minor fixes

a/assembler_nasm_linux64.asm

@@ -3,14 +3,15 @@ section	.text
 
 _start:					;tell linker entry point
 
-	xor	eax,eax		; eax:=0
+	xor	eax,eax		;eax:=0
 	lea	edx,[rax+len]	;message length
 	mov	al,1		;system call number (sys_write)
 	mov	esi,msg		;message to write
 	mov	edi,eax		;file descriptor (stdout)
 	syscall			;call kernel
 	
-	mov	al,60		;system call number (sys_exit)
+	xor	edi, edi	;return 0 (EXIT_SUCCESS)
+	lea	eax,[rdi+60]	;system call number (sys_exit)
 	syscall			;call kernel
 
 section	.rodata