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magnetizable rings located at the intersections of a grid of
wires. Magnetization in the north direction could repre-
sent binary 1, and this could be flipped to a south magne-
tization, representing binary 0, by changing the current in
the wire. By having a rectangular grid of wires and locating
the cores at the intersections, it was possible to access each
core individually. This allowed genuine random access of
the individual memory locations - as opposed to having to
go through the bits in sequence to get to the desired bit, as
would be the case if the bits are stored on a magnetic tape.
Forrester's technology was first tested in the construction
of the Memory Test Computer at MIT in 1952. Compared to
memory based on Williams Tubes, magnetic core memory
proved much faster and far more reliable.
The first device to provide almost random access to data was not
Forrester's magnetic core memory. It was a spinning drum with a magne-
tizable surface that allowed fast access to information stored in magne-
tized bands on the drum. This was invented in 1948 by Andrew Booth of
Birkbeck College in England ( B.2.9 ). Booth had made a visit to Princeton
and had seen the progress von Neumann's team was making toward build-
ing the IAS stored program computer. Booth's prototype magnetic drum
device was only two inches in diameter and could store ten bits per inch
( Fig. 2.25 ). He followed up this prototype with larger drums that featured
thirty-two magnetized bands, each divided into thirty-two words of thirty-
two bits. A read/write head simply read off the values as the drum spun.
Booth's drum memory was soon taken up by others and was adopted as sec-
ondary memory for Williams and Kilburn's scaled up version of the Baby,
the Manchester Mark 1 machine. Magnetic drum memory was widely used
for secondary memory in the 1950s and 1960s, until it was gradually super-
seded by magnetic disks.
The first hard disk
was introduced by IBM
in 1956 and hard disks
soon became ubiquitous.
Hard disk drives consist
of a number of flat circu-
lar disks, called platters,
mounted on a spindle
( Fig. 2.26 ). The platters are
coated with a thin film of
magnetic material, and
changes in the direction of
magnetization record the
required pattern of binary
digits. The engineering of
these drives is impressive.
The platters can rotate at
Fig. 2.24. Forrester's Magnetic Core Memory.
B.2.8. Jay Forrester holding a frame
of core memory from the Whirlwind
computer. He invented magnetic
core memory while working at MIT
in the early 1950s. His invention
proved much faster and more reli-
able than the earlier Williams Tubes
or delay-line memory technologies.
B.2.9. Andrew Booth (1918-
2009), together with his assis-
tant and future wife, Kathleen
Britten, developed magnetic
drum storage. He also invented
a fast multiplication algorithm
that is used in modern Intel
microprocessors.
Fig. 2.25. Andrew Booth's magnetic drum memory.
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