Reference count page tables

kernel/platform/i386_pc/sys/mm.h

@@ -48,11 +48,11 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
                                  I use this here, because when (un)mapping an entire page range
                                  it would be inefficient to constantly (un)lock. */
 
-#define KERNEL_HEAP_START 0xF0000000
-#define MEM_PROBE_HEAP_START 0xE0000000
+#define KERNEL_HEAP_START       0xF0000000
 
 struct page_dir {
   volatile uint32_t *pd;
+  uint16_t pt_refcount[1024];
   struct spinlock sl;
 };