Page 67 -

42 CHAPTER 2 / COMPUTER EVOLUTION AND PERFORMANCE

examples of typical peripheral devices in use on personal computers and worksta-
tions.These devices create tremendous data throughput demands.While the current
generation of processors can handle the data pumped out by these devices, there re-
mains the problem of getting that data moved between processor and peripheral.
Strategies here include caching and buffering schemes plus the use of higher-speed
interconnection buses and more elaborate structures of buses. In addition, the use of
multiple-processor configurations can aid in satisfying I/O demands.
The key in all this is balance. Designers constantly strive to balance the
throughput and processing demands of the processor components, main memory,
I/O devices, and the interconnection structures. This design must constantly be
rethought to cope with two constantly evolving factors:
• The rate at which performance is changing in the various technology areas
(processor, buses, memory, peripherals) differs greatly from one type of ele-
ment to another.
• New applications and new peripheral devices constantly change the nature of
the demand on the system in terms of typical instruction profile and the data
access patterns.
Thus, computer design is a constantly evolving art form.This book attempts to
present the fundamentals on which this art form is based and to present a survey of
the current state of that art.

Improvements in Chip Organization and Architecture
As designers wrestle with the challenge of balancing processor performance with that
of main memory and other computer components, the need to increase processor
speed remains.There are three approaches to achieving increased processor speed:
• Increase the hardware speed of the processor. This increase is fundamentally
due to shrinking the size of the logic gates on the processor chip, so that more
gates can be packed together more tightly and to increasing the clock rate.
With gates closer together, the propagation time for signals is significantly re-
duced, enabling a speeding up of the processor. An increase in clock rate
means that individual operations are executed more rapidly.
• Increase the size and speed of caches that are interposed between the proces-
sor and main memory. In particular, by dedicating a portion of the processor
chip itself to the cache, cache access times drop significantly.
• Make changes to the processor organization and architecture that increase the
effective speed of instruction execution. Typically, this involves using paral-
lelism in one form or another.
Traditionally, the dominant factor in performance gains has been in increases
in clock speed due and logic density. Figure 2.12 illustrates this trend for Intel
processor chips. However, as clock speed and logic density increase, a number of ob-
stacles become more significant [INTE04b]:

• Power: As the density of logic and the clock speed on a chip increase, so does
2
the power density (Watts/cm ).The difficulty of dissipating the heat generated