assembler

An assembler is a software tool that translates assembly language, a human-readable low-level representation of a computer's instruction set, into machine code or object code that a processor can execute. Assembly language uses mnemonics for opcodes, symbolic names for registers and memory addresses, and a variety of directives to control assembly and data layout. The output is typically object code or an executable, which may require a linker to resolve symbols and produce a final binary.

Assemblers perform symbol resolution, address arithmetic, and relocation. They often support macros and conditional assembly. Most assemblers operate in one or more passes. In a two-pass assembler, forward references (labels defined later) are resolved in a second pass. They also maintain a symbol table mapping labels to addresses.

Syntax: assembly languages vary by architecture and by assembler. Two common syntaxes are Intel syntax and AT&T syntax; different assemblers implement different directives and pseudo-ops for sections, data, and constants.

Workflow: source file(s) -> assembler -> object file(s) -> linker -> executable or library.

Usage and history: assemblers emerged in the 1950s to simplify programming on early computers. They are still used for performance-critical code, embedded systems, OS kernels, bootloaders, and for reverse engineering. Popular assemblers include NASM, MASM, GAS (GNU Assembler), with architecture-specific variants.

Limitations: portability is limited by architecture; readability is lower than high-level languages; debugging can be challenging; optimization is manual.