Step 2 - let's test

Once ready, let's build our first Assembly program on Linux. Create an Assembly source file named, for example, test.S.

Fill in the following code:

/*
 This is a multiline comment.
*/
// This is a single line comment.
# Another single line comment.

# The following line is not a necessity.
   .file "test.S"
 
# Tell GAS that we are using an external function.
   .extern printf
 
# Make some data - store message in data section 0
   .data
msg:
   .ascii "Hello from Assembly language!xaxdx0"
 
# Begin the actual code
   .text
# Make main() publicly visible
   .globl main
/*
 This is our main() function. 
 It is important to mention, 
 that we can't begin the program with 
 'main()' when using GAS alone. We have then
 to begin the program with 'start' or '_start' 
 function.
*/

main:
   pushl %ebp
   movl %esp, %ebp
   pushl $msg          # Pass parameter (pointer
                       # to message) to output_message function.
   call output_message # Print the message
   movl $0, %eax
   leave
   ret
 
# This function simply prints out a message to the Terminal
output_message:
   pushl %ebp
   movl %esp, %ebp
   subl $8, %esp
   movl 8(%ebp), %eax
   movl %eax, (%esp)
   call _printf        # Here we call printf
   addl $4, %esp
   movl $0, %eax
   leave
   ret $4

If on Linux, build the code with the following command:

gcc -o test test.S

If you're on Windows, change the name of the output executable accordingly:

gcc -o test.exe test.S

Run the executable in the Terminal. You should see the message followed by a new line:

Hello from Assembly language!

As you may have noticed, the syntax of this Assembly source is different from that supported by MASM. While MASM supports what is called Intel syntax, GAS originally supported only the AT&T syntax. However, the support for Intel syntax was added at some point, thus making the life of new adepts significantly easier.