asm-5: fix single-argument PRINT macro with %strlen

content/asm_5.md

@@ -283,20 +283,31 @@ Using the `%strlen` directive, we can calculate the length of a string. Using `%
 ```assembly
 ;; Definition of the PRINT macro.
 %macro PRINT 1
+section .data
+        ;; Define the string to print with a label local to this macro call.
+        %%msg db %1
+section .text
         ;; Specify the number of the system call (1 is `sys_write`).
-    	mov rax, 1
+        mov rax, 1
         ;; Set the first argument of `sys_write` to 1 (`stdout`).
         mov rdi, 1
         ;; Set the second argument of `sys_write` to the reference of the string to print.
-        ;; The reference will be stored in the first argument of the macro.
-        mov rsi, %1
+        mov rsi, %%msg
         ;; Set the third argument of `sys_write` to the length of the string to print.
         mov rdx, %strlen(%1)
         ;; Call the `sys_write` system call.
         syscall
 %endmacro
 ```
 
+Now we can pass a string directly to the macro:
+
+```assembly
+PRINT "Hello world!"
+```
+
+You might be wondering why we define the string inside the macro instead of passing a label, as we did with the implementation of the `PRINT` macro above. The reason is that `%strlen` is a preprocessor directive. The preprocessor runs before the assembler and knows nothing about labels or the data behind them. It sees only the given text. So if we passed `msg_1` to our macro, `%strlen` would happily return `5`, which is the length of the word `msg_1` itself. As a result, our program would print only the first five characters of the message instead of the whole message.
+
 ### %rotate directive
 
 The `%rotate` directive allows you to rotate the input arguments given to the macro. The arguments are rotated to the left by one position. This directive is usually used together with the next `%rep` directive.