diff --git a/reginfo.c b/reginfo.c
index 6498459..be7bcdb 100644
--- a/reginfo.c
+++ b/reginfo.c
@@ -24,10 +24,20 @@ static int packet_mismatch = 0;
 int send_register_info(write_fn write_fn, void *uc)
 {
    struct reginfo ri;
+   trace_header_t header;
    int op;
+
    reginfo_init(&ri, uc);
    op = get_risuop(&ri);
 
+   /* Write a header with PC/op to keep in sync */
+   header.pc = get_pc(&ri);
+   header.risu_op = op;
+   if (write_fn(&header, sizeof(header)) != 0) {
+      fprintf(stderr,"%s: failed header write\n", __func__);
+      return -1;
+   }
+
    switch (op) {
       case OP_TESTEND:
          write_fn(&ri, sizeof(ri));
@@ -63,51 +73,72 @@ int send_register_info(write_fn write_fn, void *uc)
 int recv_and_compare_register_info(read_fn read_fn, respond_fn resp_fn, void *uc)
 {
    int resp = 0, op;
+   trace_header_t header;
 
    reginfo_init(&master_ri, uc);
    op = get_risuop(&master_ri);
 
-   switch (op) {
-      case OP_COMPARE:
-      case OP_TESTEND:
-      default:
-         /* Do a simple register compare on (a) explicit request
-          * (b) end of test (c) a non-risuop UNDEF
-          */
-         if (read_fn(&apprentice_ri, sizeof(apprentice_ri))) {
-            packet_mismatch = 1;
-            resp = 2;
-         } else if (!reginfo_is_eq(&master_ri, &apprentice_ri)) {
-            /* register mismatch */
-            resp = 2;
-         } else if (op == OP_TESTEND) {
-            resp = 1;
-         }
-         resp_fn(resp);
-         break;
-      case OP_SETMEMBLOCK:
-         memblock = (void *)(uintptr_t)get_reginfo_paramreg(&master_ri);
-         break;
-      case OP_GETMEMBLOCK:
-         set_ucontext_paramreg(uc, get_reginfo_paramreg(&master_ri) +
-                               (uintptr_t)memblock);
-         break;
-      case OP_COMPAREMEM:
-         mem_used = 1;
-         if (read_fn(apprentice_memblock, MEMBLOCKLEN)) {
-            packet_mismatch = 1;
-            resp = 2;
-         } else if (memcmp(memblock, apprentice_memblock, MEMBLOCKLEN) != 0) {
-            /* memory mismatch */
-            resp = 2;
-         }
-         resp_fn(resp);
-         break;
+   if (read_fn(&header, sizeof(header)) != 0) {
+      fprintf(stderr,"%s: failed header read\n", __func__);
+      return -1;
+   }
+
+   if (header.risu_op == op ) {
+
+      /* send OK for the header */
+      resp_fn(0);
+
+      switch (op) {
+         case OP_COMPARE:
+         case OP_TESTEND:
+         default:
+            /* Do a simple register compare on (a) explicit request
+             * (b) end of test (c) a non-risuop UNDEF
+             */
+            if (read_fn(&apprentice_ri, sizeof(apprentice_ri))) {
+               packet_mismatch = 1;
+               resp = 2;
+
+            } else if (!reginfo_is_eq(&master_ri, &apprentice_ri)) {
+               /* register mismatch */
+               resp = 2;
+
+            } else if (op == OP_TESTEND) {
+               resp = 1;
+            }
+            resp_fn(resp);
+            break;
+         case OP_SETMEMBLOCK:
+            memblock = (void *)(uintptr_t)get_reginfo_paramreg(&master_ri);
+            break;
+         case OP_GETMEMBLOCK:
+            set_ucontext_paramreg(uc, get_reginfo_paramreg(&master_ri) +
+                                  (uintptr_t)memblock);
+            break;
+         case OP_COMPAREMEM:
+            mem_used = 1;
+            if (read_fn(apprentice_memblock, MEMBLOCKLEN)) {
+               packet_mismatch = 1;
+               resp = 2;
+            } else if (memcmp(memblock, apprentice_memblock, MEMBLOCKLEN) != 0) {
+               /* memory mismatch */
+               resp = 2;
+            }
+            resp_fn(resp);
+            break;
+      }
+   } else {
+      fprintf(stderr, "out of sync %" PRIxPTR "/%" PRIxPTR " %d/%d\n",
+              get_pc(&master_ri), header.pc,
+              op, header.risu_op);
+      resp = 2;
+      resp_fn(resp);
    }
 
    return resp;
 }
 
+
 /* Print a useful report on the status of the last comparison
  * done in recv_and_compare_register_info(). This is called on
  * exit, so need not restrict itself to signal-safe functions.
diff --git a/risu.h b/risu.h
index 71ea94f..c01a9c3 100644
--- a/risu.h
+++ b/risu.h
@@ -51,6 +51,12 @@ extern int ismaster;
 
 struct reginfo;
 
+typedef struct
+{
+   uintptr_t pc;
+   uint32_t risu_op;
+} trace_header_t;
+
 /* Functions operating on reginfo */
 
 /* To keep the read/write logic from multiplying across all arches
@@ -102,6 +108,9 @@ uint64_t get_reginfo_paramreg(struct reginfo *ri);
  */
 int get_risuop(struct reginfo *ri);
 
+/* Return the PC from a reginfo */
+uintptr_t get_pc(struct reginfo *ri);
+
 /* initialize structure from a ucontext */
 void reginfo_init(struct reginfo *ri, ucontext_t *uc);
 
diff --git a/risu_aarch64.c b/risu_aarch64.c
index 5625979..312bd6a 100644
--- a/risu_aarch64.c
+++ b/risu_aarch64.c
@@ -43,3 +43,8 @@ int get_risuop(struct reginfo *ri)
     uint32_t risukey = 0x00005af0;
     return (key != risukey) ? -1 : op;
 }
+
+uintptr_t get_pc(struct reginfo *ri)
+{
+   return ri->pc;
+}
diff --git a/risu_arm.c b/risu_arm.c
index eaf4f6c..a61256b 100644
--- a/risu_arm.c
+++ b/risu_arm.c
@@ -73,3 +73,8 @@ int get_risuop(struct reginfo *ri)
    uint32_t risukey = (isz == 2) ? 0xdee0 : 0xe7fe5af0;
    return (key != risukey) ? -1 : op;
 }
+
+uintptr_t get_pc(struct reginfo *ri)
+{
+   return ri->gpreg[15];
+}
diff --git a/risu_m68k.c b/risu_m68k.c
index f84ac7a..35b3c03 100644
--- a/risu_m68k.c
+++ b/risu_m68k.c
@@ -33,3 +33,8 @@ int get_risuop(struct reginfo *ri)
     uint32_t risukey = 0x4afc7000;
     return (key != risukey) ? -1 : op;
 }
+
+uintptr_t get_pc(struct reginfo *ri)
+{
+    return ri->gregs[R_PC];
+}
diff --git a/risu_ppc64.c b/risu_ppc64.c
index b575078..e702a00 100644
--- a/risu_ppc64.c
+++ b/risu_ppc64.c
@@ -38,3 +38,8 @@ int get_risuop(struct reginfo *ri)
     uint32_t risukey = 0x00005af0;
     return (key != risukey) ? -1 : op;
 }
+
+uintptr_t get_pc(struct reginfo *ri)
+{
+   return ri->nip;
+}
