1.5 Instruction Set Architectures

An instruction set architecture (ISA) abstracts the interface between a computer's hardware and the lowest-level software for the programmer or compiler-writer. Thorough knowledge of an ISA involves not only the appropriate ways to operate the programmer-accessible registers used for calculations and storage of intermediate results, but also how to move data between those registers and memory, storage, or other attached devices. With knowledge of a particular ISA, one knows in principle what the computer can do and as we shall sketch out in this book what simple studies to conduct when seeking improved performance with particular implementations.

A well-conceived ISA leaves considerable latitude for implementations that realize the architectural concept in numerous actual products that serve different purposes with different cost implications. Ideally, the ISA will not be rendered prematurely obsolete by any unforeseen technological developments, because the effort and cost to design a new ISA and gain its adoption in the marketplace are very high.

Certain interrelated facets of the design of an ISA usually cannot be changed through new implementations; instead, they require extension or replacement of the architecture. Those intrinsic components of design include the following: the bit width(s) of data to be easily manipulated; the types of data to be represented and manipulated; the number and nature of registers; the amount of memory to be readily accessible, and in how many ways it can be accessed; the way that external devices are to be accessed; and the numbers and classes of machine instructions.

In the next section, we show some of the very different choices the computer industry has made for those and other architectural facets. We give more details in Chapter 2 and throughout the book.