diff --git a/Makefile b/Makefile
index 33bfb0a..20cb884 100644
--- a/Makefile
+++ b/Makefile
@@ -148,6 +148,12 @@ _forktest: forktest.o $(ULIB)
 mkfs: mkfs.c fs.h
 	gcc -Werror -Wall -o mkfs mkfs.c
 
+# Prevent deletion of intermediate files, e.g. cat.o, after first build, so
+# that disk image changes after first build are persistent until clean.  More
+# details:
+# http://www.gnu.org/software/make/manual/html_node/Chained-Rules.html
+.PRECIOUS: %.o
+
 UPROGS=\
 	_cat\
 	_echo\
diff --git a/mmu.h b/mmu.h
index 5c9ab60..685f51d 100644
--- a/mmu.h
+++ b/mmu.h
@@ -142,6 +142,7 @@ struct segdesc {
 
 // Address in page table or page directory entry
 #define PTE_ADDR(pte)   ((uint)(pte) & ~0xFFF)
+#define PTE_FLAGS(pte)  ((uint)(pte) &  0xFFF)
 
 #ifndef __ASSEMBLER__
 typedef uint pte_t;
diff --git a/vm.c b/vm.c
index dde56b7..4cffb58 100644
--- a/vm.c
+++ b/vm.c
@@ -311,7 +311,7 @@ copyuvm(pde_t *pgdir, uint sz)
 {
   pde_t *d;
   pte_t *pte;
-  uint pa, i;
+  uint pa, i, flags;
   char *mem;
 
   if((d = setupkvm()) == 0)
@@ -322,10 +322,11 @@ copyuvm(pde_t *pgdir, uint sz)
     if(!(*pte & PTE_P))
       panic("copyuvm: page not present");
     pa = PTE_ADDR(*pte);
+    flags = PTE_FLAGS(*pte);
     if((mem = kalloc()) == 0)
       goto bad;
     memmove(mem, (char*)p2v(pa), PGSIZE);
-    if(mappages(d, (void*)i, PGSIZE, v2p(mem), PTE_W|PTE_U) < 0)
+    if(mappages(d, (void*)i, PGSIZE, v2p(mem), flags) < 0)
       goto bad;
   }
   return d;