comment fixes

mp.c

@@ -196,7 +196,8 @@ mp_bcpu(void)
 
 extern void mpmain(void);
 
-#define APBOOTCODE 0x7000 // XXX hack
+// Write bootstrap code to unused memory at 0x7000.
+#define APBOOTCODE 0x7000
 
 void
 mp_startthem(void)

syscall.c

@@ -168,7 +168,7 @@ syscall(void)
     break;
   default:
     cprintf("unknown sys call %d\n", num);
-    // XXX fault
+    // Maybe kill the process?
     break;
   }
   cp->tf->eax = ret;