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### 1.5 Input Files

We use the phrase source program, abbreviated source, to describe the program input to one run of `as`. The program may be in one or more files; how the source is partitioned into files doesn't change the meaning of the source.

The source program is a concatenation of the text in all the files, in the order specified.

Each time you run `as` it assembles exactly one source program. The source program is made up of one or more files. (The standard input is also a file.)

You give `as` a command line that has zero or more input file names. The input files are read (from left file name to right). A command line argument (in any position) that has no special meaning is taken to be an input file name.

If you give `as` no file names it attempts to read one input file from the `as` standard input, which is normally your terminal. You may have to type <ctl-D> to tell `as` there is no more program to assemble.

Use ``--`' if you need to explicitly name the standard input file in your command line.

If the source is empty, `as` produces a small, empty object file.

#### Filenames and Line-numbers

There are two ways of locating a line in the input file (or files) and either may be used in reporting error messages. One way refers to a line number in a physical file; the other refers to a line number in a “logical” file. See Error and Warning Messages.

Physical files are those files named in the command line given to `as`.

Logical files are simply names declared explicitly by assembler directives; they bear no relation to physical files. Logical file names help error messages reflect the original source file, when `as` source is itself synthesized from other files. `as` understands the ``#`' directives emitted by the `gcc` preprocessor. See also `.file`.