diff --git a/mps/code/arena.c b/mps/code/arena.c
index df709753143..58bd0652e3b 100644
--- a/mps/code/arena.c
+++ b/mps/code/arena.c
@@ -8,6 +8,8 @@
 #include "tract.h"
 #include "poolmv.h"
 #include "mpm.h"
+#include "cbs.h"
+
 
 SRCID(arena, "$Id$");
 
@@ -142,6 +144,9 @@ Bool ArenaCheck(Arena arena)
   CHECKL(RingCheck(&arena->chunkRing));
   /* nothing to check for chunkSerial */
   CHECKD(ChunkCacheEntry, &arena->chunkCache);
+  
+  /* FIXME: Can't check freeCBS until it's initialised */
+  /* CHECKD(CBS, &arena->freeCBS); */
 
   CHECKL(LocusCheck(arena));
   
@@ -183,7 +188,7 @@ Res ArenaInit(Arena arena, ArenaClass class)
   RingInit(&arena->chunkRing);
   arena->chunkSerial = (Serial)0;
   ChunkCacheEntryInit(&arena->chunkCache);
-
+  
   LocusInit(arena);
   
   res = GlobalsInit(ArenaGlobals(arena));
@@ -192,6 +197,14 @@ Res ArenaInit(Arena arena, ArenaClass class)
 
   arena->sig = ArenaSig;
 
+  MPS_ARGS_BEGIN(cbsiArgs) {
+    MPS_ARGS_ADD(cbsiArgs, MPS_KEY_CBS_EXTEND_BY, 0); /* FIXME: explain why we never extend */
+    MPS_ARGS_DONE(cbsiArgs);
+    res = CBSInit(arena, &arena->freeCBS, arena, arena->alignment, TRUE, cbsiArgs);
+  } MPS_ARGS_END(cbsiArgs);
+  if (res != ResOK)
+    goto failCBSInit;
+
   /* initialize the reservoir, <design/reservoir/> */
   res = ReservoirInit(&arena->reservoirStruct, arena);
   if (res != ResOK)
@@ -201,6 +214,8 @@ Res ArenaInit(Arena arena, ArenaClass class)
   return ResOK;
 
 failReservoirInit:
+  CBSFinish(&arena->freeCBS);
+failCBSInit:
   GlobalsFinish(ArenaGlobals(arena));
 failGlobalsInit:
   return res;
@@ -278,6 +293,7 @@ failInit:
 void ArenaFinish(Arena arena)
 {
   ReservoirFinish(ArenaReservoir(arena));
+  CBSFinish(&arena->freeCBS);
   arena->sig = SigInvalid;
   GlobalsFinish(ArenaGlobals(arena));
   LocusFinish(arena);
diff --git a/mps/code/cbs.h b/mps/code/cbs.h
index 78f516727f8..857847816f3 100644
--- a/mps/code/cbs.h
+++ b/mps/code/cbs.h
@@ -10,7 +10,6 @@
 #define cbs_h
 
 #include "arg.h"
-#include "meter.h"
 #include "mpmtypes.h"
 #include "mpmst.h"
 #include "range.h"
@@ -21,21 +20,6 @@ typedef struct CBSStruct *CBS;
 typedef Bool (*CBSIterateMethod)(CBS cbs, Range range,
                                  void *closureP, Size closureS);
 
-
-#define CBSSig ((Sig)0x519CB599) /* SIGnature CBS */
-
-typedef struct CBSStruct {
-  SplayTreeStruct splayTree;
-  Count splayTreeSize;
-  MFSStruct blockPoolStruct;    /* FIXME: ref to why this is inlined */
-  Align alignment;
-  Bool fastFind;
-  Bool inCBS;                   /* prevent reentrance */
-  /* meters for sizes of search structures at each op */
-  METER_DECL(splaySearch);
-  Sig sig; /* sig at end because embeded */
-} CBSStruct;
-
 extern Bool CBSCheck(CBS cbs);
 
 extern Res CBSInit(Arena arena, CBS cbs, void *owner,
diff --git a/mps/code/mpmst.h b/mps/code/mpmst.h
index a95083b32b5..01f0f8e7df8 100644
--- a/mps/code/mpmst.h
+++ b/mps/code/mpmst.h
@@ -27,6 +27,8 @@
 #include "protocol.h"
 #include "ring.h"
 #include "chain.h"
+#include "splay.h"
+#include "meter.h"
 
 
 /* PoolClassStruct -- pool class structure
@@ -600,6 +602,26 @@ typedef struct GlobalsStruct {
 } GlobalsStruct;
 
 
+/* CBSStruct -- coalescing block structure
+ *
+ * See <code/cbs.c>.
+ */
+
+#define CBSSig ((Sig)0x519CB599) /* SIGnature CBS */
+
+typedef struct CBSStruct {
+  SplayTreeStruct splayTree;
+  Count splayTreeSize;
+  MFSStruct blockPoolStruct;    /* FIXME: ref to why this is inlined */
+  Align alignment;
+  Bool fastFind;
+  Bool inCBS;                   /* prevent reentrance */
+  /* meters for sizes of search structures at each op */
+  METER_DECL(splaySearch);
+  Sig sig; /* sig at end because embeded */
+} CBSStruct;
+
+
 /* ArenaStruct -- generic arena
  *
  * See <code/arena.c>.  */
@@ -633,6 +655,8 @@ typedef struct mps_arena_s {
   RingStruct chunkRing;         /* all the chunks */
   Serial chunkSerial;           /* next chunk number */
   ChunkCacheEntryStruct chunkCache; /* just one entry */
+  
+  CBSStruct freeCBS;            /* CBS of free address space */
 
   /* locus fields (<code/locus.c>) */
   GenDescStruct topGen;         /* generation descriptor for dynamic gen */
diff --git a/mps/code/poolmfs.c b/mps/code/poolmfs.c
index 3f596c5d251..accfdabb46f 100644
--- a/mps/code/poolmfs.c
+++ b/mps/code/poolmfs.c
@@ -111,7 +111,7 @@ static Res MFSInit(Pool pool, ArgList args)
       extendBy = unitSize;
   }
 
-  AVER(extendBy >= unitSize);
+  AVER(extendBy == 0 || extendBy >= unitSize);
  
   mfs = PoolPoolMFS(pool);
   arena = PoolArena(pool);
@@ -189,6 +189,9 @@ static Res MFSAlloc(Addr *pReturn, Pool pool, Size size,
     Size unitSize;
     Addr base;
     Header header = NULL, next;
+    
+    if (mfs->extendBy == 0)
+      return ResLIMIT;
 
     /* Create a new region and attach it to the pool. */
     res = ArenaAlloc(&base, SegPrefDefault(), mfs->extendBy, pool,
@@ -322,7 +325,7 @@ Bool MFSCheck(MFS mfs)
   CHECKD(Pool, &mfs->poolStruct);
   CHECKL(mfs->poolStruct.class == EnsureMFSPoolClass());
   CHECKL(mfs->unroundedUnitSize >= UNIT_MIN);
-  CHECKL(mfs->extendBy >= UNIT_MIN);
+  CHECKL(mfs->extendBy == 0 || mfs->extendBy >= UNIT_MIN);
   arena = PoolArena(&mfs->poolStruct);
   CHECKL(SizeIsAligned(mfs->extendBy, ArenaAlign(arena)));
   CHECKL(SizeAlignUp(mfs->unroundedUnitSize, mfs->poolStruct.alignment) ==