diff --git a/mps/code/airtest.c b/mps/code/airtest.c
index 302f91bb716..eea742def46 100644
--- a/mps/code/airtest.c
+++ b/mps/code/airtest.c
@@ -128,7 +128,8 @@ static void test_main(void *marker, int interior, int stack)
 
   if (stack) {
     res = mps_root_create_stack(&reg_root, scheme_arena, mps_rank_ambig(),
-                                0, thread, sizeof(mps_word_t) - 1, 0, marker);
+                                0, thread, mps_scan_area_tagged,
+				sizeof(mps_word_t) - 1, 0, marker);
     if (res != MPS_RES_OK)
       error("Couldn't create root");
   }
diff --git a/mps/code/amcssth.c b/mps/code/amcssth.c
index bd06dc9b2c6..80320261aec 100644
--- a/mps/code/amcssth.c
+++ b/mps/code/amcssth.c
@@ -140,7 +140,8 @@ static void *kid_thread(void *arg)
 
   die(mps_thread_reg(&thread, (mps_arena_t)arena), "thread_reg");
   die(mps_root_create_stack(&reg_root, arena, mps_rank_ambig(),
-                            0, thread, sizeof(mps_word_t) - 1, 0, marker),
+                            0, thread, mps_scan_area_tagged,
+			    sizeof(mps_word_t) - 1, 0, marker),
       "root_create");
 
   die(mps_ap_create(&ap, cl->pool, mps_rank_exact()), "BufferCreate(fooey)");
@@ -318,7 +319,8 @@ static void test_arena(int mode)
       "root_create_table(ambig)");
   die(mps_thread_reg(&thread, arena), "thread_reg");
   die(mps_root_create_stack(&reg_root, arena, mps_rank_ambig(),
-                            0, thread, sizeof(mps_word_t) - 1, 0, marker),
+                            0, thread, mps_scan_area_tagged,
+			    sizeof(mps_word_t) - 1, 0, marker),
       "root_create");
 
   die(mps_pool_create(&amc_pool, arena, mps_class_amc(), format, chain),
diff --git a/mps/code/awlut.c b/mps/code/awlut.c
index 93575df854a..1b2a430af47 100644
--- a/mps/code/awlut.c
+++ b/mps/code/awlut.c
@@ -268,7 +268,8 @@ static void *setup(void *v, size_t s)
   thr = guff->thr;
 
   die(mps_root_create_stack(&stack, arena, mps_rank_ambig(),
-                            0, thr, sizeof(mps_word_t) - 1, 0, v),
+                            0, thr, mps_scan_area_tagged,
+			    sizeof(mps_word_t) - 1, 0, v),
       "Root Create\n");
   die(mps_fmt_create_A(&dylanfmt, arena, dylan_fmt_A()),
       "Format Create\n");
diff --git a/mps/code/awluthe.c b/mps/code/awluthe.c
index e8b22f6ebb9..b5b4f4db07f 100644
--- a/mps/code/awluthe.c
+++ b/mps/code/awluthe.c
@@ -272,7 +272,8 @@ static void *setup(void *v, size_t s)
   thr = guff->thr;
 
   die(mps_root_create_stack(&stack, arena, mps_rank_ambig(),
-                            0, thr, sizeof(mps_word_t) - 1, 0, v),
+                            0, thr, mps_scan_area_tagged,
+			    sizeof(mps_word_t) - 1, 0, v),
       "Root Create\n");
   die(EnsureHeaderFormat(&dylanfmt, arena), "EnsureHeaderFormat");
   die(EnsureHeaderWeakFormat(&dylanweakfmt, arena), "EnsureHeaderWeakFormat");
diff --git a/mps/code/awlutth.c b/mps/code/awlutth.c
index b9b60aff962..fad46a29844 100644
--- a/mps/code/awlutth.c
+++ b/mps/code/awlutth.c
@@ -255,7 +255,8 @@ static void *setup(void *v, size_t s)
   thr = guff->thr;
 
   die(mps_root_create_stack(&stack, arena, mps_rank_ambig(),
-                            0, thr, sizeof(mps_word_t) - 1, 0, v),
+                            0, thr, mps_scan_area_tagged,
+			    sizeof(mps_word_t) - 1, 0, v),
       "Root Create\n");
   die(mps_fmt_create_A(&dylanfmt, arena, dylan_fmt_A()),
       "Format Create\n");
diff --git a/mps/code/gcbench.c b/mps/code/gcbench.c
index 2a61a8662c5..0451cd9fbc0 100644
--- a/mps/code/gcbench.c
+++ b/mps/code/gcbench.c
@@ -178,7 +178,8 @@ static void *start(void *p) {
   RESMUST(mps_thread_reg(&thread->mps_thread, arena));
   RESMUST(mps_root_create_stack(&thread->reg_root, arena, mps_rank_ambig(),
                                 (mps_rm_t)0, thread->mps_thread,
-                                sizeof(mps_word_t) - 1, 0, &marker));
+                                mps_scan_area_tagged,
+				sizeof(mps_word_t) - 1, 0, &marker));
   RESMUST(mps_ap_create_k(&thread->ap, pool, mps_args_none));
   thread->fn(thread);
   mps_ap_destroy(thread->ap);
diff --git a/mps/code/mpm.h b/mps/code/mpm.h
index 3fcd56d2c77..fb5f772ae99 100644
--- a/mps/code/mpm.h
+++ b/mps/code/mpm.h
@@ -963,6 +963,7 @@ extern Res RootCreateReg(Root *rootReturn, Arena arena,
                            void *p, size_t s);
 extern Res RootCreateRegMasked(Root *rootReturn, Arena arena,
                                Rank rank, Thread thread,
+			       mps_area_scan_t scan_area,
                                Word mask, Word pattern,
                                Word *stackBot);
 extern Res RootCreateFmt(Root *rootReturn, Arena arena,
diff --git a/mps/code/mps.h b/mps/code/mps.h
index 92a02399f10..c1fd1e5f0eb 100644
--- a/mps/code/mps.h
+++ b/mps/code/mps.h
@@ -693,6 +693,7 @@ extern mps_res_t mps_root_create_reg(mps_root_t *, mps_arena_t,
                                      mps_reg_scan_t, void *, size_t);
 extern mps_res_t mps_root_create_stack(mps_root_t *, mps_arena_t,
                                        mps_rank_t, mps_rm_t, mps_thr_t,
+				       mps_area_scan_t,
                                        mps_word_t, mps_word_t,
                                        void *);
 extern void mps_root_destroy(mps_root_t);
diff --git a/mps/code/mpsi.c b/mps/code/mpsi.c
index c55b4763890..42db2008cfc 100644
--- a/mps/code/mpsi.c
+++ b/mps/code/mpsi.c
@@ -1450,10 +1450,13 @@ mps_res_t mps_root_create_reg(mps_root_t *mps_root_o, mps_arena_t arena,
 }
 
 
+/* FIXME: re-document */
 mps_res_t mps_root_create_stack(mps_root_t *mps_root_o, mps_arena_t arena,
                                 mps_rank_t mps_rank, mps_rm_t mps_rm,
-                                mps_thr_t thread, mps_word_t mask,
-                                mps_word_t pattern, void *reg_scan_p)
+                                mps_thr_t thread,
+				mps_area_scan_t scan_area,
+				mps_word_t mask, mps_word_t pattern,
+				void *stack)
 {
   Rank rank = (Rank)mps_rank;
   Root root;
@@ -1462,15 +1465,16 @@ mps_res_t mps_root_create_stack(mps_root_t *mps_root_o, mps_arena_t arena,
   ArenaEnter(arena);
 
   AVER(mps_root_o != NULL);
-  AVER(reg_scan_p != NULL); /* stackBot */
-  AVER(AddrIsAligned(reg_scan_p, sizeof(Word)));
+  AVER(stack != NULL); /* stackBot */
+  AVER(AddrIsAligned(stack, sizeof(Word)));
   AVER(rank == mps_rank_ambig());
   AVER(mps_rm == (mps_rm_t)0);
   AVER((~mask & pattern) == 0);
 
   /* See .root-mode. */
   res = RootCreateRegMasked(&root, arena, rank, thread,
-                            mask, pattern, (Word *)reg_scan_p);
+                            scan_area, mask, pattern,
+			    (Word *)stack);
 
   ArenaLeave(arena);
 
diff --git a/mps/code/mpsicv.c b/mps/code/mpsicv.c
index aa563683f4f..10b98060ce5 100644
--- a/mps/code/mpsicv.c
+++ b/mps/code/mpsicv.c
@@ -604,7 +604,8 @@ int main(int argc, char *argv[])
   } else {
     die(mps_root_create_stack(&reg_root, arena,
                               mps_rank_ambig(), (mps_rm_t)0,
-                              thread, sizeof(mps_word_t) - 1, 0, marker),
+                              thread, mps_scan_area_tagged,
+			      sizeof(mps_word_t) - 1, 0, marker),
         "root_create_stack");
   }
 
diff --git a/mps/code/root.c b/mps/code/root.c
index 4433df89033..f1fb45f82d5 100644
--- a/mps/code/root.c
+++ b/mps/code/root.c
@@ -352,7 +352,9 @@ Res RootCreateReg(Root *rootReturn, Arena arena,
 
 Res RootCreateRegMasked(Root *rootReturn, Arena arena,
                         Rank rank, Thread thread,
-                        Word mask, Word pattern, Word *stackBot)
+			mps_area_scan_t scan_area,
+                        Word mask, Word pattern,
+			Word *stackBot)
 {
   union RootUnion theUnion;
 
@@ -364,7 +366,7 @@ Res RootCreateRegMasked(Root *rootReturn, Arena arena,
   AVER((~mask & pattern) == 0);
 
   theUnion.regMasked.thread = thread;
-  theUnion.regMasked.scan_area = mps_scan_area_tagged;
+  theUnion.regMasked.scan_area = scan_area;
   theUnion.regMasked.tag.mask = mask;
   theUnion.regMasked.tag.pattern = pattern;
   theUnion.regMasked.stackBot = stackBot;
diff --git a/mps/code/tagtest.c b/mps/code/tagtest.c
index 76d8b5ec792..7f27d45fc09 100644
--- a/mps/code/tagtest.c
+++ b/mps/code/tagtest.c
@@ -209,13 +209,15 @@ static void test(int mode, void *marker)
   case MODE_CONS:
     /* Scan words tagged "cons" -- everything will live. */
     die(mps_root_create_stack(&root, arena, mps_rank_ambig(), 0, thread,
-                              TAG_MASK, tag_cons, marker), "root");
+                              mps_scan_area_tagged, TAG_MASK, tag_cons,
+			      marker), "root");
     expected = 0;
     break;
   case MODE_INVALID:
     /* Scan words tagged "invalid" -- everything will die. */
     die(mps_root_create_stack(&root, arena, mps_rank_ambig(), 0, thread,
-                              TAG_MASK, tag_invalid, marker), "root");
+                              mps_scan_area_tagged, TAG_MASK, tag_invalid,
+			      marker), "root");
     expected = OBJCOUNT;
     break;
   }
diff --git a/mps/code/zcoll.c b/mps/code/zcoll.c
index 64fd5406018..16b403be2f4 100644
--- a/mps/code/zcoll.c
+++ b/mps/code/zcoll.c
@@ -576,7 +576,8 @@ static void StackScan(mps_arena_t arena, int on)
     Insist(root_stackreg == NULL);
     die(mps_root_create_stack(&root_stackreg, arena,
                               mps_rank_ambig(), (mps_rm_t)0, stack_thr,
-                              sizeof(mps_word_t) - 1, 0, stack_start),
+                              mps_scan_area_tagged,
+			      sizeof(mps_word_t) - 1, 0, stack_start),
         "root_stackreg");
     Insist(root_stackreg != NULL);
   } else {
@@ -764,7 +765,8 @@ static void *testscriptB(void *arg, size_t s)
   stack_thr = thr;
   die(mps_root_create_stack(&root_stackreg, arena,
                             mps_rank_ambig(), (mps_rm_t)0, stack_thr,
-                            sizeof(mps_word_t) - 1, 0, stack_start),
+                            mps_scan_area_tagged,
+			    sizeof(mps_word_t) - 1, 0, stack_start),
       "root_stackreg");
 
 
diff --git a/mps/code/zmess.c b/mps/code/zmess.c
index ff7cb526509..60a9c4a1e99 100644
--- a/mps/code/zmess.c
+++ b/mps/code/zmess.c
@@ -332,6 +332,7 @@ static void *testscriptB(void *arg, size_t s)
   /* root_stackreg: stack & registers are ambiguous roots = mutator's workspace */
   die(mps_root_create_stack(&root_stackreg, arena,
                             mps_rank_ambig(), (mps_rm_t)0, thr,
+			    mps_scan_area_tagged, 
                             sizeof(mps_word_t) - 1, 0, &stack_starts_here),
       "root_stackreg");