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36 CHAPTER 2 / COMPUTER EVOLUTION AND PERFORMANCE

Console Main I/O I/O
CPU • • •
controller memory module module

Omnibus

Figure 2.9 PDP-8 Bus Structure

In the 1950s and 1960s, most computer memory was constructed from tiny
rings of ferromagnetic material, each about a sixteenth of an inch in diameter.These
rings were strung up on grids of fine wires suspended on small screens inside the
computer. Magnetized one way, a ring (called a core) represented a one; magnetized
the other way, it stood for a zero. Magnetic-core memory was rather fast; it took as
little as a millionth of a second to read a bit stored in memory. But it was expensive,
bulky, and used destructive readout:The simple act of reading a core erased the data
stored in it. It was therefore necessary to install circuits to restore the data as soon as
it had been extracted.
Then, in 1970, Fairchild produced the first relatively capacious semiconductor
memory. This chip, about the size of a single core, could hold 256 bits of memory. It
was nondestructive and much faster than core. It took only 70 billionths of a second
to read a bit. However, the cost per bit was higher than for that of core.
In 1974, a seminal event occurred: The price per bit of semiconductor memory
dropped below the price per bit of core memory. Following this, there has been a con-
tinuing and rapid decline in memory cost accompanied by a corresponding increase in
physical memory density. This has led the way to smaller, faster machines with mem-
ory sizes of larger and more expensive machines from just a few years earlier. Devel-
opments in memory technology, together with developments in processor technology
to be discussed next, changed the nature of computers in less than a decade.Although
bulky, expensive computers remain a part of the landscape, the computer has also
been brought out to the “end user,” with office machines and personal computers.
Since 1970, semiconductor memory has been through 13 generations: 1K, 4K,
16K, 64K, 256K, 1M, 4M, 16M, 64M, 256M, 1G, 4G, and, as of this writing, 16 Gbits
on a single chip (1K = 2 ,1M = 2 ,1G = 2 ). Each generation has provided four
20
30
10
times the storage density of the previous generation, accompanied by declining cost
per bit and declining access time.
MICROPROCESSORS Just as the density of elements on memory chips has continued
to rise, so has the density of elements on processor chips.As time went on, more and
more elements were placed on each chip, so that fewer and fewer chips were needed
to construct a single computer processor.
A breakthrough was achieved in 1971, when Intel developed its 4004.The 4004
was the first chip to contain all of the components of a CPU on a single chip:The mi-
croprocessor was born.
The 4004 can add two 4-bit numbers and can multiply only by repeated addi-
tion. By today’s standards, the 4004 is hopelessly primitive, but it marked the begin-
ning of a continuing evolution of microprocessor capability and power.