instruction-set architecture Processor Architecture

Computer Systems A Programmer's Perspective Second Edition

We have seen that a processor must execute a sequence of instructions,

where each instruction performs some primitive operation, such as adding

two numbers. An instruction is encoded in binary form as a sequence of 1 or more bytes.

The instructions supported by a particular processor and their byte-level encodings

are known as its instruction-set architecture (ISA). Different

“families” of processors, such as Intel IA32, IBM/Freescale PowerPC, and the

ARM processor family have different ISAs. A program compiled for one type

of machine will not run on another. On the other hand, there are many different

models of processors within a single family. Each manufacturer produces proces-

sors of ever-growing performance and complexity, but the different models remain

compatible at the ISA level. Popular families, such as IA32, have processors sup-

plied by multiple manufacturers. Thus, the ISA provides a conceptual layer of

abstraction between compiler writers, who need only know what instructions are

permitted and how they are encoded, and processor designers, who must build

machines that execute those instructions.