diff --git a/mps/src/mpm.h b/mps/src/mpm.h
index 680e7ff4748..8ab2864e818 100644
--- a/mps/src/mpm.h
+++ b/mps/src/mpm.h
@@ -1,6 +1,6 @@
 /* impl.h.mpm: MEMORY POOL MANAGER DEFINITIONS
  *
- * $HopeName: MMsrc!mpm.h(trunk.41) $
+ * $HopeName: MMsrc!mpm.h(trunk.42) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  */
 
@@ -322,6 +322,8 @@ extern TraceSet (TraceSetUnion)(TraceSet ts1, TraceSet ts2);
 extern Bool (TraceSetIsMember)(TraceSet ts, TraceId id);
 
 extern Bool ScanStateCheck(ScanState ss);
+extern void ScanStateSetSummary(ScanState ss, RefSet summary);
+extern RefSet ScanStateSummary(ScanState ss);
 extern Bool TraceIdCheck(TraceId id);
 extern Bool TraceSetCheck(TraceSet ts);
 extern Bool TraceCheck(Trace trace);
@@ -340,7 +342,7 @@ extern Size TraceGreyEstimate(Arena arena, RefSet refSet);
   BEGIN \
     Shift SCANzoneShift = (ss)->zoneShift; \
     RefSet SCANwhite = (ss)->white; \
-    RefSet SCANsummary = (ss)->summary; \
+    RefSet SCANsummary = (ss)->unfixedSummary; \
     Word SCANt; \
     {
 
@@ -366,7 +368,7 @@ extern Size TraceGreyEstimate(Arena arena, RefSet refSet);
 
 #define TRACE_SCAN_END(ss) \
    } \
-   (ss)->summary = SCANsummary; \
+   (ss)->unfixedSummary = SCANsummary; \
   END
 
 extern Res TraceScanArea(ScanState ss, Addr *base, Addr *limit);
diff --git a/mps/src/mpmst.h b/mps/src/mpmst.h
index 1ffe75e7de4..bc8377197fd 100644
--- a/mps/src/mpmst.h
+++ b/mps/src/mpmst.h
@@ -1,6 +1,6 @@
 /* impl.h.mpmst: MEMORY POOL MANAGER DATA STRUCTURES
  *
- * $HopeName: MMsrc!mpmst.h(trunk.31) $
+ * $HopeName: MMsrc!mpmst.h(trunk.32) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  *
  * .readership: MM developers.
@@ -412,7 +412,7 @@ typedef struct RootStruct {
  *   ss->fix            mps_ss->fix
  *   ss->zoneShift      mps_ss->w0
  *   ss->white          mps_ss->w1
- *   ss->summary        mps_ss->w2
+ *   ss->unfixedSummary mps_ss->w2
  * See impl.h.mps.ss and impl.c.mpsi.check.ss.  This is why the
  * Sig field is in the middle of this structure.
  *
@@ -426,13 +426,13 @@ typedef struct ScanStateStruct {
   Res (*fix)(ScanState, Addr *);/* fix function */
   Word zoneShift;               /* copy of arena->zoneShift.  See .ss.zone */
   RefSet white;                 /* white set, for inline fix test */
-  RefSet summary;               /* accumulated summary of scanned references */
+  RefSet unfixedSummary;        /* accumulated summary of scanned references */
   Sig sig;                      /* design.mps.sig */
   Arena arena;                  /* owning arena */
   TraceSet traces;              /* traces to scan for */
   Rank rank;                    /* reference rank of scanning */
   Bool wasMarked;               /* design.mps.fix.protocol.was-ready */
-  RefSet fixed;                 /* accumulated summary of fixed references */
+  RefSet fixedSummary;          /* accumulated summary of fixed references */
 } ScanStateStruct;
 
 
diff --git a/mps/src/mpsi.c b/mps/src/mpsi.c
index a47a36c430d..2a7ff1de2b4 100644
--- a/mps/src/mpsi.c
+++ b/mps/src/mpsi.c
@@ -1,6 +1,6 @@
 /* impl.c.mpsi: MEMORY POOL SYSTEM C INTERFACE LAYER
  *
- * $HopeName: MMsrc!mpsi.c(trunk.26) $
+ * $HopeName: MMsrc!mpsi.c(trunk.27) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  *
  * .purpose: This code bridges between the MPS interface to C,
@@ -52,7 +52,7 @@
 #include "mps.h"
 #include "mpsavm.h" /* only for mps_space_create */
 
-SRCID(mpsi, "$HopeName: MMsrc!mpsi.c(trunk.26) $");
+SRCID(mpsi, "$HopeName: MMsrc!mpsi.c(trunk.27) $");
 
 
 /* mpsi_check -- check consistency of interface mappings
@@ -140,7 +140,7 @@ static Bool mpsi_check(void)
   CHECKL(CHECKFIELDAPPROX(mps_ss_s, fix, ScanStateStruct, fix));
   CHECKL(CHECKFIELD(mps_ss_s, w0, ScanStateStruct, zoneShift));
   CHECKL(CHECKFIELD(mps_ss_s, w1, ScanStateStruct, white));
-  CHECKL(CHECKFIELD(mps_ss_s, w2, ScanStateStruct, summary));
+  CHECKL(CHECKFIELD(mps_ss_s, w2, ScanStateStruct, unfixedSummary));
 
   /* Check ld_s/LDStruct compatibility by hand */
   /* .check.ld: See also impl.h.mpmst.ld.struct and impl.h.mps.ld */
diff --git a/mps/src/poolams.c b/mps/src/poolams.c
index 7244a1ab47c..e40925bad01 100644
--- a/mps/src/poolams.c
+++ b/mps/src/poolams.c
@@ -1,6 +1,6 @@
 /* impl.c.poolams: AUTOMATIC MARK & SWEEP POOL CLASS
  *
- * $HopeName: MMsrc!poolams.c(trunk.4) $
+ * $HopeName: MMsrc!poolams.c(trunk.5) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  * 
  * NOTES
@@ -19,7 +19,7 @@
 #include "mpm.h"
 #include "mpscams.h"
 
-SRCID(poolams, "$HopeName: MMsrc!poolams.c(trunk.4) $");
+SRCID(poolams, "$HopeName: MMsrc!poolams.c(trunk.5) $");
 
 /* These two BT utility functions should be in the BT module.
  * See design.mps.poolams.bt.utilities */
@@ -632,9 +632,6 @@ static Res AMSScan(ScanState ss, Pool pool, Seg seg)
   Bool scanAllObjects;
 
   AVERT(ScanState, ss);
-  AVER(ss->summary == RefSetEMPTY); /* to make afterSummary correct */
-  AVER(ss->fixed == RefSetEMPTY);   /* to make afterSummary correct */
-  
   AVERT(Pool, pool);
   ams = PoolPoolAMS(pool);
   AVERT(AMS, ams);
@@ -651,6 +648,15 @@ static Res AMSScan(ScanState ss, Pool pool, Seg seg)
 
   AVER(!scanOnce || scanAllObjects); /* scanOnce implies scanAllObjects */
   
+
+  /* If the scanner isn't going to scan all the objects then the */
+  /* summary of the unscanned objects must be added into the scan */
+  /* state summary, so that it's a valid summary of the entire */
+  /* segment on return.  See design.mps.poolams.summary.scan.part. */
+  if(!scanAllObjects)
+    ScanStateSetSummary(ss, RefSetUnion(ScanStateSummary(ss),
+                                        SegSummary(seg)));
+
   if (scanOnce) {
 
     Bool wasMarked = group->marked; /* for checking */
@@ -675,16 +681,6 @@ static Res AMSScan(ScanState ss, Pool pool, Seg seg)
 
   }
 
-  AVER(RefSetSub(ss->summary, SegSummary(seg)));
-
-  if (!scanAllObjects) { /* design.mps.poolams.summary */
-    /* design.mps.poolams.summary.scan.part.summary */
-    ss->summary = RefSetUnion(ss->summary, SegSummary(seg));
-    /* design.mps.poolams.summary.scan.part.fixed */
-    ss->fixed = RefSetUnion(ss->fixed, RefSetInter(SegSummary(seg),
-                                                   ss->white));
-  }
-
   return ResOK;
 }
 
diff --git a/mps/src/poolawl.c b/mps/src/poolawl.c
index d2faad15ab7..c47dee78cdc 100644
--- a/mps/src/poolawl.c
+++ b/mps/src/poolawl.c
@@ -1,6 +1,6 @@
 /* impl.c.poolawl: AUTOMATIC WEAK LINKED POOL CLASS
  *
- * $HopeName: MMsrc!poolawl.c(trunk.14) $
+ * $HopeName: MMsrc!poolawl.c(trunk.15) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  *
  * READERSHIP
@@ -16,7 +16,7 @@
 #include "mpm.h"
 #include "mpscawl.h"
 
-SRCID(poolawl, "$HopeName: MMsrc!poolawl.c(trunk.14) $");
+SRCID(poolawl, "$HopeName: MMsrc!poolawl.c(trunk.15) $");
 
 
 #define AWLSig	((Sig)0x519b7a37)	/* SIGPooLAWL */
@@ -445,6 +445,14 @@ static Res AWLScan(ScanState ss, Pool pool, Seg seg)
   base = SegBase(arena, seg);
   limit = SegLimit(arena, seg);
 
+  /* If the scanner isn't going to scan all the objects then the */
+  /* summary of the unscanned objects must be added into the scan */
+  /* state summary, so that it's a valid summary of the entire */
+  /* segment on return. */
+  if(SegWhite(seg) != RefSetEMPTY)
+    ScanStateSetSummary(ss, RefSetUnion(ScanStateSummary(ss),
+                                        SegSummary(seg)));
+
 notFinished:
   finished = TRUE;
   p = base;
@@ -500,7 +508,7 @@ notFinished:
   }
   if(!finished)
     goto notFinished;
-  
+
   return ResOK;
 }
 
diff --git a/mps/src/trace.c b/mps/src/trace.c
index a5718b719ac..7f8ff82a0c5 100644
--- a/mps/src/trace.c
+++ b/mps/src/trace.c
@@ -1,12 +1,12 @@
 /* impl.c.trace: GENERIC TRACER IMPLEMENTATION
  *
- * $HopeName: MMsrc!trace.c(trunk.39) $
+ * $HopeName: MMsrc!trace.c(trunk.40) $
  * Copyright (C) 1997 The Harlequin Group Limited.  All rights reserved.
  */
 
 #include "mpm.h"
 
-SRCID(trace, "$HopeName: MMsrc!trace.c(trunk.39) $");
+SRCID(trace, "$HopeName: MMsrc!trace.c(trunk.40) $");
 
 
 /* ScanStateCheck -- check consistency of a ScanState object */
@@ -390,7 +390,6 @@ static Res TraceFlip(Trace trace)
   ss.fix = TraceFix;
   ss.zoneShift = ArenaZoneShift(arena);
   ss.white = trace->white;
-  ss.summary = RefSetEMPTY;
   ss.arena = arena;
   ss.traces = TraceSetSingle(trace->ti);
   ss.wasMarked = TRUE;
@@ -535,6 +534,40 @@ static Bool traceFindGrey(Seg *segReturn, Rank *rankReturn,
 }
 
 
+/* ScanStateSetSummary -- set the summary of scanned references
+ *
+ * This function sets unfixedSummary and fixedSummary such that
+ * ScanStateSummary will return the summary passed.  Subsequently
+ * fixed references are accumulated into this result.
+ */
+
+void ScanStateSetSummary(ScanState ss, RefSet summary)
+{
+  AVERT(ScanState, ss);
+
+  ss->fixedSummary = RefSetEMPTY;
+  ss->unfixedSummary = summary;
+}
+
+
+/* ScanStateSummary -- calculate the summary of scanned references
+ *
+ * The summary of the scanned references is the summary of the
+ * unfixed references, minus the white set, plus the summary of the
+ * fixed references.  This is because TraceFix is called for all
+ * references in the white set, and accumulates a summary of
+ * references after they have been fixed.
+ */
+
+RefSet ScanStateSummary(ScanState ss)
+{
+  AVERT(ScanState, ss);
+
+  return TraceSetUnion(ss->fixedSummary,
+                       TraceSetDiff(ss->unfixedSummary, ss->white));
+}
+
+
 /* TraceScan -- scan a segment to remove greyness
  *
  * @@@@ During scanning, the segment should be write-shielded to
@@ -562,8 +595,8 @@ static Res TraceScan(TraceSet ts, Rank rank,
   ss.traces = ts;
   ss.fix = TraceFix;
   ss.zoneShift = arena->zoneShift;
-  ss.summary = RefSetEMPTY;
-  ss.fixed = RefSetEMPTY;
+  ss.unfixedSummary = RefSetEMPTY;
+  ss.fixedSummary = RefSetEMPTY;
   ss.arena = arena;
   ss.wasMarked = TRUE;
   ss.white = RefSetEMPTY;
@@ -580,28 +613,12 @@ static Res TraceScan(TraceSet ts, Rank rank,
   if(res != ResOK)
     goto failScan;
 
-  /* .scan.post-condition: */ 
-  /* The summary of reference seens by scan (ss.summary) is a subset */
-  /* of the summary previously computed (SegSummary).  There are two */
-  /* reasons that it is not an equality relation: */
+  /* See design.mps.scan.summary.subset. */
+  AVER(RefSetSub(ss.unfixedSummary, SegSummary(seg)));
 
-  /* 1. if the segment has had objects forwarded onto it then its summary */
-  /* will get unioned with the summary of the segment that the object was */
-  /* forwarded from.  This may increase the summary.  The forwarded object */
-  /* of course may have a smaller summary (if such a thing were to be */
-  /* computed) and so subsequent scanning of the segment may reduce the */
-  /* summmary.  (The forwarding process may erroneously introduce zones */
-  /* into the destination's summary). */
-
-  /* 2. A write barrier hit will set the summary to RefSetUNIV. */
-
-  /* The reason that ss.summary is always a subset of the previous summary */
-  /* is due to an "optimization" which has not been made in TraceFix.  See */
-  /* .fix.fixed.all */
-
-  AVER(RefSetSub(ss.summary, SegSummary(seg)));
-  SegSetSummary(seg, TraceSetUnion(ss.fixed,
-                                   TraceSetDiff(ss.summary, ss.white)));
+  /* All objects on the segment have been scanned, so the scanned */
+  /* summary should replace the segment summary. */
+  SegSetSummary(seg, ScanStateSummary(&ss));
 
   ss.sig = SigInvalid;                  /* just in case */
 
@@ -779,16 +796,16 @@ Res TraceFix(ScanState ss, Ref *refIO)
   }
 
   /* .fix.fixed.all: */
-  /* ss->fixed is accumulated for all the pointers whether or not they are */
-  /* genuine references.  We could accumulate fewer pointers here, if a */
-  /* pointer fails the SegOfAddr test then we know it isn't a reference, so */
-  /* we needn't accumulate it into the fixed summary.  The design allows */
-  /* this, but it breaks a useful post-condition on scanning.  See */
-  /* .scan.post-condition.  (if the accumulation of ss->fixed was moved the */
-  /* accuracy of ss->fixed would vary according to the "width" of the white */
-  /* summary). */
-
-  ss->fixed = RefSetAdd(ss->arena, ss->fixed, *refIO);
+  /* ss->fixedSummary is accumulated for all the pointers whether */
+  /* or not they are genuine references.  We could accumulate fewer */
+  /* pointers here, if a pointer fails the SegOfAddr test then we */
+  /* know it isn't a reference, so we needn't accumulate it into the */
+  /* fixed summary.  The design allows this, but it breaks a useful */
+  /* post-condition on scanning.  See .scan.post-condition.  (if */
+  /* the accumulation of ss->fixedSummary was moved the accuracy */
+  /* of ss->fixedSummary would vary according to the "width" of the */
+  /* white summary). */
+  ss->fixedSummary = RefSetAdd(ss->arena, ss->fixedSummary, *refIO);
 
   return ResOK;
 }