Fundamentals of Quantitative Design and Analysis - Computer Architecture: A Quantitative Approach - page 8

average revenue per hour was almost $6M. During a peak hour for Christmas shopping, the
potential loss would be many times higher. As Chapter 6 explains, the difference from servers
is that WSCs use redundant inexpensive components as the building blocks, relying on a soft-
ware layer to catch and isolate the many failures that will happen with computing at this scale.
Note that scalability for a WSC is handled by the local area network connecting the computers
and not by integrated computer hardware, as in the case of servers.
Supercomputers are related to WSCs in that they are equally expensive, costing hundreds of
millions of dollars, but supercomputers differ by emphasizing floating-point performance and
by running large, communication-intensive batch programs that can run for weeks at a time.
This tight coupling leads to use of much faster internal networks. In contrast, WSCs emphas-
ize interactive applications, large-scale storage, dependability, and high Internet bandwidth.
Embedded Computers
Embedded computers are found in everyday machines; microwaves, washing machines, most
printers, most networking switches, and all cars contain simple embedded microprocessors.
The processors in a PMD are often considered embedded computers, but we are keeping
them as a separate category because PMDs are platforms that can run externally developed
software and they share many of the characteristics of desktop computers. Other embedded
devices are more limited in hardware and software sophistication. We use the ability to run
third-party software as the dividing line between non-embedded and embedded computers.
Embedded computers have the widest spread of processing power and cost. They include
8-bit and 16-bit processors that may cost less than a dime, 32-bit microprocessors that execute
100 million instructions per second and cost under $5, and high-end processors for network
switches that cost $100 and can execute billions of instructions per second. Although the range
of computing power in the embedded computing market is very large, price is a key factor in
the design of computers for this space. Performance requirements do exist, of course, but the
primary goal is often meeting the performance need at a minimum price, rather than achiev-
ing higher performance at a higher price.
Most of this topic applies to the design, use, and performance of embedded processors,
whether they are off-the-shelf microprocessors or microprocessor cores that will be assembled
with other special-purpose hardware. Indeed, the third edition of this topic included examples
from embedded computing to illustrate the ideas in every chapter.
Alas, most readers found these examples unsatisfactory, as the data that drive the quantit-
ative design and evaluation of other classes of computers have not yet been extended well to
embedded computing (see the challenges with EEMBC, for example, in Section 1.8 ). Hence,
we are left for now with qualitative descriptions, which do not fit well with the rest of the
book. As a result, in this and the prior edition we consolidated the embedded material into
Appendix E. We believe a separate appendix improves the flow of ideas in the text while all
lowing readers to see how the differing requirements affect embedded computing.
Classes Of Parallelism And Parallel Architectures
Parallelism at multiple levels is now the driving force of computer design across all four
classes of computers, with energy and cost being the primary constraints. There are basically
two kinds of parallelism in applications:
1. Data-Level Parallelism (DLP) arises because there are many data items that can be operated
on at the same time.