Leaf function calls added

x86_64/4_leaf.asm

@@ -0,0 +1,63 @@
+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