hello_world.s

# Prepare a new stack frame (save the old base pointer, and set
the new base pointer to the current stack pointer)
pushq %rbp
movq %rsp, %rbp

# Get the address of your string (lea = load effective address)
# and put it into the rdi register as a parameter to printf.
leaq L_.str(%rip), %rdi

# Not sure why this is needed - I believe it sets eax to zero.
# must be another parameter to printf?
xorl %eax, %eax
callq _printf

# Same thing here - set eax to zero, which is now the
# return value of the function.
xorl %eax, %eax

# Restore the old stack frame and return to the caller
popq %rbp
retq

# And this is the string literal that is passed to printf.
L_.str: ## @.str
.asciz "Hello World!\n"