I’m new to assembly coding and I have a question about the null character in .data section.

I tested a few codes:

Code 1:

section .data
        out: db "%s",10,0
        mes1: db "a",0
        mes2: db "b",0
section .text
        extern printf
        global main
main:
        push rbp
        mov rdi,out
        mov rsi,mes1
        mov rax,0
        call printf
        mov rdi,out
        mov rsi,mes2
        mov rax,0
        call printf
        pop rbp
        mov rax,0
        ret

Output is:

a
b

Code 2: changed the .data section to:

section .data
        out: db "%s",10 ; no 0
        mes1: db "a",0
        mes2: db "b",0

Output is:

a
ab
a

Code 3: changed the .data section to:

section .data
        out: db "%s",10,0
        mes1: db "a"
        mes2: db "b"

Output is:

ab
b

So what does the null character do?

I tried to debug it in pwndbg but I didn’t get anything interesting.