Small Hello World repeater

repeat_hello.asm

; nasm -f elf64 -g -F dwarf repeat_hello.asm && gcc -g -no-pie repeat_hello.o -o repeat_hello && ./repeat_hello 5
; Intended for use on linux x86-864 machines; to run on OS X you'd have to change the syscalls.
extern atoi

global main

section .data
message:	db	"Hello, world!", 0x0A
length:		equ	14
err_msg:	db	"Wrong!", 0x0A
err_msg_len:	equ 	7

section .text
main:		push	r12			; Make an odd number of pushes to align stack to 16 byte boundary (call gives you one).
		push 	r13
		push 	r14	
		cmp 	rdi, 2			; Argc goes into RDI. Test for two arguments, if not two, jump to error message.
		jne	error			; Jump to error if we don't have two arguments.
		mov 	rdi, [rsi+8]		; Load the string at argv[1] (address [rsi+8]) into rdi - this should be the number of iterations. 
						; Note: argv[0] is at [rsi], but that's the name of the program, not the actual number of iters.
		call	atoi			; Convert the string stored at rdi to an integer in eax using the C library function atoi().
						; Make sure you use GCC and link against the C library.
		cmp	eax, 0			; Gotta make sure that we have a positive number of times to print.
		jl	error			; If the value is too small, go to the error block.
							
						; Now we're confident that EAX >= 0.
		mov 	r15d, eax		; Set the iteration cap to EAX.
		cmp	r15, 0			; Compare against zero.
		je	exit			; If the iteration cap is zero, go to exit; we've got no work to do.
		mov 	r14, 0			; Otherweise, we'll set a counter variable in r14 to zero. 
		
display:					; Main "write" loop. Prints `message` zero or more times.
		mov 	rax, 1			; Set up syscall WRITE with it's magic number 1 on linux.
		mov 	rdi, 1			; We're writing to the special STDOUT file, so put it as the first parameter in RDI.
		mov 	rsi, message		; Set RSI to address of message.
		mov 	rdx, length		; Set RDX to length of message.
		syscall				; Fire syscall for WRITE.
						
						; Now we handle loop logic.
		inc 	r14			; Increment count variable, we've done a run through the code.	
		cmp	r14, r15		; Check if count is greater than the iteration threshold.
		je	exit			; If count = iters, we're done. Jump to exit label.
		jmp	display			; Otherwise, jump back to display label.

error:						; Print out "Wrong!" when we jump here.
		mov	rax, 1			 
		mov	rdi, 1			
		mov	rsi, err_msg		 
		mov	rdx, err_msg_len	
		syscall
		jmp exit

exit:						; Send EXIT syscall (different magic number than OS X) with return code zero.	
		mov	rax, 60			
		mov	rdi, 0			 
		syscall

mrdmnd@supernova:~/code/arch_demos$ ./repeat_hello 
Wrong!
mrdmnd@supernova:~/code/arch_demos$ ./repeat_hello -1
Wrong!
mrdmnd@supernova:~/code/arch_demos$ ./repeat_hello 0
mrdmnd@supernova:~/code/arch_demos$ ./repeat_hello 5
Hello, world!
Hello, world!
Hello, world!
Hello, world!
Hello, world!
mrdmnd@supernova:~/code/arch_demos$