfer is inversely proportional to the rotational speed of the drive.
• Transfer time is inversely proportional to the physical length of a sector.
• Transfer time is roughly inversely proportional to the number of sectors per track.
• Data is always read or written in complete blocks.