global _start

section .data
	STR: db 'this string will be reversed',0xA
	STRSIZE: equ $ - STR

section .text
_start:
; Function calls follow the Unix System V ABI on Linux.

; This spec defines the INTEGER class "functions" (not sure what they mean)
; that use the registers:
; %rdi, %rsi, %rdx, %rcx, %r8 %r9
; Most data types fit the INTEGER class; pointers and integers all fit within
; a single register. So nearly all functions are of this class..

mov rdi, STR ; first arg
lea rsi, [STRSIZE - 1] ; second arg (exclude \n)

; Call jumps to the label supplied, but first sets the return point
; in the stack (hence recursion can cause stack overflows)
call revstr

mov rax, 1 ; write
mov rdi, 1 ; stdout
mov rsi, STR
mov rdx, STRSIZE
syscall

mov rax, 60 ; see ya later
xor rdi, rdi ; ret 0
syscall

; Functions MUST NOT modify the registers:
; rbx and r12 to r15

; Function arguments go into:
; %rdi, %rsi, %rdx, %rcx, %r8 %r9

; Function return value(s) end up in:
; %rax, %rdx

; void revstr(char * s, size_t length)
; Reverses the bytes given, e.g. s[length] = s[0], s[length-1] = s[1], etc. 
revstr:
	xor r8, r8 ; loop counter
	lea r9, [rsi - 1] ; loop limit

.loop: ; labels prefixed with a dot (.) are LOCAL labels
	cmp r8, r9
	jge .done

	; The actual swapping
	mov cl, byte [rdi + r8] ; one value needs to be a in a register
	xchg cl, byte[rdi + r9] ; swap s[loop counter] with s[loop limit]
	mov byte [rdi, r8], cl  ; store s[loop counter] manually

	inc r8 ; ++loop counter
	dec r9 ; --loop limit
	jmp .loop

.done:
	ret