从x86汇编器调用C函数

Question

I am trying to write a function that converts decimal numbers into binary in assembler. Since printing is so troublesome in there, I have decided to make a separate function in C that just prints the numbers. But when I run the code, it always prints '0110101110110100'

Heres the C function (both print and conversion):

void printBin(int x) {
  printf("%d", x);
}

void DecToBin(int n)
{
    // Size of an integer is assumed to be 16 bits
    for (int i = 15; i >= 0; i--) {
        int k = n >> i;
        printBin(k & 1);
}

heres the code in asm:

.globl _DecToBin
.extern _printBin

_DecToBin:
  pushl %ebp
  movl %esp, %ebp

  movl 8(%ebp),%eax
  movl $15, %ebx
  cmpl $0, %ebx
  jl end

  start:
  movl %ebx, %ecx
  movl %eax, %edx
  shrl %cl, %eax
  andl $1, %eax
  pushl %eax
  call _printBin
  movl %edx, %eax
  dec %ebx
  cmpl $0, %ebx
  jge start

  end:

  movl %ebp, %esp
  popl %ebp
  ret

Cant figure out where the mistake is. Any help would be appreciated

disassembled code using online program

Answer 1

Your principle problem is that it is very unlikely that %edx is preserved across the function call to printBin. Also:

%ebx is not a volatile register in most (any?) C calling convention rules. You need to check your compilers documentation and conform to it. If you are going to use ebx, you need to save and restore it.

The stack pointer needs to be kept aligned to 16 bytes. On my machine (macos), it SEGVs under printBin if you don't.