Hardware Reference
In-Depth Information
Chapter 8. Magnetic Storage Principles
Magnetic Storage
Most permanent or semipermanent computer data is stored magnetically, meaning a stream of binary
computer data bits (0s and 1s) is stored by magnetizing tiny pieces of metal embedded on the surface
of a disk or tape in a pattern that represents the data. Later, this magnetic 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, IBM announced
the IBM 726 Tape Unit with the IBM701 Defense Calculator, marking 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, California,
introduced the first computer disk storage system as part of the 305 RAMAC (Random Access
Method of Accounting and Control) computer.
The 305 RAMAC drive could store 5 million characters (that's right, only 5MB!) of data on 50 disks,
each a whopping 24 inches in diameter. Individual bits were stored at a density 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 4TB (4,000GB) 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 actually flow from negative to
positive, as shown in the figure, although we normally think of current flowing in the other direction.
