system call example, use gcc -m32 32-{name}.S or gcc 64-{name}.S

32-int80.S

.intel_syntax noprefix

.data
hello_string:
	.ascii "Hello World!\n"
len:
	.long len-hello_string

.text
.global main
main:
	push ebp
	mov ebp, esp

	mov edx, len
	lea ecx, hello_string
	mov ebx, 1
	mov eax, 4
	int 0x80

	mov eax, 0
	leave
	ret

32-sysenter.S

.intel_syntax noprefix

.data
hello_string:
	.ascii "Hello World!\n"
len:
	.long len-hello_string

.text
.global main
main:
	push ebp
	mov ebp, esp

	mov edx, len
	lea ecx, hello_string
	mov ebx, 1
	mov eax, 4
	call get_selfaddr
get_selfaddr:
	add dword ptr [esp], sysenter_ret-get_selfaddr
	push ecx
	push edx
	push ebp
	mov ebp, esp
	sysenter

sysenter_ret:
	mov eax, 0
	leave
	ret

64-syscall.S

.intel_syntax noprefix

.data
hello_string:
	.ascii "Hello World!\n"
len:
	.long len-hello_string

.text
.global main
main:
	push rbp
	mov rbp, rsp

	mov rdx, len[rip]
	lea rsi, hello_string[rip]
	mov rdi, 1
	mov rax, 1
	syscall

	mov rax, 0
	leave
	ret