diff --git a/mps/src/sslii3.c b/mps/src/sslii3.c
index 382a8e174c0..23e0f25c2a2 100644
--- a/mps/src/sslii3.c
+++ b/mps/src/sslii3.c
@@ -1,6 +1,6 @@
 /* impl.c.sslii3: LINUX/INTEL STACK SCANNING
  *
- * $HopeName: $
+ * $HopeName: MMsrc!sslii3.c(trunk.1) $
  * Copyright (C) 1999.  Harlequin Group plc.  All rights reserved.
  *
  *  This scans the stack and fixes the registers which may contain 
@@ -10,17 +10,31 @@
  *  across function calls and therefore may contain roots.
  *  These are pushed on the stack for scanning.
  *
+ * SOURCES
+ *
+ * .source.callees.saves: Set of callee-saved registers taken from
+ * CALL_USED_REGISTERS in <gcc-sources>/config/i386/i386.h.
+ *
  * ASSUMPTIONS
  *
- * .align: The stack pointer is assumed to be aligned on a word
+ * .assume.align: The stack pointer is assumed to be aligned on a word
  * boundary.
+ *
+ * .assume.asm.stack: The compiler must not do wacky things with the
+ * stack pointer around a call since we need to ensure that the
+ * callee-save regs are visible during TraceScanArea.
+ *
+ * .assume.asm.order: The volatile modifier should prevent movement
+ * of code, which might break .assume.asm.stack.
+ *
  */
 
 
 #include "mpm.h"
 
-SRCID(sslii3, "$HopeName: $");
+SRCID(sslii3, "$HopeName: MMsrc!sslii3.c(trunk.1) $");
 
+/* .assume.asm.order */
 #define ASMV(x) __asm__ volatile (x)
 
 
@@ -29,15 +43,16 @@ Res StackScan(ScanState ss, Addr *stackBot)
   Addr *stackTop;
   Res res;
 
-  ASMV("push %ebx");    /* these registers are the save registers  */
+  /* .assume.asm.stack */
+  ASMV("push %ebx");    /* These registers are callee-saved */
   ASMV("push %esi");    /* and so may contain roots.  They are pushed */
-  ASMV("push %edi");    /* for scanning */
-  ASMV("mov %%esp, %0" : "=r" (stackTop) :);    /* stack pointer */
+  ASMV("push %edi");    /* for scanning.  See .source.callees.saves. */
+  ASMV("mov %%esp, %0" : "=r" (stackTop) :);    /* stackTop = esp */
 
-  AVER(AddrIsAligned((Addr)stackTop, sizeof(Addr)));  /* .align */
+  AVER(AddrIsAligned((Addr)stackTop, sizeof(Addr)));  /* .assume.align */
   res = TraceScanArea(ss, stackTop, stackBot);
 
-  ASMV("add $12, %esp");  /* pop 3 registers to restore the stack pointer */
+  ASMV("add $12, %esp");  /* pop 3 regs to restore the stack pointer */
 
   return res;
 }