Question

I’m reading Paul Carter’s pcasm book. It uses NASM, a C driver app that calls my assembly code, and a companion library that makes it easy to do basic I/O in assembly.

This is what my function that will be called from C looks like:

segment .text
    global  _asm_main
_asm_main:
    enter   0,0               ; setup routine
    pusha

    mov bx, 0034h   ; bx = 52 (stored in 16 bits)
    mov cl, bl      ; cl = lower 8-bits of bx
    mov eax, ecx
    call print_int

    popa
    mov     eax, 0            ; return back to C
    leave                     
    ret

The print_int function prints the value store at eax as an integer. But this prints out garbage to stdout:

4200244

If I initialize the ecx register to 0 with mov ecx, 0000h before using it, I will get the expected output:

52

Is initialization always required, and if so, is there a quick way of initializing all the registers to 0 (or a user-defined initializer), from either C or NASM?

I’m using XP32, MinGW 4.4.0, and NASM 2.09.04.

Answer 1

The function print_int prints out the value of eax. In your code you only assign to the lowest of the four bytes of eax (a.k.a. al) via the following chain of assignments: bl->cl->al. The remaining three bytes of eax are left uninitialized. Your code inherits whatever values happened to be in those three bytes at the start of your routine. This is why you get garbage.

You have to initialize all the registers and memory locations that you use.

My x86 assembly is a bit rusty, but I am pretty sure there isn’t a single instruction that would set all general-purpose registers to zero. If you were so inclined, you could probably write a macro that would do this for you.