The Linux Assembly template (32-bit)

On Linux, although files on disc are divided into sections, executables in memory are divided into code and data segments. The following is our template for the ELF 32-bit executable for Linux:

; File: src/template_lin.asm

; Just as in the Windows template - we tell the assembler which type 
; of output we expect. 
; In this case it is 32-bit executable ELF
format ELF executable

; Tell the assembler where the entry point is
entry _start

; On *nix based systems, when in memory, the space is arranged into 
; segments, rather than in sections, therefore, we define 
; two segments:
; Code segment (executable segment)
segment readable executable

; Here is our entry point
_start:


   ; Set return value to 0
   xor ebx, ebx
   mov eax, ebx
 
   ; Set eax to 1 - 32-bit Linux SYS_exit system call number
   inc eax
 
   ; Call kernel
   int 0x80


; Data segment
segment readable writeable
   db 0


; As you see, there is no import/export segment here. The structure 
; of an ELF executable/object file will be covered in more detail 
; in chapters 8 and 9

As was mentioned in the preceding code, these two templates will be used as a starting point for any code we will write in this book.