Question

I’m learning assembly and I have a very basic loop here

segment .data
    msg: db '%d',10,0

segment .text

    global  _asm_main
    extern _printf

_asm_main:

    push DWORD 5 ; Should loop 5 times
    call dump_stack
    add esp,4

    ret

dump_stack:
    push ebp
    mov ebp, esp

    mov ecx, 0

loop_start:
    cmp ecx,[ebp+8] ;compare to the first param of dump_stack, (5)
    jnle loop_end

    push ecx ;push the value of my loop onto the stack
    push DWORD msg ;push the msg (%d) should just print the value of my loop
    call _printf
    add esp, 8 ;clear the stack

    inc ecx ;increment ecx
    jmp loop_start ; go back to my loop start

loop_end:

    mov esp, ebp
    pop ebp
    ret

My output looks something like this

program.exe
0

Just 0, then a newline. I tried to verify the loop was executing by moving my printf to the loop_end part, and it came out with ecx as 6, which is correct. So the loop is executing but printf is not… What am I doing wrong?

(Also, the function is called dump stack because it was initially supposed to dump the details of the stack, but that didn’t work because of the same reason here)

And I am compiling with nasm -f win32 program.asm -o program.o
Then I have a cpp file that includes windows.h, and I compiled it with gcc -c include
and finally I linked them with gcc -o program program.o include.o
and I run program.exe

Answer 1

My guess is that printf() modifies ecx, it becomes >= [ebp+8] and your loop body executes only once. If that’s the case, you need to read up on the calling conventions used in your compiler and manually preserve and restore the so-called volatile registers (which a called function can freely modify without restoring). Note that there may be several different calling conventions. Use the debugger!