prosessorikäskyihin

Prosessorikäsky (engl. processor instruction) is a single operation that a central processing unit (CPU) can execute. Each instruction is encoded in binary as an opcode followed by optional operands. The set of available instructions is defined by an instruction set architecture (ISA), which specifies the syntax, semantics, and binary encoding of all commands a processor can perform. Common ISA families include CISC (Complex Instruction Set Computing) examples such as x86, and RISC (Reduced Instruction Set Computing) examples such as ARM and RISC‑V. CISC architectures typically use variable-length instructions that may perform multiple micro‑operations, whereas RISC designs employ fixed-length instructions that map more directly to a small set of simple operations.

During execution a processor follows a fetch‑decode‑execute cycle. In the fetch phase the instruction word is read from memory into an instruction register. The decode phase interprets the opcode, determines required operands, and generates control signals. The execute phase performs the actual operation, such as arithmetic, logical, memory access, or control transfer. Many modern CPUs execute multiple instructions in parallel through techniques such as pipelining, superscalar dispatch, and out‑of‑order execution.

Microcode is sometimes used to implement complex ISA commands on simpler underlying hardware. In microcoded engines, a high‑level instruction expands into a sequence of micro‑operations handled by a microcode sequencer. Some architectures, like many RISC processors, eschew microcode in favor of a pure hardware implementation of the ISA.

Processor instructions can be grouped into categories: data movement (load/store), arithmetic and logic (add, sub, and, or), control flow (branch, jump), and system instructions (interrupt, system call). Understanding the instruction set of a given processor is essential for low‑level programming, compiler design, and performance optimisation.