NULL mag Issue 06 22 Assembly tutorial 6

eZine lover (@eZine)

Published in 

null magazine

 · 26 Dec 2020

  

 

    NULL terminating bytes 
    Ok so why did our second message print twice when we only called our 
    sprint function on msg2 once? Well actually it did only print once. You 
    can see what I mean if you comment out our second call to sprint. The 
    output will be both of our message strings. 
    But how is this possible? 
    What is happening is we weren't properly terminating our strings. In 
    assembly, variables are stored one after another in memory so the last 
    byte of our msg1 variable is right next to the first byte of our msg2 
    variable. We know our string length calculation is looking for a zero 
    byte so unless our msg2 variable starts with a zero byte it keeps 
    counting as if it's the same string (and as far as assembly is concerned 
    it is the same string). So we need to put a zero byte or 0h after our 
    strings to let assembly know where to stop counting. 
    Note: In programming 0h denotes a null byte and a null byte after a 
    string tells assembly where it ends in memory. 

    helloworld-inc.asm 

    ; Hello World Program (NULL terminating bytes) 
    ; 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' 

    SECTION .data 
    msg1    db      'Hello, brave new world!', 0Ah, 0h 
    msg2    db      'This is how we recycle in NASM.', 0Ah, 0h 

    SECTION .text 
    global  _start 

    _start: 

        mov     eax, msg1 
        call    sprint 

        mov     eax, msg2 
        call    sprint 

        call    quit 
    ~$ 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.