How to write assembly language hello world program for 64 bit Mac OS X using printf?

Question

I am trying to learn writing assembly language for 64 bit Mac OS. I have no problem with 32 bit Mac OS and both 32 bit and 64 bit Linux.

However, Mac OS 64 bit is di

Answer 1

You didn't say exactly what the problem you're seeing is, but I'm guessing that you're crashing at the point of the call to printf. This is because OS X (both 32- and 64-bit) requires that the stack pointer have 16-byte alignment at the point of any external function call.

The stack pointer was 16-byte aligned when _main was called; that call pushed an eight-byte return address onto the stack, so the stack is not 16-byte aligned at the point of the call to _printf. Subtract eight from %rsp before making the call in order to properly align it.

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So I went ahead and debugged this for you (no magic involved, just use gdb, break main, display/5i $pc, stepi, etc). The other problem you're having is here:

movq _hello(%rip), %rdi

This loads the first eight bytes of your string into %rdi, which isn't what you want at all (in particular, the first eight bytes of your string are exceedingly unlikely to constitute a valid pointer to a format string, which results in a crash in printf). Instead, you want to load the address of the string. A debugged version of your program is:

.cstring
_hello: .asciz "Hello, world\n"

.text
.globl _main
_main:
    sub  $8, %rsp           // align rsp to 16B boundary
    mov  $0, %rax
    lea  _hello(%rip), %rdi // load address of format string
    call _printf            // call printf
    add  $8, %rsp           // restore rsp
    ret