Aout

Aout, commonly written a.out, is the name of the original executable file format used by Unix systems in the 1970s and 1980s. It arose as the direct output of the assembler and linker and served as the standard binary format for many early Unix programs. Over time it was supplanted by more modern formats such as COFF and ELF, but persisted in various forms for compatibility on some systems.

The a.out file has a simple layout centered on a header that describes the program’s memory layout and metadata. The header typically includes fields such as a_magic (the magic value that indicates loading characteristics), a_text (size of the code segment), a_data (size of the initialized data segment), a_bss (size of uninitialized data), a_syms (size of the symbol table), a_trsize (size of the relocation information), a_drsize (size of any debugging information), and a_entry (the entry point address). After the header come the actual segments: the text segment containing code, the data segment with initialized data, and the uninitialized data region (the BSS) reserved in memory. Depending on the variant, a symbol table and relocation data may follow. The magic value determines how the program is loaded—whether text and data are mapped together or separately and whether the program is loaded at a fixed address or mapped dynamically.

Variants of a.out were distinguished by these magic numbers, affecting loading and addressing. As Unix evolved, the format was largely replaced, but a.out is still discussed in historical and compatibility contexts, reflecting the lineage of binary formats in modern systems. The term aout also appears as a shorthand in discussions for the original assembler-produced output.