Hardware Reference
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puter 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 beter
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 ad-
dition, 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 sim-
pler instructions, called RISC (Reduced Instruction Set Computer) architectures, in the early
1980s. The RISC-based machines focused the atention of designers on two critical perform-
ance techniques, the exploitation of instruction-level 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 ar-
chitecture. 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 trans-
lating 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.
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