How to use the command "as" (with examples)

The “as” command is a portable GNU assembler primarily intended to assemble output from “gcc” and prepare it to be used by “ld”. It is a vital tool in the compilation process, converting assembly language code into object files that can be linked to create executables. This article will provide examples and explanations for various use cases of the “as” command.

Use case 1: Assemble a file, writing the output to a.out

Code:

as path/to/file.s

Motivation: The code above assembles the assembly language code in the file file.s and produces an output file named a.out. The motivation for using this example is to demonstrate the basic usage of the “as” command to assemble a file without specifying an explicit output file.

Explanation:

• as: This is the command itself.
• path/to/file.s: Specifies the path to the source file containing assembly language code.

Example output: If the assembly process was successful, no output will be displayed. The output file a.out will be generated in the current working directory.

Use case 2: Assemble the output to a given file

Code:

as path/to/file.s -o path/to/output_file.o

Motivation: The code above assembles the assembly language code in the file file.s and specifies that the output should be written to the file output_file.o. The motivation for using this example is to demonstrate how to customize the name and location of the output file.

Explanation:

• -o path/to/output_file.o: The -o option is used to specify the output file. The path and filename provided after the -o flag determine where the resulting object file will be written.

Example output: If the assembly process was successful, no output will be displayed. The output file output_file.o will be generated at the specified location.

Use case 3: Generate output faster by skipping whitespace and comment preprocessing

Code:

as -f path/to/file.s

Motivation: The code above instructs the “as” command to skip whitespace and comment preprocessing while assembling the assembly language code. This option is useful when using a trusted compiler and can improve the assembly speed.

Explanation:

• -f: The -f option is used to enable faster assembly by skipping whitespace and comment preprocessing.

Example output: If the assembly process was successful, no output will be displayed. The output file will be generated without any changes made to whitespace or comments.

Use case 4: Include a given path to the list of directories to search for files specified in .include directives

Code:

as -I path/to/directory path/to/file.s

Motivation: The code above allows the “as” command to include a given directory path in the search list for files specified in .include directives within the assembly code. This is particularly useful when external files need to be included during the assembly process.

Explanation:

• -I path/to/directory: The -I option is used to specify a directory to include in the search list for files specified in .include directives.
• path/to/file.s: Specifies the path to the source file containing assembly language code.

Example output: If the assembly process was successful, no output will be displayed. The “as” command will search in the specified directory for files included using .include directives in the assembly code.