diff --git a/mps/src/lock.h b/mps/src/lock.h
new file mode 100644
index 00000000000..ae32e78ecb3
--- /dev/null
+++ b/mps/src/lock.h
@@ -0,0 +1,139 @@
+/*  impl.h.lock
+ *
+ *                          RECURSIVE LOCKS
+ *
+ *  $HopeName$
+ *
+ *  Copyright (C) 1995 Harlequin Group, all rights reserved
+ *
+ *  This defines the type Lock, which supports simple recursive 
+ *  locking.  Locking ensures that only a single thread may be running
+ *  with a lock held.  By claiming a lock in some code, this ensures
+ *  that only one thread can be running in that code at a time.  This
+ *  in turn can be used to protect different threads from trying to
+ *  read or update data structures which are in a transitional state.
+ *  
+ *  At most one thread may own a lock at a time.  A lock is initialised
+ *  without an owner.  A lock should not have an owner when it is
+ *  finished.  Claiming the lock will wait until the lock is not owned
+ *  by another thread and then cause the current thread to become the 
+ *  owner.  Releasing the the lock will relinquish ownership if the
+ *  number of releases matches the number of claims.
+ *
+ *  To use a lock a structure of type LockStruct must be allocated.
+ *  This is defined in impl.h.lockst.  Sources which allocate such a
+ *  structure will need to include "lockst.h".  A lock of type Lock is
+ *  a pointer to such an allocated structure.
+ * 
+ *  A lock must be Inited before use and should be Finished after use,
+ *  using LockInit and LockFinish.
+ *
+ *  LockClaimRecursive & LockReleaseRecursive are for claiming and
+ *  releasing the lock.  These may be used recursively.
+ *
+ *  There is a limit on the number of recursive claims which
+ *  depends on the implementation.  See issue.lock-claim-limit.
+ *
+ *  LockClaim and LockRelease are the same as the Recursive versions,
+ *  except that LockClaim may only be used by a thread that doesn't
+ *  already own the lock, and LockRelease may only be used to release
+ *  a lock with one claim.  LockClaim and LockRelease if used, must
+ *  be used symmetrically in pairs.
+ *  
+ *  There are two intended uses.  Here is an example:
+ *  #include "lock.h"
+ *  #include "lockst.h"
+ *  static LockStruct lockStruct;
+ *  binaryUse()
+ *  { ;; lock not owned by this thread.
+ *    LockClaim(&lockStruct);
+ *    ;; lock owned by this thread.
+ *    ;; Cannot call binaryUse() at this point.
+ *    ;; only one thread at a time may be at this point.
+ *    LockRelease(&lockStruct);
+ *    ;; lock not owned by this thread.
+ *  }
+ *
+ *  recursiveUse()
+ *  { ;; lock may already be owned by this thread.
+ *    LockClaimRecursive(&lockStruct);
+ *    ;; lock held by this thread.
+ *    ;; only one thread at a time may be at this point.
+ *    LockReleaseRecursive(&lockStruct);
+ *    ;; lock owned by this thread if it was before.
+ *  }
+ *  LockInit(&lockStruct) must be called before calling binaryUse()
+ *  or recursiveUse().
+ *  LockFinish(&lockStruct) should be called when lock is no longer
+ *  needed.
+ *  recursiveUse() may be called by both functions.
+ *  binaryUse() may only be called where lock is known not to be
+ *  already owned by this thread.  In particular, it may not be
+ *  called by recursiveUse().
+ *
+ */
+ 
+#ifndef lock_h
+#define lock_h
+
+#include "std.h"
+
+typedef struct LockStruct *Lock;
+
+/*  == LockInit/Finish == 
+ *
+ *  lock points to the allocated lock structure.  A lock has no
+ *  owner after initialisation.
+ */
+
+extern void LockInit(Lock lock);
+extern void LockFinish(Lock lock);
+
+
+/*  == LockClaimRecursive ==
+ *
+ *  This is called to increase the number of claims on the lock.
+ *  LockClaimRecursive will wait until the lock is not owned by another
+ *  thread and return with the lock owned.
+ *  This can be called recursively.
+ */
+
+extern void LockClaimRecursive(Lock lock);
+
+
+/*  == LockReleaseRecursive ==
+ *
+ *  This is called to reduce the number of claims on the lock.
+ *  If the number of claims drops to zero, ownership is relinquished.
+ *  This must not be called without possession of the lock.
+ */
+
+extern void LockReleaseRecursive(Lock lock);
+
+
+/*  == LockClaim ==
+ *
+ *  This may only be used when the lock is not already owned by 
+ *  the calling thread.
+ *  When used it behaves like LockClaimRecursive, but must be
+ *  matched by a call to LockRelease.
+ */
+
+extern void LockClaim(Lock lock);
+
+
+/*  == LockRelease ==
+ *
+ *  This must any may only be used to release a Lock symmetrically
+ *  with LockClaim.  It therefore should only be called with
+ *  a single claim.
+ */
+
+extern void LockRelease(Lock lock);
+
+
+/*  == Validation == */
+
+extern Bool LockIsValid(Lock lock, ValidationType validParam);
+
+#endif /* lock_h */
diff --git a/mps/src/lockcov.c b/mps/src/lockcov.c
new file mode 100644
index 00000000000..0fa7df6730b
--- /dev/null
+++ b/mps/src/lockcov.c
@@ -0,0 +1,32 @@
+/* impl.c.lockcov
+ *                     LOCK COVERAGE TEST
+ *
+ * $HopeName$
+ */
+
+#include "std.h"
+#include "lock.h"
+#include "lockst.h"
+
+int main(void){
+  LockStruct a,b;
+
+  LockInit(&a);
+  LockInit(&b);
+  LockClaim(&a);
+  LockClaimRecursive(&b);
+  LockRelease(&a);
+  LockClaimRecursive(&b);
+  LockFinish(&a);
+  LockReleaseRecursive(&b);
+  LockReleaseRecursive(&b);
+  LockFinish(&b);
+  LockInit(&a);
+  LockClaim(&a);
+  LockClaimRecursive(&a);
+  LockReleaseRecursive(&a);
+  LockRelease(&a);
+  LockFinish(&a);
+
+  return 0;
+}