Question

I am reading some linux assembly manuals and found idea about using printf() function. I need it to output register values for debugging reasons in binary form to terminal, but now I am tried simply to test that function with text.

I am stuck, because of segfault when I am using pushq instead of pushl. How can I change this program to output strings and binary form of registers?

.data
input_prompt:
    .string "Hello, world!"

printf_format:
    .string "%5d "

printf_newline:
    .string "\n"

size:
    .long 0

.text
.globl main
main:
    pushq $input_prompt
    call  printf

    movl  $0, %eax
    ret

It was compiled by GCC as:

gcc tmp.S -o tmp

Answer 1

Linux (and Windows) x86-64 calling convention has the first few arguments not on the stack, but in registers instead

See http://www.x86-64.org/documentation/abi.pdf (page 20)

Specifically:

1. If the class is MEMORY, pass the argument on the stack.
2. If the class is INTEGER, the next available register of the sequence %rdi, %rsi, %rdx, %rcx, %r8 and %r9 is used.
3. If the class is SSE, the next available vector register is used, the registers are taken in the order from %xmm0 to %xmm7.
4. If the class is SSEUP, the eightbyte is passed in the next available eightbyte chunk of the last used vector register.
5. If the class is X87, X87UP or COMPLEX_X87, it is passed in memory.

The INTEGER class is anything that will fit in a general purpose register, so that’s what you would use for string pointers as well.