diff --git a/basis/Foreign.581.sml b/basis/Foreign.581.sml
index 2cdfba5f..6b2fc9a2 100644
--- a/basis/Foreign.581.sml
+++ b/basis/Foreign.581.sml
@@ -1,23 +1,88 @@
 (*
     Title:      Foreign Function Interface: main part
     Author:     David Matthews
-    Copyright   David Matthews 2015-16, 2018
+    Copyright   David Matthews 2015-16, 2018, 2020
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Lesser General Public
     License version 2.1 as published by the Free Software Foundation.
     
     This library is distributed in the hope that it will be useful,
     but WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     Lesser General Public License for more details.
     
     You should have received a copy of the GNU Lesser General Public
     License along with this library; if not, write to the Free Software
     Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 *)
 
+(* A subset of the main Foreign structure for booting.  We need
+   memoise in the interpreter. *)
+
 structure Foreign =
 struct
     exception Foreign = RunCall.Foreign
+    
+    structure Memory :>
+    sig
+        eqtype volatileRef
+        val volatileRef: SysWord.word -> volatileRef
+        val setVolatileRef: volatileRef * SysWord.word -> unit
+        val getVolatileRef: volatileRef -> SysWord.word
+        
+        eqtype voidStar
+        (* Remember an address except across loads. *)
+        val memoise: ('a -> voidStar) ->'a -> unit -> voidStar
+    end
+    =
+    struct
+        open ForeignConstants
+    
+        (* Both volatileRef and SysWord.word are the ADDRESSes of the actual value. *)
+        type volatileRef = word ref
+
+        val memMove: SysWord.word * SysWord.word * word * word* word -> unit = RunCall.moveBytes
+   
+        fun volatileRef init =
+        let
+            (* Allocate a single word marked as mutable, weak, no-overwrite, byte. *)
+            (* A weak byte cell is cleared to zero when it is read in either from the
+               executable or from a saved state.  Using the no-overwrite bit ensures
+               that if it is contained in the executable it won't be changed by loading
+               a saved state but there's a problem if it is contained in a parent state.
+               Then loading a child state will clear it because we reload all the parents
+               when we load a child. *)
+            val v = RunCall.allocateWordMemory(sysWordSize div wordSize, 0wx69, 0w0)
+            (* Copy the SysWord into it. *)
+            val () = memMove(init, RunCall.unsafeCast v, 0w0, 0w0, sysWordSize)
+        in
+            v
+        end
+
+        fun setVolatileRef(v, i) = memMove(i, RunCall.unsafeCast v, 0w0, 0w0, sysWordSize)
+
+        fun getVolatileRef var =
+        let
+            (* Allocate a single word marked as mutable, byte. *)
+            val v = RunCall.allocateByteMemory(sysWordSize div wordSize, 0wx41)
+            val () = memMove(RunCall.unsafeCast var, v, 0w0, 0w0, sysWordSize)
+            val () = RunCall.clearMutableBit v
+        in
+            v
+        end
+
+        type voidStar = SysWord.word
+
+        fun 'a memoise(f: 'a -> voidStar) (a: 'a) : unit -> voidStar =
+        let
+            (* Initialise to zero.  That means the function won't be
+               executed until we actually want the result. *)
+            val v = volatileRef 0w0
+        in
+            (* If we've reloaded the volatile ref it will have been reset to zero.
+               We need to execute the function and set it. *)
+            fn () => (case getVolatileRef v of 0w0 => let val r = f a in setVolatileRef(v, r); r end | r => r)
+        end
+    end
 end;
diff --git a/basis/ForeignConstants.581.sml b/basis/ForeignConstants.581.sml
deleted file mode 100644
index 7cc051cf..00000000
--- a/basis/ForeignConstants.581.sml
+++ /dev/null
@@ -1,21 +0,0 @@
-(*
-    Title:      Foreign Function Interface: constants
-    Author:     David Matthews
-    Copyright   David Matthews 2015, 2016-17
-
-    This library is free software; you can redistribute it and/or
-    modify it under the terms of the GNU Lesser General Public
-    License version 2.1 as published by the Free Software Foundation.
-    
-    This library is distributed in the hope that it will be useful,
-    but WITHOUT ANY WARRANTY; without even the implied warranty of
-    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-    Lesser General Public License for more details.
-    
-    You should have received a copy of the GNU Lesser General Public
-    License along with this library; if not, write to the Free Software
-    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
-*)
-
-(* This is defined separately so that the values are computed and
-   available as constants for the Foreign structure. *)