NULL mag Issue 08 26 Assembly tutorial 8

eZine lover (@eZine)

Published in 

null magazine

 · 26 Dec 2020

  

 

 
    Passing arguments to your program from the command line is as easy 
    as popping them off the stack in NASM. When we run our program, any 
    passed arguments are loaded onto the stack in reverse order. The name 
    of the program is then loaded onto the stack and lastly the total 
    number of arguments is loaded onto the stack. The last two stack items 
    for a NASM compiled program are always the name of the program and the 
    number of passed arguments. 

    So all we have to do to use them is pop the number of arguments off 
    the stack first, then iterate once for each argument and perform our 
    logic. In our program that means calling our print function. 

 
    NOTE  
    We are using the ECX register as our counter for the loop. Although it's 
    a general-purpose register it's original intention was to be used as a   
    counter.                                                                 

helloworld-args.asm 
; Hello World Program (Passing arguments from the command line) 
; Compile with: nasm -f elf helloworld-args.asm 
; Link with (64 bit systems require elf_i386 option): 
;       ld -m elf_i386 helloworld-args.o -o helloworld-args 
; Run with: ./helloworld-args 

%include        'functions.asm' 

SECTION .text 
global  _start 

_start: 

    pop     ecx         ; first value on the stack is the number of arguments 

nextArg: 
    cmp     ecx, 0h     ; check to see if we have any arguments left 

; if zero flag is set jump to noMoreArgs label (jumping over the end of 
; the loop) 

    jz      noMoreArgs 
    pop     eax         ; pop the next argument off the stack 
    call    sprintLF    ; call our print with linefeed function 
    dec     ecx         ; decrease ecx (number of arguments left) by 1 
    jmp     nextArg     ; jump to nextArg label 

noMoreArgs: 
    call    quit 

 

 
~$ nasm -f elf helloworld-args.asm 
~$ ld -m elf_i386 helloworld-lf.o -o helloworld-args 
~$ ./helloworld-args "This is one argument" "This is another" 101 
./helloworld-args 
This is one argument 
This is another 
101