Hardware Reference
In-Depth Information
Chapter 8. Magnetic Storage Principles
Magnetic Storage
Mostpermanent orsemipermanent computer data isstored magnetically,meaning astream
of binary computer data bits (0s and 1s) is stored by magnetizing tiny pieces of metal em-
bedded on the surface of a disk or tape in a pattern that represents the data. Later, this mag-
netic pattern can be read and converted back into the same original stream of bits. This is
the principle of magnetic storage and the subject of this chapter.
History of Magnetic Storage
Before magnetic storage, the primary computer storage medium was punch cards (paper
cards with holes punched in them to indicate character or binary data), originally invented
by Herman Hollerith for use in the 1890 Census.
The history of magnetic storage dates back to June 1949, when a group of IBM engineers
and scientists began working on a new storage device. What they were working on was the
first magnetic storage device for computers, and it revolutionized the industry. On May 21,
1952,IBMannouncedtheIBM726TapeUnitwiththeIBM701Defense Calculator,mark-
ing the transition from punched-card calculators to electronic computers.
Four years later, on September 13, 1956, a small team of IBM engineers in San Jose, Cali-
fornia, introduced the first computer disk storage system as part of the 305 RAMAC (Ran-
dom Access Method of Accounting and Control) computer.
The 305 RAMAC drive could store 5 million characters (that's right, only 5MB!) of data
on50disks,eachawhopping24inchesindiameter.Individualbitswerestoredatadensity
of only 2Kb/sq. inch. Unlike tape drives, RAMAC's recording heads could go directly to
any location on a disk surface without reading all the information in between. This random
accessibility had a profound effect on computer performance at the time, enabling data to
be stored and retrieved significantly faster than if it were on tape.
From these beginnings, in just over 60 years the magnetic storage industry has progressed
such that today you can store 3TB (3,000GB) or more on tiny 3 1/2-inch drives that fit into
a single computer drive bay.
How Magnetic Fields Are Used to Store Data
All magnetic storage devices read and write data by using electromagnetism. This basic
principle of physics states that as an electric current flows through a conductor (wire), a
magnetic field is generated around the conductor (see
Figure 8.1
). Note that electrons actu-
