NULL mag Issue 05 16 Assembly tutor 005

Published in

· 26 Dec 2020

  

External include files allow us to move code from our program and put it into 
separate files. This technique is useful for writing clean, easy to maintain  
programs. Reusable bits of code can be written as subroutines and stored in  
separate files called libraries. When you need a piece of logic you can  
include the file in your program and use it as if they are part of the same  
file. 

In this lesson we will move our string length calculating subroutine into an  
external file. We fill also make our string printing logic and program exit  
logic a subroutine and we will move them into this external file. Once it's  
completed our actual program will be clean and easier to read. 

We can then declare another message variable and call our print function  
twice in order to demonstrate how we can reuse code. 

functions.asm 
;------------------------------------------ 
; int slen(String message) 
; String length calculation function 
slen: 
    push    ebx 
    mov     ebx, eax 
  
nextchar: 
    cmp     byte [eax], 0 
    jz      finished 
    inc     eax 
    jmp     nextchar 
  
finished: 
    sub     eax, ebx 
    pop     ebx 
    ret 
  
  
;------------------------------------------ 
; void sprint(String message) 
; String printing function 
sprint: 
    push    edx 
    push    ecx 
    push    ebx 
    push    eax 
    call    slen 
  
    mov     edx, eax 
    pop     eax 
  
    mov     ecx, eax 
    mov     ebx, 1 
    mov     eax, 4 
    int     80h 
  
    pop     ebx 
    pop     ecx 
    pop     edx 
    ret 
  
helloworld.asm 
;------------------------------------------ 
; void exit() 
; Exit program and restore resources 
quit: 
    mov     ebx, 0 
    mov     eax, 1 
    int     80h 
    ret 
     
     
; Hello World Program (External file include) 
; Compile with: nasm -f elf helloworld-inc.asm 
; Link with (64 bit systems require elf_i386 option):  
;   ld -m elf_i386 helloworld-inc.o -o helloworld-inc 
; Run with: ./helloworld-inc 
  
%include        'functions.asm'         ; include our external file 
  
SECTION .data 
                ; our first message string 
msg1    db      'Hello, brave new world!', 0Ah               
                ; our second message string 
msg2    db      'This is how we recycle in NASM.', 0Ah       
  
SECTION .text 
global  _start 
  
_start: 
    ; move the address of our first message string into EAX 
    mov     eax, msg1        
    call    sprint          ; call our string printing function 
  
    ; move the address of our second message string into EAX 
    mov     eax, msg2        
    call    sprint          ; call our string printing function 
  
    call    quit            ; call our quit function 
     

~$ nasm -f elf helloworld-inc.asm  
~$ ld -m elf_i386 helloworld-inc.o -o helloworld-inc  
~$ ./helloworld-inc  
Hello, brave new world!  
This is how we recycle in NASM.  
This is how we recycle in NASM.