MAGNETIC DRUM

Like the magnetic disk, the magnetic drum is another example of a direct-access storage device. Although the magnetic drum was once used as main (or primary) storage, it is now used as secondary (or auxiliary) storage. Unlike some disk packs, the magnetic drum cannot be physically removed. The drum is permanently mounted in the device.

Magnetic drum storage devices consist of either a hollow cylinder (thus, the name drum) or a solid cylinder that rotates at a constant velocity (from 600 to 6,000 rpm). The outer surface is coated with an iron-oxide material capable of being magnetized.

A magnetic drum differs from a magnetic disk in that the tracks in which the data is stored are assigned to channels located around the circumference of the drum as shown in figure 2-16. That is, the channels form circular bands around the drum. The coded representation of data in figure 2-16 is similar to that used on 9-track magnetic tape, 8-bit code. The basic functions of the read/write heads are to place magnetized spots (those little binary 0's and 1's) on the drum during a writing operation and to sense these spots during a reading operation. The read/write heads of a drum perform in a manner similar to the read/write heads of a magnetic tape unit or disk drive unit.

Figure 2-16. - Magnetic drum.

The tracks on each channel are grouped into sectors as illustrated in figure 2-16. Does this sound familiar to you? It sounds almost like the format used on disk packs when referring to tracks (or cylinders) and sectors. As the drum rotates, the reading or writing occurs when the specified sector of a given channel passes under the read/write head for that channel.

Some drums are mounted in a horizontal position, such as the one shown in figure 2-16, while others are mounted in a vertical position. Another major difference in the design is the number of read/write heads. Some drums use only one read/write head, which services all channels on the drum. In this case, the head moves back and forth (or up and down) over the surface of the drum as required. Other drums, using multiple read/write heads, have one principal advantage over drums with the single-head type. Since one read/write head is assigned to each channel, no read/write head movement is required. That is, the time required for head positioning is zero. The only significant time required when reading or writing is the rotational delay that occurs in reaching a desired record location.

To give you some idea of speed and storage capacities, some high-speed drums are capable of transferring over one million characters of data per second, which is roughly equivalent to reading a stack of punched cards 8 feet high in one second. The storage capacities of magnetic drums range from 20 million to more than 150,000 million characters (or bytes) of data.

Q.26 Why are disk storage devices popular?
Q.27 How is data stored on all disks?
Q.28 What precedes each record on a disk?
Q.29 How is the storage capacity of a disk determined?
Q.30 What two ways can data be physically organized on a disk pack?
Q.31 The amount of data that can be stored on a linear inch of tape is known by what term?
Q.32 The length of tape between BOT and EOT is referred to by what term?
Q.33 How does a magnetic drum differ from a magnetic disk?
Q.34 Tracks on each channel of a magnetic drum are grouped into what?