First Hello world syscall

kernel/amd64/debug.c

@@ -13,6 +13,8 @@
 /// Lock, which ensures that prints to the serial port are atomic
 static spin_lock_t serial_lock = SPIN_LOCK_INIT;
 
+static bool debug_init = false;
+
 /// Block until TX buffer is empty
 static bool amd64_debug_serial_tx_empty (void) {
   return (bool)(amd64_io_inb (PORT_COM1 + 5) & 0x20);
@@ -30,6 +32,9 @@ static void amd64_debug_serial_write (char x) {
  * all prints are atomic.
  */
 void debugprintf (const char* fmt, ...) {
+  if (!debug_init)
+    return;
+
   char buffer[BUFFER_SIZE];
   memset (buffer, 0, sizeof (buffer));
 
@@ -61,4 +66,6 @@ void amd64_debug_init (void) {
   amd64_io_outb (PORT_COM1 + 3, 0x03);
   amd64_io_outb (PORT_COM1 + 2, 0xC7);
   amd64_io_outb (PORT_COM1 + 4, 0x0B);
+
+  debug_init = true;
 }