architectures and multiple execution
engines (cores) but have grown to include all sorts of additional functions,
including floating-point units, caches,
memory controllers, and media-pro-cessing engines. However, the defining characteristics of a microprocessor
remain—a single semiconductor chip
embodying the primary computation
(data transformation) engine in a computing system.
Because our own greatest access
and insight involves Intel designs and
data, our graphs and estimates draw
heavily on them. In some cases, they
may not be representative of the entire
industry but certainly represent a large
fraction. Such a forthright view, solidly
grounded, best supports our goals for
this article.
20 Years of exponential
Performance Gains
For the past 20 years, rapid growth in
microprocessor performance has been
enabled by three key technology drivers—transistor-speed scaling, core microarchitecture techniques, and cache
memories—discussed in turn in the
following sections:
Transistor-speed scaling. The MOS
transistor has been the workhorse for
decades, scaling in performance by
nearly five orders of magnitude and
providing the foundation for today’s
unprecedented compute performance.
The basic recipe for technology scaling
was laid down by Robert N. Dennard of
IBM17 in the early 1970s and followed
for the past three decades. The scaling recipe calls for reducing transistor
figure 1. evolution of Intel microprocessors 1971–2009.
Intel 4004, 1971
1 core, no cache
23K transistors
Intel 8088, 1978
1 core, no cache
29K transistors
Intel Mehalem-eX, 2009
8 cores, 24MB cache
2.3B transistors
figure 2. architecture advances and energy efficiency.
Die area
Integer Performance (X)
FP Performance (X)
Int Performance/ Watt (X)
386 to 486
4
486 to Pentium
3
Increase (x)
2
Pentium to P6 P6 to Pentium 4
Pentium 4
to Core
1
0
on-die cache,
pipelined
Super-scalar
ooo-Speculative
Deep pipeline
Back to non-deep
pipeline
dimensions by 30% every generation
(two years) and keeping electric fields
constant everywhere in the transistor to maintain reliability. This might
sound simple but is increasingly difficult to continue for reasons discussed
later. Classical transistor scaling provided three major benefits that made
possible rapid growth in compute performance.
First, the transistor dimensions are
scaled by 30% (0.7x), their area shrinks
50%, doubling the transistor density
every technology generation—the fundamental reason behind Moore’s Law.
Second, as the transistor is scaled, its
performance increases by about 40%
(0.7x delay reduction, or 1.4x frequency increase), providing higher system
performance. Third, to keep the electric field constant, supply voltage is reduced by 30%, reducing energy by 65%,
or power (at 1.4x frequency) by 50%
(active power = CV2f). Putting it all together, in every technology generation
transistor integration doubles, circuits
are 40% faster, and system power consumption (with twice as many transistors) stays the same. This serendipitous scaling (almost too good to be
true) enabled three-orders-of-magnitude increase in microprocessor performance over the past 20 years. Chip
architects exploited transistor density
to create complex architectures and
transistor speed to increase frequency,
achieving it all within a reasonable
power and energy envelope.
Core microarchitecture techniques. Advanced microarchitectures
have deployed the abundance of transistor-integration capacity, employing
a dizzying array of techniques, including pipelining, branch prediction,
out-of-order execution, and speculation, to deliver ever-increasing performance. Figure 2 outlines advances in
microarchitecture, showing increases
in die area and performance and energy efficiency (performance/watt),
all normalized in the same process
technology. It uses characteristics of
Intel microprocessors (such as 386,
486, Pentium, Pentium Pro, and Pentium 4), with performance measured
by benchmark SpecInt ( 92, 95, and
2000 representing the current benchmark for the era) at each data point.
It compares each microarchitecture
advance with a design without the ad-