as – Assembly Language Compiler

The as command is a GNU assembler that converts assembly code into machine code. It is used to create object files that can be linked to form executable programs. The as command is a part of the GNU Binutils package, which also includes other important tools such as ld (the linker), nm (the symbol table manager), and objdump (the object file dumper).


The as command is used to assemble assembly language source code into object files. The syntax for the as command is as follows:

as [options] file

Where file is the name of the assembly language source code file, and [options] are optional command line arguments that modify the behavior of the as command. The as command can accept input from either a file or standard input.

Here is an example of how to use the as command to assemble an assembly language source code file:

as -o hello.o hello.s

This command will assemble the hello.s assembly language source code file and create an object file named hello.o.

Use Cases

The as command is commonly used in the development of low-level system software, such as device drivers, operating systems, and firmware. It can also be used for performance-critical applications that require fine-grained control over the generated machine code.


The as command supports several command line options that modify its behavior. Here is a table of the most commonly used options:

Option Description
-o file Specifies the output file name.
-g Generates debugging information in the object file.
-f format Specifies the output format.
-march=arch Specifies the target architecture.
-mfpu=fpu Specifies the target floating-point unit.
-mthumb Generates code for the Thumb instruction set.
-marm Generates code for the ARM instruction set.
-Wall Enables all warning messages.
-Werror Treats warning messages as errors.

Troubleshooting Tips

One common issue with the as command is incorrect assembly language syntax. If the assembler encounters syntax errors in the source code, it will produce error messages and halt the assembly process. To fix these errors, you should carefully review the assembly language source code and correct any syntax errors.

Another issue that can arise is linker errors. If the linker is unable to find the required symbols in the object files, it will produce error messages and halt the linking process. To fix these errors, you should ensure that all required object files are included in the linker command line, and that all required symbols are defined in the object files.


The as command is a powerful tool for low-level system software development, but it requires a deep understanding of assembly language and machine code. It is recommended that you have a strong understanding of these topics before attempting to use the as command. Additionally, the as command is just one part of the GNU Binutils package, which includes several other important tools that are required for low-level system software development.