;;
;; initialized data
;;
section .data
	SYS_WRITE	equ 1
	STD_OUT		equ 1
	SYS_EXIT	equ 60
	EXIT_CODE	equ 0

	NEW_LINE	db 0xa
	INPUT		db "Hello world!"

;;
;; non initialized data
;;
section .bss
	OUTPUT	resb 1

;;
;; code
;;
section .text
	global	_start

;;
;; main routine
;;
_start:
	;; get addres of INPUT
	mov	rsi, INPUT
	;; zeroize rcx for counter
	xor	rcx, rcx
	; df = 0 si++
	cld
	; remember place after function call
	mov	rdi, $ + 15
	;; get string lengt
	call	calculateStrLength
	;; write zeros to rax
	xor	rax, rax
	;; additional counter for reverseStr
	xor	rdi, rdi
	;; reverse string
	jmp	reverseStr

;;
;; calculate length of string
;;
calculateStrLength:
	;; check is it end of string
	cmp	byte [rsi], 0
	;; if yes exit from function
	je	exitFromRoutine
	;; load byte from rsi to al and inc rsi
	lodsb
	;; push symbol to stack
	push	rax
	;; increase counter
	inc	rcx
	;; loop again
	jmp	calculateStrLength

;;
;; back to _start
;;
exitFromRoutine:
	;; push return addres to stack again
	push	rdi
	;; return to _start
	ret

;;
;; reverse string
;;
;; 31 in stack
reverseStr:
	;; check is it end of string
	cmp	rcx, 0
	;; if yes print result string
	je	printResult
	;; get symbol from stack
	pop	rax
	;; write it to output buffer
	mov	[OUTPUT + rdi], rax
	;; decrease length counter
	dec	rcx
	;; increase additional length counter (for write syscall)
	inc	rdi
	;; loop again
	jmp	reverseStr

;;
;; Print result string
;;
printResult:
	mov	rdx, rdi
	mov	rax, 1
	mov	rdi, 1
	mov	rsi, OUTPUT
	syscall
	jmp	printNewLine

;;
;; Print new line
;;
printNewLine:
	mov	rax, SYS_WRITE
	mov	rdi, STD_OUT
	mov	rsi, NEW_LINE
	mov	rdx, 1
	syscall
	jmp	exit

;;
;; Exit from program
;;
exit:
	;; syscall number
	mov	rax, SYS_EXIT
	;; exit code
	mov	rdi, EXIT_CODE
	;; call sys_exit
	syscall