Hello World!

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

NOTE: sys_write is a function enabled by the operating system or the kernel. So the way to call this function may vary from each operating system to another. This example is for x86 Linux calls only.

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 $-$$.

Blog Comments powered by Disqus.