assembly - Segfault when loading from [esp] in 64-bit code

Question

I'm quite new to x86 assembly, and I'm trying to build off a hello world program. I'm trying to make a subroutine, that writes a single byte to stdout, but i've hit a problem.

The line mov ebx, [esp+1] (to load the byte passed, when I call the subroutine) causes a segfault.

I've tried xoring the ebx register with itself, to make sure that it is empty, to make sure, that it doesn't mess with the syscall

_start:
    push 32h
    call _writeByte

    ; This just jumps to an exit routine
    jmp  _exit

_writeByte:
    ; This line causes the problem. If I remove it the program works fine
    mov  ebx, [esp+1]
    xor  ebx, ebx

    mov  eax, 1
    mov  edi, 1
    mov  esi, tmp
    mov  edx, 1
    syscall

    ret

Why is the program segfaulting?

Answer 1

I'm in x64 mode, and like a bunch of people suggested in the comments using mov ebx, [rsp+8] worked, because esp are just the 4 lower bytes of the register. The stack is outside the low 4 GiB of virtual address space, so ESP != RSP and [esp] will be an unmapped page.

Note that x86-64 calling conventions pass the first few args in register, not on the stack, so you normally don't want to do this at all (unless your function has lots of args).