Auxiliary Memory

The most common auxiliary memory devices used in computer systems are magnetic disks and tapes. Other components used, but not as commonly, they are magnetic drums, magnetic bubble memory, and optical disks.

Although the physical properties of these storage devices can be quite complex, their logical properties can be characterized and compared by a few parameters. The important characteristics of any device are its access mode, access time, transfer rate, capacity, and cost.

Magnetic Disks:

A magnetic disk is a circular plate constructed of metal or plastic coated with magnetized material.

Bits are stored in the magnetized surface in spots along concentric circles called tracks.

The tracks are commonly divided into sections called sectors. In most systems, the minimum quantity of information which can be transferred is a sector.

Some units use a single read/write head for each disk surface.

In other disk systems, separate read/write heads are provided for each track in each surface.This type of unit is more expensive and is found only in very large computer systems.  

Disks that are permanently attached to the unit assembly and cannot be removed by the occasional user are called hard disks. A disk drive with removable disks is called a floppy disk.

Magnetic Tape:

A magnetic tape transport consists of the electrical, mechanical, and electronic components to provide the parts and control mechanism for a magnetic-tape unit.

In magnetic tape information is recorded in blocks referred to as records.  

The tape itself is a strip of plastic coated with a magnetic recordingmedium. 

Bits are recorded as magnetic spots on the tape along several tracks. 

Usually, seven or nine bits are recorded simultaneously to form a character together with a parity bit.

Read/write heads are mounted one in each track so that data can be recorded and read as a sequence of characters. 

Magnetic tape units can be stopped, started to move forward or in reverse, or can be rewound.