16-Bit Microprocessor Handbook

Thus a low-to-high transition of one signal triggering changes in two other signal levels would be illustrated as
follows:
10) All signal level changes are shown as square waves. Thus rise and fall times are ignored. These times are given in
the data sheets which appear at the end of every chapter.
INSTRUCTION SET CONVENTIONS
Every microcomputer instruction set is described with two tables. One table identifies the operations which
occur when the instruction set is executed, while the second table defines object codes and instruction times.
Because of the wide differences that exist between one instruction set and another, we have elected not to
use a single set of codes and symbols to describe the operations for all instructions in all instruction sets. We
believe any type of universal convention is like to confuse rather than clarify; therefore each instruction set
table is preceded by a list of symbols as used within the table alone.
A short benchmark program is given to illustrate each instruction set. Some comments regarding benchmark
programs in general are, however, in order. We are not attempting to highlight strengths or weaknesses of
different devices, nor does this book make any attempt to comparative analyses, since the criteria which make
one microcomputer better than another are simply too dependent on the application.
Consider an application which requires relatively high speed processing. The only important criterion
will be program execution speed, which may limit the choice to just one of the microcomputers
we are describing.
COMPARATIVE
ANALYSIS
Execution speeds of all of the microcomputers may, on the other hand, be quite adequate for a second application; in
this case, price may be the only overriding factor. In a third application, a manufacturer may have already invested in a
great deal of engineering development expense, using one particular microcomputer that was available in quantity earlier
than any others; the advantages or disadvantages of using a different microcomputer, based on minor cost of performance
advantages, will likely be overwhelmed by the extra expense and time delays involved with switching in
midstream.
And what about benchmark programs 7
There have been a number of benchmark programs in the literature, purporting to show the
strengths or weaknesses of one microcomputer versus another; individual manufacturers
have added to the confusion by putting out their own competing benchmarks, aimed at showing their product to
be superior to an immediate rival.
Benchmark programs are misleading, irrelevant and worthless for these reasons:
1) In a majority of microcomputer applications, program execution speed, and minor variations in program
length, are simply overwhelmed by pricing considerations.
2) Even assuming that for some specific application, program length and execution speed are important, trivial
changes in the benchmark program definition can profoundly alter the results that are obtained. This is one
point we will demonstrate in this book, while describing individual instruction sets.
3) Benchmark programs are invariable written by the smartest programmers in an organization, and they take
an enormous amount of time to ensure programming accuracy and excellence. This is not the level at which
any user should anticipate "run of the mill" programmers working; indeed, a far more realistic evaluation of
a microcomputer's instruction set could be generated by giving an average programmer too little time in
which to implement an incompletely defined benchmark. This will more closely approximate the working
conditions under which real products are developed. Of course, defining the "average programmer," "too
little time" and an "incomplete specification" are all sufficiently subjective that they defy resolution.
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