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Computer technology has made incredible progress in the roughly 65 years since
the first general-purpose electronic computer was created. Today, less than $500
will purchase a mobile computer that has more performance, more main memory,
and more disk storage than a computer bought in 1985 for $1 million. This rapid
improvement has come both from advances in the technology used to build computers and from innovations in computer design.
Although technological improvements have been fairly steady, progress arising from better computer architectures has been much less consistent. During the
first 25 years of electronic computers, both forces made a major contribution,
delivering performance improvement of about 25% per year. The late 1970s saw
the emergence of the microprocessor. The ability of the microprocessor to ride
the improvements in integrated circuit technology led to a higher rate of performance improvement—roughly 35% growth per year.
This growth rate, combined with the cost advantages of a mass-produced
microprocessor, led to an increasing fraction of the computer business being
based on microprocessors. In addition, two significant changes in the computer
marketplace made it easier than ever before to succeed commercially with a new
architecture. First, the virtual elimination of assembly language programming
reduced the need for object-code compatibility. Second, the creation of standardized, vendor-independent operating systems, such as UNIX and its clone, Linux,
lowered the cost and risk of bringing out a new architecture.
These changes made it possible to develop successfully a new set of architectures with simpler instructions, called RISC (Reduced Instruction Set Computer)
architectures, in the early 1980s. The RISC-based machines focused the attention
of designers on two critical performance techniques, the exploitation of instructionlevel parallelism (initially through pipelining and later through multiple instruction
issue) and the use of caches (initially in simple forms and later using more sophisticated organizations and optimizations).
The RISC-based computers raised the performance bar, forcing prior architectures to keep up or disappear. The Digital Equipment Vax could not, and so it
was replaced by a RISC architecture. Intel rose to the challenge, primarily by
translating 80x86 instructions into RISC-like instructions internally, allowing it
to adopt many of the innovations first pioneered in the RISC designs. As transistor counts soared in the late 1990s, the hardware overhead of translating the more
complex x86 architecture became negligible. In low-end applications, such as
cell phones, the cost in power and silicon area of the x86-translation overhead
helped lead to a RISC architecture, ARM, becoming dominant.
Figure 1.1 shows that the combination of architectural and organizational
enhancements led to 17 years of sustained growth in performance at an annual
rate of over 50%—a rate that is unprecedented in the computer industry.
The effect of this dramatic growth rate in the 20th century has been fourfold.
First, it has significantly enhanced the capability available to computer users. For
many applications, the highest-performance microprocessors of today outperform the supercomputer of less than 10 years ago.
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