Hello World using x86 assembler on Mac 0SX

Question

I am trying to dive into some x86 assembly programming on my Mac, but am having trouble producing an executable. The problem seems to be at the linking stage.

helloWorld.s:

.data

    HelloWorldString:
    .ascii "Hello World\n"

.text

.globl _start

_start:
    # load all the arguments for write()
    movl $4, %eax
    movl $1, %ebx
    movl $HelloWorldString, %ecx
    movl $12, %edx
    # raises software interrupt to call write()
    int $0x80

    # call exit()
    movl $1, %eax
    movl $0, %ebx
    int $0x80

Assemble the program:

as -o helloWorld.o helloWorld.s

Link the object file:

ld -o helloWorld helloWorld.o

The error I get at this point is:

ld: could not find entry point "start" (perhaps missing crt1.o) for inferred architecture x86_64

Any advice on what I'm doing wrong / missing would be very helpful. thanks

Answer 1

You'll probably find it easier to build with gcc rather than trying to micro-manage the assembler and linker, e.g.

$ gcc helloWorld.s -o helloWorld

(You'll probably want to change _start to _main if you go this route.)

Incidentally, it can be instructive to start with a working C program, and study the generated asm from this. E.g.

#include <stdio.h>

int main(void)
{
    puts("Hello world!\n");

    return 0;
}

when compiled with gcc -Wall -O3 -m32 -fno-PIC hello.c -S -o hello.S generates:

    .cstring
LC0:
    .ascii "Hello world!\12\0"
    .text
    .align 4,0x90
.globl _main
_main:
    pushl   %ebp
    movl    %esp, %ebp
    subl    $24, %esp
    movl    $LC0, (%esp)
    call    _puts
    xorl    %eax, %eax
    leave
    ret
    .subsections_via_symbols

You might want to consider using this as a template for your own "Hello world" or other experimental asm programs, especially given that it already builds and runs:

$ gcc -m32 hello.S -o hello
$ ./hello 
Hello world!

One final comment: beware of taking examples from Linux-oriented asm books or tutorials and trying to apply them under OS X - there are important differences !

Answer 2

Try:

ld -e _start -arch x86_64 -o HelloWorld HelloWorld.S

then:

./HelloWorld

Info:

-e <entry point>
-arch <architecture>, You can check your architecture by uname -a 
-o <output file>

Answer 3

hello.asm

.data

    HelloWorldString:
    .ascii "Hello World!\n"

.text

.globl start

start:
    ; load all the arguments for write()
    movl $0x2000004, %eax
    movl $1, %ebx
    movq HelloWorldString@GOTPCREL(%rip), %rsi
    movq $100, %rdx
    ; raises software interrupt to call write()
    syscall

    ; call exit()
    movl $0x2000001, %eax
    movl $0, %ebx
    syscall

Then run:

$ as -arch x86_64  -o hello.o hello.asm
$ ld -o hello hello.o
$ ./hello

This is a working solution for Mac OS X Mach-0 GNU-based assemblers

Answer 4

The code in the question looks like it's for 32-bit Linux using the int $0x80 ABI with args in EBX, ECX, EDX.

x86-64 code on MacOS uses the syscall instruction, with arg-passing and return value similar to what's documented in the x86-64 System V ABI for Linux. It's completely different from int $0x80, the only similarity being that the call number is passed in EAX/RAX. But the call numbers are different: https://sigsegv.pl/osx-bsd-syscalls/ ORed with 0x2000000.

Args go in the same registers as for function calls. (except R10 instead of RCX.)

See also basic assembly not working on Mac (x86_64+Lion)? and How to get this simple assembly to run?

---

I think this is a lot neater and more intuitive version of what was suggested in another answer.

OS X uses start, not _start, for the process entry point.

.data
str:
  .ascii "Hello world!\n"
  len = . - str                  # length = start - end.   . = current position

.text
.globl start
start:
    movl   $0x2000004, %eax
    movl   $1, %edi
    leaq   str(%rip), %rsi  
    movq   $len, %rdx          
    syscall                       # write(1, str, len)

    movl   $0x2000001, %eax 
    movl   $0, %edi
    syscall                       # _exit(0)

Normally you'd omit the operand-size suffix when a register implies it. And use xor %edi,%edi to zero RDI.

And use mov $len, %edx because you know the size is smaller than 4GB so a more efficient 32-bit zero-extended mov-immediate will work, like you're doing to set RAX to the call number.

Notice the use of a RIP-relative LEA to get the address of static data into a register. x86-64 code on MacOS can't use 32-bit absolute addressing because the base address where your executable will be mapped is above 2^32.

There are no relocation types for 32-bit absolute addresses so you can't use them. (And you want RIP-relative, not 64-bit absolute, even though that's also supported.)

Answer 5

To assemble and link the above code on macOS 10.15 the following changes need to be made.

Change .global _start to .global main and _start to main

To assemble and link the code use:

as -arch x86_64 -o hello.o hello.asm
ld -arch x86_64 -o hello hello.o -lSystem

This is assuming that "Apple clang version 12.0.0" of "as" is being used and the corresponding "ld" is used.