instructionset

An instruction set, more formally known as an instruction set architecture (ISA), is the formal collection of instructions that a processor can execute, together with the data types, registers, addressing modes, and encoding schemes used to implement those instructions. The ISA defines the software-visible interface to the processor, including how instructions are fetched, decoded, and executed, as well as the semantics and potential side effects of each instruction. It also encompasses privileged instructions used by an operating system and the rules for interacting with memory, I/O, and other system resources.

An instruction typically consists of an opcode that specifies the operation and a set of operands that indicate the sources and destinations of data. Addressing modes determine how operands are located, whether in registers, in memory, or as immediate values embedded in the instruction stream. Instruction formats can be fixed-length, which simplifies decoding and pipelining, or variable-length, as seen in some complex instruction sets. The design of an ISA affects code density, compiler complexity, and how effectively the hardware can be optimized for speed and power.

ISAs are often categorized by design philosophy, such as CISC (complex instruction set computing) and RISC (reduced instruction set computing), with other families like VLIW and EPIC representing alternative approaches. Examples of widely used ISAs include x86, ARM, MIPS, and the open RISC-V. Modern ISAs typically support modular extensions for floating point, SIMD/vector processing, and security features, while maintaining backward compatibility with earlier instruction sets to support existing software. The ISA thus serves as the foundational blueprint guiding processor implementation and software development.