;  ------------------------------------------------------------------------
;  hellos_64.asm
;  Writes "Hello, World" to the console using only system calls.
;  Runs on 64-bit Linux only.
;  To assemble and run: using single Linux command
;
;  nasm -f elf64 hellos_64.asm && ld hellos_64.o && ./a.out   
;
;  -------------------------------------------------------------------------

	global  _start        ; standard ld main program

	section .data	      ; data section
msg:	db "Hello World",10   ; the string to print, newline 10
len:	equ $-msg	      ; "$" means "here"
			      ; len is a value, not an address

	section .text     
_start:
	;;  write(1, msg, 13)         equivalent system command
	mov  	rax, 1	      ; system call 1 is write
	mov  	rdi, 1	      ; file handle 1 is stdout
	mov  	rsi, msg      ; address of string to output
	mov	rdx, len      ; number of bytes
	syscall		      ; invoke operating system to do the write

	;;  exit(0)                   equivalent system command
	mov     rax, 60	      ; system call 60 is exit
	xor     rdi, rdi      ; exit code 0
	syscall		      ; invoke operating system to exit