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Chapter 2. Overview

2.3. Object File Formats

The gnu assembler can be configured to produce several alternative object file formats. For the most

part, this does not affect how you write assembly language programs; but directives for debugging

symbols are typically different in different file formats. Section 6.5 Symbol Attributes.

2.4. Command Line

After the program name

as

, the command line may contain options and file names. Options may

appear in any order, and may be before, after, or between file names. The order of file names is

significant.

(two hyphens) by itself names the standard input file explicitly, as one of the files for

as

to assemble.

Except for

any command line argument that begins with a hyphen (

) is an option. Each option

changes the behavior of

as

. No option changes the way another option works. An option is a

followed by one or more letters; the case of the letter is important. All options are optional.

Some options expect exactly one file name to follow them. The file name may either immediately

follow the option's letter (compatible with older assemblers) or it may be the next command argument

(gnu standard). These two command lines are equivalent:

as  o my object file.o mumble.s

as  omy object file.o mumble.s

2.5. 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.

2.5.1. 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. Section 2.7 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