Thanks Anton Burtsev <aburtsev@uci.edu>

kalloc.c

@@ -11,6 +11,7 @@
 
 void freerange(void *vstart, void *vend);
 extern char end[]; // first address after kernel loaded from ELF file
+                   // defined by the kernel linker script in kernel.ld
 
 struct run {
   struct run *next;