Subroutines

helloworld.asm

@@ -4,14 +4,36 @@
 ; Run with: ./helloworld
 
 SECTION .data
-msg     db      'Hello World!', 0Ah     ; assign msg variable with your message string
+msg     db      'Hello, brave new world!', 0Ah     ; assign msg variable with your message string
 
 SECTION .text
 global  _start
 
 _start:
-    mov     edx, 13     ; number of bytes to write - one for each letter plus 0Ah (line feed character)
-    mov     ecx, msg    ; move the memory address of our message string into ecx
-    mov     ebx, 1      ; write to the STDOUT file
-    mov     eax, 4      ; invoke SYS_WRITE (kernel opcode 4)
+    mov     eax, msg
+    call    strlen
+
+    mov     edx, eax        ; our function leaves the result in EAX
+    mov     ecx, msg
+    mov     ebx, 1          ; STDOUT
+    mov     eax, 4          ; SYS_WRITE
     int     80h
+
+    mov     ebx, 0          ; return 0
+    mov     eax, 1          ; SYS_EXIT
+    int     80h
+
+strlen:
+    push    ebx             ; preserve EBX while we use in this function
+    mov     ebx, eax        ; move the address in EAX into EBX
+
+nextchar:
+    cmp     byte [eax], 0   ; compare the byte pointed to by EAX at this address against zero (Zero is an end of string delimiter)
+    jz      finished        ; jump (if the zero flagged has been set) to the point in the code labeled 'finished'
+    inc     eax             ; increment the address in EAX by one byte (if the zero flagged has NOT been set)
+    jmp     nextchar        ; jump to the point in the code labeled 'nextchar'
+
+finished:
+    sub     eax, ebx
+    pop     ebx
+    ret