NASM Hello World for x86 and x86_64 Intel Mac OS X (get yourself an updated nasm with brew)

32.asm

; /usr/local/bin/nasm -f macho 32.asm && ld -macosx_version_min 10.7.0 -o 32 32.o && ./32

global start

section .text
start:
    push    dword msg.len
    push    dword msg
    push    dword 1
    mov     eax, 4
    sub     esp, 4
    int     0x80
    add     esp, 16

    push    dword 0
    mov     eax, 1
    sub     esp, 12
    int     0x80

section .data

msg:    db      "Hello, world!", 10
.len:   equ     $ - msg

64.asm

; /usr/local/bin/nasm -f macho64 64.asm && ld -macosx_version_min 10.7.0 -lSystem -o 64 64.o && ./64

global start


section .text

start:
    mov     rax, 0x2000004 ; write
    mov     rdi, 1 ; stdout
    mov     rsi, msg
    mov     rdx, msg.len
    syscall

    mov     rax, 0x2000001 ; exit
    mov     rdi, 0
    syscall


section .data

msg:    db      "Hello, world!", 10
.len:   equ     $ - msg

References.markdown

• NASM Manual - for the syntax
• What are the calling conventions for UNIX & Linux system calls on x86-64 - comparison
• AMD64 ABI reference - A.2.1 - for the actual x86_64 calling convention
• X86 calling conventions - moar calling conventions
• Mac OS X syscalls reference

for macOS users you have two options. You can statically link or dynamically link which will require libSystem.*
ld links dynamically to your system libraries by default.

Static linking:
ld -e "start" -static -o 64 64.o

Dynamic linking:
ld -macosx_version_min 10.7.0 -e "start" -no_pie -lSystem -L/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib -o 64 64.o

If the linker cannot find specific symbols you can inspect your object file to see all symbols
(note: your start symbol doesn't need to be renamed, just needs to be referenced as-is from the symbol table)

objdump -t ./64.o

SYMBOL TABLE:
0000000000000027 l     O __DATA,__data msg
000000000000000e l       *ABS* msg.len
0000000000000000 g     F __TEXT,__text start

A couple of things about the hello world above.

OS X deprecated syscall. It still works but if you look at the compiled code, the gas compiler is replacing the syscall with imported function calls. Since they are function calls, you are supposed to align stack. It seems to work without it, but if you want to align the stack to a 16 byte boundary without worrying about where it is when the program starts, you can AND the stack pointer with FFFFFFFFFFFFFFF0 at entry.

I think exported functions have to have _ in front of them in gas. Something to do with avoiding name collisions with imports maybe..

Mac also changed where the system libraries are for security reasons for Big Sur. If you want your program to run on a system that doesn't have the xtools command line tools installed, you have to link in a certain way. I posted a question about this on the Mac developer forums and Eskimo explained how to figure it out using -v with a c program. The link to the discussion is here: https://developer.apple.com/forums/thread/669094
The short answer is, this link command works: ld -e _myhelloworldstart -no_uuid -no_eh_labels -demangle -dynamic -arch x86_64 -platform_version macos 11.0.0 11.1 -syslibroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -o helloworld64 -L/usr/local/lib helloworld64.o -lSystem
The key is the -syslibroot command.
I think if you use OTOOL to look what the library is linking against, you can see what's going on.

This hello world works for gas with the filename helloworld64gas.s:

.text

_omyhelloworldmessage:
.asciz "\nHello World!\n\n"

.globl _myhelloworldstart
_myhelloworldstart:

/* align the stack regardless of where it is when this function is called */
andq $0xfffffffffffffff0, %rsp

movq $1, %rdi /* stdout handle /
leaq _omyhelloworldmessage(%rip), %rsi / pmessageaddress /
movq $15, %rdx / messagelength */
call _write

call _exit

with this assembly command:

as -mmacosx-version-min=11.0 ./helloworld64gas.s -o helloworld64gas.o

and this link command:

ld -e _myhelloworldstart -no_uuid -no_eh_labels -demangle -dynamic -arch x86_64 -platform_version macos 11.0.0 11.1 -syslibroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -o helloworld64gas -L/usr/local/lib helloworld64gas.o -lSystem

If you are interested, and up for something different,
I wrote my own assembler which is available on here on github under the DiaperGlu repository.
This hello world code works with that assembler:

NEW-FLAT-OSYMBOL-BUF

X86-WORDLIST >SEARCH-ORDER

DECIMAL

OSYMBOL omyhelloworldmessage
13 CODE-U8, 10 CODE-U8, // pushes crlf to the buffer
$" Hello World!" PCURRENTCOMPILEBUFFER @ $>BUF // pushes the message to the buffer
13 CODE-U8, 10 CODE-U8, // appends another crlf
13 CODE-U8, 10 CODE-U8, // and another crlf

OSYMBOL myhelloworldstart
// align the stack regardless of where it is when this function is called
HEX -10 N RSP AND,

1 N RDI MOV, // stdout handle
EH. omyhelloworldmessage [O] RSI LEA, // pmessageaddress
EH. myhelloworldstart EH. omyhelloworldmessage - N RDX MOV, // messagelength

IMP CALL, OSYMBOL-IMPORT write

IMP CALL, OSYMBOL-IMPORT exit

PCURRENTCOMPILEBUFFER @
EH
1 EH[ND]
BUF>NEWEXPORTIMPORT.OBUF

DUP $" helloworld64.o" SAVEFILE$
FREEBUFFER

SEARCH-ORDER> DROP

FREE-FLAT-OSYMBOL-BUF