diff --git a/include/commodity.h b/include/commodity.h
index 1f03fa78..c6b628b3 100644
--- a/include/commodity.h
+++ b/include/commodity.h
@@ -44,6 +44,7 @@ struct comstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned com_seqno: 12;
+    unsigned com_generation: 12;
     int com_uid;
     time_t com_timestamp;
     natid com_owner;
diff --git a/include/empobj.h b/include/empobj.h
index 9574073d..183021eb 100644
--- a/include/empobj.h
+++ b/include/empobj.h
@@ -57,6 +57,7 @@ struct empobj {
      */
     signed ef_type: 8;
     unsigned seqno: 12;
+    unsigned generation: 12;
     int uid;
     time_t timestamp;
     /* end of part matching struct emptypedstr */
diff --git a/include/file.h b/include/file.h
index 4c0424b4..58146d31 100644
--- a/include/file.h
+++ b/include/file.h
@@ -87,6 +87,7 @@ struct empfile {
 struct emptypedstr {
     signed ef_type: 8;
     unsigned seqno: 12;
+    unsigned generation: 12;
     int uid;
     time_t timestamp;
 };
@@ -203,6 +204,8 @@ enum {
 
 extern struct castr *ef_cadef(int);
 extern int ef_read(int, int, void *);
+extern void ef_make_stale(void);
+extern void ef_mark_fresh(int, void *);
 extern void *ef_ptr(int, int);
 extern char *ef_nameof(int);
 extern time_t ef_mtime(int);
diff --git a/include/game.h b/include/game.h
index a58d1bdb..109920d2 100644
--- a/include/game.h
+++ b/include/game.h
@@ -40,6 +40,7 @@ struct gamestr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned game_seqno: 12;
+    unsigned game_generation: 12;
     int game_uid;
     time_t game_timestamp;
     /* end of part matching struct empobj */
diff --git a/include/land.h b/include/land.h
index 5b1fb707..d3c174a0 100644
--- a/include/land.h
+++ b/include/land.h
@@ -51,6 +51,7 @@ struct lndstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned lnd_seqno: 12;
+    unsigned lnd_generation: 12;
     int lnd_uid;		/* unit id (land #) */
     time_t lnd_timestamp;	/* Last time this unit was touched */
     natid lnd_own;		/* owner's country num */
diff --git a/include/loan.h b/include/loan.h
index d4fae4c7..713d88c2 100644
--- a/include/loan.h
+++ b/include/loan.h
@@ -44,6 +44,7 @@ struct lonstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned l_seqno: 12;
+    unsigned l_generation: 12;
     int l_uid;
     time_t l_timestamp;
     /* end of part matching struct empobj */
diff --git a/include/lost.h b/include/lost.h
index 0e40470c..9fa7ab13 100644
--- a/include/lost.h
+++ b/include/lost.h
@@ -41,6 +41,7 @@ struct loststr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned lost_seqno: 12;
+    unsigned lost_generation: 12;
     int lost_uid;
     time_t lost_timestamp;	/* When it was lost */
     natid lost_owner;		/* Who lost it */
diff --git a/include/nat.h b/include/nat.h
index 5b658bdb..d64f8036 100644
--- a/include/nat.h
+++ b/include/nat.h
@@ -72,6 +72,7 @@ struct realmstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned r_seqno: 12;
+    unsigned r_generation: 12;
     int r_uid;			/* realm table index */
     time_t r_timestamp;		/* Last time this realm was touched */
     natid r_cnum;		/* country number */
@@ -85,6 +86,7 @@ struct natstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned nat_seqno: 12;
+    unsigned nat_generation: 12;
     int nat_uid;		/* equals nat_cnum */
     time_t nat_timestamp;
     natid nat_cnum;		/* our country number */
diff --git a/include/nuke.h b/include/nuke.h
index 6a0db83f..ec697c71 100644
--- a/include/nuke.h
+++ b/include/nuke.h
@@ -44,6 +44,7 @@ struct nukstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned nuk_seqno: 12;
+    unsigned nuk_generation: 12;
     int nuk_uid;		/* unit id (nuke #) */
     time_t nuk_timestamp;	/* Last time this nuke was touched */
     natid nuk_own;
diff --git a/include/plane.h b/include/plane.h
index e77ee93a..e080099b 100644
--- a/include/plane.h
+++ b/include/plane.h
@@ -48,6 +48,7 @@ struct plnstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned pln_seqno: 12;
+    unsigned pln_generation: 12;
     int pln_uid;		/* unit id (plane #) */
     time_t pln_timestamp;	/* Last time this plane was touched */
     natid pln_own;		/* owning country */
diff --git a/include/sect.h b/include/sect.h
index 9b6fe8d8..62e634ce 100644
--- a/include/sect.h
+++ b/include/sect.h
@@ -46,6 +46,7 @@ struct sctstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned sct_seqno: 12;
+    unsigned sct_generation: 12;
     int sct_uid;		/* equals XYOFFSET(sct_x, sct_y) */
     time_t sct_timestamp;	/* Last time this sector was written to */
     natid sct_own;		/* owner's country num */
diff --git a/include/ship.h b/include/ship.h
index 23cefa1f..c4425b80 100644
--- a/include/ship.h
+++ b/include/ship.h
@@ -65,6 +65,7 @@ struct shpstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned shp_seqno: 12;
+    unsigned shp_generation: 12;
     int shp_uid;		/* unit it (ship #) */
     time_t shp_timestamp;	/* Last time this ship was touched. */
     natid shp_own;		/* owner's country num */
diff --git a/include/trade.h b/include/trade.h
index 72ebdbbb..41889265 100644
--- a/include/trade.h
+++ b/include/trade.h
@@ -44,6 +44,7 @@ struct trdstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned trd_seqno: 12;
+    unsigned trd_generation: 12;
     int trd_uid;
     time_t trd_timestamp;
     natid trd_owner;
diff --git a/include/treaty.h b/include/treaty.h
index 440d7f53..f1c4cbb0 100644
--- a/include/treaty.h
+++ b/include/treaty.h
@@ -41,6 +41,7 @@ struct trtstr {
     /* initial part must match struct empobj */
     signed ef_type: 8;
     unsigned trt_seqno: 12;
+    unsigned trt_generation: 12;
     int trt_uid;
     time_t trt_timestamp;
     /* end of part matching struct empobj */
diff --git a/src/lib/common/file.c b/src/lib/common/file.c
index f32bafa4..b59e1ab5 100644
--- a/src/lib/common/file.c
+++ b/src/lib/common/file.c
@@ -57,9 +57,12 @@ static int do_read(struct empfile *, void *, int, int);
 static int do_write(struct empfile *, void *, int, int);
 static unsigned get_seqno(struct empfile *, int);
 static void new_seqno(struct empfile *, void *);
+static void must_be_fresh(struct empfile *, void *);
 static void do_blank(struct empfile *, void *, int, int);
 static int ef_check(int);
 
+static unsigned ef_generation;
+
 /*
  * Open the file-backed table TYPE (EF_SECTOR, ...).
  * HOW are flags to control operation.  Naturally, immutable flags are
@@ -360,6 +363,7 @@ ef_read(int type, int id, void *into)
 	cachep = ep->cache + (id - ep->baseid) * ep->size;
     }
     memcpy(into, cachep, ep->size);
+    ef_mark_fresh(type, into);
 
     if (ep->postread)
 	ep->postread(id, into);
@@ -522,9 +526,11 @@ ef_write(int type, int id, void *from)
 	if (ep->onresize && ep->onresize(type) < 0)
 	    return 0;
     }
-    if (id >= ep->baseid && id < ep->baseid + ep->cids)
+    if (id >= ep->baseid && id < ep->baseid + ep->cids) {
 	cachep = ep->cache + (id - ep->baseid) * ep->size;
-    else
+	if (cachep != from)
+	    must_be_fresh(ep, from);
+    } else
 	cachep = NULL;
     if (ep->prewrite)
 	ep->prewrite(id, cachep, from);
@@ -607,6 +613,35 @@ new_seqno(struct empfile *ep, void *buf)
     elt->seqno = old_seqno + 1;
 }
 
+void
+ef_make_stale(void)
+{
+    ef_generation++;
+}
+
+void
+ef_mark_fresh(int type, void *buf)
+{
+    struct empfile *ep;
+
+    if (ef_check(type) < 0)
+	return;
+    ep = &empfile[type];
+    if (!(ep->flags & EFF_TYPED))
+	return;
+    ((struct emptypedstr *)buf)->generation = ef_generation;
+}
+
+static void
+must_be_fresh(struct empfile *ep, void *buf)
+{
+    struct emptypedstr *elt = buf;
+
+    if (!(ep->flags & EFF_TYPED))
+	return;
+    CANT_HAPPEN(elt->generation != (ef_generation & 0xfff));
+}
+
 /*
  * Extend table TYPE by COUNT elements.
  * Any pointers obtained from ef_ptr() become invalid.
@@ -684,6 +719,7 @@ ef_blank(int type, int id, void *buf)
 	elt = buf;
 	elt->seqno = get_seqno(ep, elt->uid);
     }
+    ef_mark_fresh(type, buf);
 }
 
 /*
diff --git a/src/lib/empthread/io.c b/src/lib/empthread/io.c
index 6679dfc6..8016044d 100644
--- a/src/lib/empthread/io.c
+++ b/src/lib/empthread/io.c
@@ -51,6 +51,7 @@
 #include <unistd.h>
 #include "empio.h"
 #include "empthread.h"
+#include "file.h"
 #include "ioqueue.h"
 #include "misc.h"
 #include "queue.h"
@@ -196,6 +197,9 @@ io_output(struct iop *iop, int wait)
     struct iovec iov[16];
     int n, res, cc;
 
+    if (wait)
+	ef_make_stale();
+
     if (!ioq_qsize(iop->output))
 	return 0;
 
diff --git a/src/lib/empthread/lwp.c b/src/lib/empthread/lwp.c
index 5889c5f1..d1373712 100644
--- a/src/lib/empthread/lwp.c
+++ b/src/lib/empthread/lwp.c
@@ -37,6 +37,7 @@
 #include <signal.h>
 #include <time.h>
 #include "empthread.h"
+#include "file.h"
 #include "misc.h"
 
 /* Flags that were passed to empth_init() */
@@ -65,6 +66,7 @@ empth_create(void (*entry)(void *), int size, int flags,
 {
     if (!flags)
 	flags = empth_flags;
+    ef_make_stale();
     return lwpCreate(1, entry, size, flags, name, 0, NULL, ud);
 }
 
@@ -89,18 +91,21 @@ empth_set_name(empth_t *thread, char *name)
 void
 empth_exit(void)
 {
+    ef_make_stale();
     lwpExit();
 }
 
 void
 empth_yield(void)
 {
+    ef_make_stale();
     lwpYield();
 }
 
 int
 empth_select(int fd, int flags, struct timeval *timeout)
 {
+    ef_make_stale();
     return lwpSleepFd(fd, flags, timeout);
 }
 
@@ -113,6 +118,7 @@ empth_wakeup(empth_t *a)
 int
 empth_sleep(time_t until)
 {
+    ef_make_stale();
     return lwpSleepUntil(until);
 }
 
@@ -123,6 +129,7 @@ empth_wait_for_signal(void)
     int sig, err;
     time_t now;
 
+    ef_make_stale();
     sigemptyset(&set);
     sigaddset(&set, SIGHUP);
     sigaddset(&set, SIGINT);
@@ -153,12 +160,14 @@ empth_rwlock_destroy(empth_rwlock_t *rwlock)
 void
 empth_rwlock_wrlock(empth_rwlock_t *rwlock)
 {
+    ef_make_stale();
     lwp_rwlock_wrlock(rwlock);
 }
 
 void
 empth_rwlock_rdlock(empth_rwlock_t *rwlock)
 {
+    ef_make_stale();
     lwp_rwlock_rdlock(rwlock);
 }
 
diff --git a/src/lib/empthread/ntthread.c b/src/lib/empthread/ntthread.c
index 57967220..67ae3ef4 100644
--- a/src/lib/empthread/ntthread.c
+++ b/src/lib/empthread/ntthread.c
@@ -56,6 +56,7 @@
 #include <process.h>
 #include "misc.h"
 #include "empthread.h"
+#include "file.h"
 #include "prototypes.h"
 #include "server.h"
 #include "sys/socket.h"
@@ -423,6 +424,7 @@ empth_create(void (*entry)(void *), int size, int flags,
     empth_t *pThread = NULL;
 
     loc_debug("creating new thread %s", name);
+    ef_make_stale();
 
     pThread = malloc(sizeof(*pThread));
     if (!pThread) {
@@ -501,6 +503,7 @@ empth_exit(void)
     empth_t *pThread = TlsGetValue(dwTLSIndex);
 
     loc_debug("empth_exit");
+    ef_make_stale();
     loc_BlockThisThread();
 
     TlsSetValue(dwTLSIndex, NULL);
@@ -516,6 +519,7 @@ empth_exit(void)
 void
 empth_yield(void)
 {
+    ef_make_stale();
     loc_BlockThisThread();
     Sleep(0);
     loc_RunThisThread(NULL);
@@ -541,6 +545,7 @@ empth_select(int fd, int flags, struct timeval *timeout)
 
     loc_debug("%s select on %d",
 	      flags == EMPTH_FD_READ ? "read" : "write", fd);
+    ef_make_stale();
     loc_BlockThisThread();
 
     hEventObject[0] = WSACreateEvent();
@@ -615,6 +620,7 @@ empth_sleep(time_t until)
     empth_t *pThread = TlsGetValue(dwTLSIndex);
     DWORD result;
 
+    ef_make_stale();
     loc_BlockThisThread();
 
     do {
@@ -645,6 +651,7 @@ empth_request_shutdown(void)
 int
 empth_wait_for_signal(void)
 {
+    ef_make_stale();
     loc_BlockThisThread();
     loc_RunThisThread(hShutdownEvent);
     return SIGTERM;
@@ -696,6 +703,7 @@ empth_rwlock_destroy(empth_rwlock_t *rwlock)
 void
 empth_rwlock_wrlock(empth_rwlock_t *rwlock)
 {
+    ef_make_stale();
     /* block any new readers */
     ResetEvent(rwlock->can_read);
     rwlock->nwrite++;
@@ -707,6 +715,7 @@ empth_rwlock_wrlock(empth_rwlock_t *rwlock)
 void
 empth_rwlock_rdlock(empth_rwlock_t *rwlock)
 {
+    ef_make_stale();
     loc_BlockThisThread();
     loc_RunThisThread(rwlock->can_read);
     ResetEvent(rwlock->can_write);
diff --git a/src/lib/empthread/pthread.c b/src/lib/empthread/pthread.c
index 3378b50b..32293aec 100644
--- a/src/lib/empthread/pthread.c
+++ b/src/lib/empthread/pthread.c
@@ -51,6 +51,7 @@
 #include <unistd.h>
 #include "misc.h"
 #include "empthread.h"
+#include "file.h"
 #include "prototypes.h"
 
 struct empth_t {
@@ -187,6 +188,7 @@ empth_create(void (*entry)(void *), int size, int flags,
     int eno;
 
     empth_status("creating new thread %s", name);
+    ef_make_stale();
 
     ctx = malloc(sizeof(empth_t));
     if (!ctx) {
@@ -262,6 +264,7 @@ empth_exit(void)
     empth_t *ctx = pthread_getspecific(ctx_key);
 
     empth_status("empth_exit");
+    ef_make_stale();
     pthread_mutex_unlock(&mtx_ctxsw);
     free(ctx->name);
     free(ctx);
@@ -271,6 +274,7 @@ empth_exit(void)
 void
 empth_yield(void)
 {
+    ef_make_stale();
     pthread_mutex_unlock(&mtx_ctxsw);
     pthread_mutex_lock(&mtx_ctxsw);
     empth_restorectx();
@@ -286,6 +290,7 @@ empth_select(int fd, int flags, struct timeval *timeout)
     empth_t *ctx;
     int res = 0;
 
+    ef_make_stale();
     pthread_mutex_unlock(&mtx_ctxsw);
     empth_status("select on %d for %d", fd, flags);
 
@@ -353,6 +358,7 @@ empth_sleep(time_t until)
     struct timeval tv;
     int res;
 
+    ef_make_stale();
     pthread_mutex_unlock(&mtx_ctxsw);
     do {
 	now = time(NULL);
@@ -373,6 +379,7 @@ empth_wait_for_signal(void)
     sigset_t set;
     int sig, err;
 
+    ef_make_stale();
     sigemptyset(&set);
     sigaddset(&set, SIGHUP);
     sigaddset(&set, SIGINT);
@@ -426,6 +433,7 @@ empth_rwlock_wrlock(empth_rwlock_t *rwlock)
 {
     empth_status("wrlock %s %d %d",
 		 rwlock->name, rwlock->nread, rwlock->nwrite);
+    ef_make_stale();
     rwlock->nwrite++;
     while (rwlock->nread != 0 || rwlock->nwrite != 1) {
 	empth_status("waiting for wrlock %s", rwlock->name);
@@ -441,6 +449,7 @@ empth_rwlock_rdlock(empth_rwlock_t *rwlock)
 {
     empth_status("rdlock %s %d %d",
 		 rwlock->name, rwlock->nread, rwlock->nwrite);
+    ef_make_stale();
     while (rwlock->nwrite) {
 	empth_status("waiting for rdlock %s", rwlock->name);
 	pthread_cond_wait(&rwlock->can_read, &mtx_ctxsw);
diff --git a/src/lib/subs/check.c b/src/lib/subs/check.c
index 77f91b72..360ce913 100644
--- a/src/lib/subs/check.c
+++ b/src/lib/subs/check.c
@@ -53,7 +53,11 @@ obj_changed(struct empobj *obj, size_t sz)
     get_empobj(obj->ef_type, obj->uid, &old);
     memcpy(&tobj, obj, sz);
     old.gen.timestamp = tobj.gen.timestamp = 0;
-    return memcmp(&tobj, &old, sz);
+    old.gen.generation = tobj.gen.generation = 0;
+    if (memcmp(&tobj, &old, sz))
+	return 1;
+    ef_mark_fresh(obj->ef_type, obj);
+    return 0;
 }
 
 int
diff --git a/src/lib/update/nav_ship.c b/src/lib/update/nav_ship.c
index 32c01c08..0a020698 100644
--- a/src/lib/update/nav_ship.c
+++ b/src/lib/update/nav_ship.c
@@ -268,6 +268,7 @@ nav_ship(struct shpstr *sp)
     mlp = malloc(sizeof(struct ulist));
     mlp->chrp = (struct empobj_chr *)(mchr + sp->shp_type);
     mlp->unit.ship = *sp;
+    ef_mark_fresh(EF_SHIP, &mlp->unit.ship);
     mlp->mobil = sp->shp_mobil;
     emp_insque(&mlp->queue, &ship_list);
 
diff --git a/src/lib/update/sail.c b/src/lib/update/sail.c
index c10c211e..a5240833 100644
--- a/src/lib/update/sail.c
+++ b/src/lib/update/sail.c
@@ -347,6 +347,7 @@ fltp_to_list(struct fltheadstr *fltp, struct emp_qelem *list)
 	sp = getshipp(fe->num);
 	mlp->chrp = (struct empobj_chr *)(mchr + sp->shp_type);
 	mlp->unit.ship = *sp;
+	ef_mark_fresh(EF_SHIP, &mlp->unit.ship);
 	mlp->mobil = fe->mobil;
 	emp_insque(&mlp->queue, list);
     }