Characteristics of Some CISCs, RISCs, and Superscalar Processors
One of the most visible forms of evolution associated with computers is that of pro-
gramming languages. As the cost of hardware has dropped, the relative cost of soft-
ware has risen. Along with that, a chronic shortage of programmers has driven up
software costs in absolute terms. Thus, the major cost in the life cycle of a system is
software, not hardware. Adding to the cost, and to the inconvenience, is the element
of unreliability: it is common for programs, both system and application, to continue
to exhibit new bugs after years of operation.
The response from researchers and industry has been to develop ever more
powerful and complex high-level programming languages. These high-level lan-
guages (HLLs): (1) allow the programmer to express algorithms more concisely,
(2) allow the compiler to take care of details that are not important in the program-
mer’s expression of algorithms, and (3) often support naturally the use of structured
programming and/or object-oriented design.
Alas, this solution gave rise to a perceived problem, known as the semantic
gap, the difference between the operations provided in HLLs and those provided
in computer architecture. Symptoms of this gap are alleged to include execution
inefficiency, excessive machine program size, and compiler complexity. Designers
responded with architectures intended to close this gap. Key features include large
instruction sets, dozens of addressing modes, and various HLL statements imple-
mented in hardware. An example of the latter is the CASE machine instruction on
the VAX. Such complex instruction sets are intended to
• Ease the task of the compiler writer.
• Improve execution efficiency, because complex sequences of operations can
be implemented in microcode.
• Provide support for even more complex and sophisticated HLLs.
Meanwhile, a number of studies have been done over the years to determine
the characteristics and patterns of execution of machine instructions generated
from HLL programs. The results of these studies inspired some researchers to look
for a different approach: namely, to make the architecture that supports the HLL
simpler, rather than more complex.
To understand the line of reasoning of the RISC advocates, we begin with a
brief review of instruction execution characteristics. The aspects of computation of
interest are as follows:
• Operations performed: These determine the functions to be performed by the
processor and its interaction with memory.
• Operands used: The types of operands and the frequency of their use deter-
mine the memory organization for storing them and the addressing modes for
accessing them.
• Execution sequencing: This determines the control and pipeline organization.
In the remainder of this section, we summarize the results of a number of
studies of high-level-language programs. All of the results are based on dynamic
measurements. That is, measurements are collected by executing the program and
counting the number of times some feature has appeared or a particular property
has held true. In contrast, static measurements merely perform these counts on the
source text of a program. They give no useful information on performance, because
they are not weighted relative to the number of times each statement is executed.
