Hello World!

Now we will be doing the classic "Hello World!" program in which we will print that string to STDOUT using sys_write.

x86 System Calls

The System Call Table provides you with the information you need on how to call a specific system function whereas the function in general is called like this:

mov eax, SYSCALL_NUM
mov ebx, arg1
mov ecx, arg2
mov edx, arg3
int 0x80

ASM Code

helloworld.asm

segment .data                   ;section declaration

msg db     "Hello, world!", 0xA, 0x0 ;our NULL-terminated string, ending with 0xA (lf) or 0xD (CR)
len equ     $ - msg             ;length of our dear string

segment .text                   ;section declaration
    global  asm_main            ;we must export the entry point to the ELF linker or loader

asm_main:
    enter 0,0
    pusha
                                ;write our string to stdout
    mov     edx,len             ;third argument: message length
    mov     ecx,msg             ;second argument: pointer to message to write
    mov     ebx,1               ;first argument: file handle (stdout)
    mov     eax,4               ;system call number (sys_write)
    int     0x80                ;call kernel

    popa                       ; return to the C caller with 0 status
    mov eax, 0
    leave       ; If you have a segmentation fault, check where you enter and leave
    ret

Execution

After compiling to heloworld.o and helloworld, we can now run the program.

$ ./helloworld
Hello World!

Explanation

EQU defines a symbol to a given constant value: when EQU is used, the source line must contain a label. The action of EQU is to define the given label name to the value of its (only) operand. This definition is absolute, and cannot change later. So, for example:

msg    db     "Hello, world!"
len    equ    $ - msg

defines len to be the constant 13. len may not then be redefined later. This is not a preprocessor definition either: the value of len is evaluated once, using the value of $ at the point of definition, rather than being evaluated wherever it is referenced and using the value of $ at the point of reference.

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $. $$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using $-$$.