FreeBSD Developers' Handbook
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29.6 Our First Program

We are now ready for our first program, the mandatory Hello, World!

     1:    %include    'system.inc'
     2:
     3: section .data
     4: hello   db  'Hello, World!', 0Ah
     5: hbytes  equ $-hello
     6:
     7: section .text
     8: global  _start
     9: _start:
    10: push    dword hbytes
    11: push    dword hello
    12: push    dword stdout
    13: sys.write
    14:
    15: push    dword 0
    16: sys.exit

Here is what it does: Line 1 includes the defines, the macros, and the code from system.inc.

Lines 3-5 are the data: Line 3 starts the data section/segment. Line 4 contains the string "Hello, World!" followed by a new line (0Ah). Line 5 creates a constant that contains the length of the string from line 4 in bytes.

Lines 7-16 contain the code. Note that FreeBSD uses the elf file format for its executables, which requires every program to start at the point labeled _start (or, more precisely, the linker expects that). This label has to be global.

Lines 10-13 ask the system to write hbytes bytes of the hello string to stdout.

Lines 15-16 ask the system to end the program with the return value of 0. The SYS_exit syscall never returns, so the code ends there.

Note: If you have come to Unix from MS DOS assembly language background, you may be used to writing directly to the video hardware. You will never have to worry about this in FreeBSD, or any other flavor of Unix. As far as you are concerned, you are writing to a file known as stdout. This can be the video screen, or a telnet terminal, or an actual file, or even the input of another program. Which one it is, is for the system to figure out.

29.6.1 Assembling the Code

Type the code (except the line numbers) in an editor, and save it in a file named hello.asm. You need nasm to assemble it.

29.6.1.1 Installing nasm

If you do not have nasm, type:

    % su
    Password:your root password
    # cd /usr/ports/devel/nasm
    # make install
    # exit
    %

You may type make install clean instead of just make install if you do not want to keep nasm source code.

Either way, FreeBSD will automatically download nasm from the Internet, compile it, and install it on your system.

Note: If your system is not FreeBSD, you need to get nasm from its home page. You can still use it to assemble FreeBSD code.

Now you can assemble, link, and run the code:

    % nasm -f elf hello.asm
    % ld -s -o hello hello.o
    % ./hello
    Hello, World!
    %

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