| Output (Object) File

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Every time you run `as`, it produces an output file, which is your assembly language program translated into numbers. This file is the object file. Its default name is `a.out`. You can give it another name by using the '-o' option. Conventionally, object file names end with `.o`. The default name is used for historical reasons: older assemblers were capable of assembling self-contained programs directly into a runnable program. (For some formats, this isn't currently possible, but it can be done for the `a.out` format.)
The object file is meant for input to the linker `ld`. It contains assembled program code, information to help `ld` integrate the assembled program into a runnable file, and (optionally) symbolic information for the debugger.