Why does this assembly program produce no output?

Question

Being new to x86_64 assembly, I am trying to write a basic "hello" program on my laptop running 64-bit OpenBSD. The program runs to completion with exit code 0, but seems to ignore the system call to write text to stdout. Why?

I am using the GNU assembler and creating the executable with:

as -o hello.o hello.s; ld -Bstatic hello.o

# OpenBSD ELF identification
.section ".note.opensd.ident", "a"
.p2align 2
.long 0x8
.long 0x4
.long 0x1
.ascii "OpenBSD\0"
.long 0x0
.p2align 2

.section .data
msg: .ascii "hello"

.section .text
.globl _start
_start:
    push $5 # number of bytes to write
    push $msg # message address
    push $1 # file descriptor 1 for stdout
    mov $4, %eax # write is system call 4
    syscall

    push $0 # exit code 0
    mov $1, %eax # exit is system call 1
    syscall

Answer 1

Since you tag x86_64 and probably are on an x86_64 system. Therefore you need to:

• as --64
• use pushq instead of pushl to push 64bit values on the stack

• transfer those values to the appropriate registers before syscall

  .section ".note.opensd.ident", "a"
  .p2align 2
  .long 0x8
  .long 0x4
  .long 0x1
  .ascii "OpenBSD\0"
  .long 0x0
  .p2align 2
  
  .section .data
   msg: .ascii "hello"
  
  .section .text
  .globl _start
  _start:
          pushq $0x4
          popq %rax               # 4 (write syscall) into rax
          pushq $0x1
          popq %rdi               # 1 (STDOUT) into rdi
          pushq $msg
          popq %rsi               # address of hello msg into rsi
          pushq $0x5
          popq %rdx               # length of hello msg into rdx
          syscall
          pushq $1
          popq %rax
          pushq $0
          popq %rdi
          syscall
  

the following articles offer some good info:

x64 asm on FreeBSD

differences between x86 and x64 asm