diff --git a/src/sim.c b/src/sim.c
index 6439c0a9..0a5421a3 100644
--- a/src/sim.c
+++ b/src/sim.c
@@ -114,6 +114,12 @@ int run_instruction(struct sim_state *s, const struct element *i, void *run_data
             g->dd == 1 || g->dd == 2, g->dd == 1);
 }
 
+static int updates_P(const struct insn_or_data i)
+{
+    struct instruction_typeany t = i.u.typeany;
+    return (t.z == 15) && (t.dd == 0 || t.dd == 3);
+}
+
 int interp_step_sim(struct sim_state *s, const struct run_ops *ops,
         void **run_data, void *ops_data)
 {
@@ -137,10 +143,25 @@ int interp_step_sim(struct sim_state *s, const struct run_ops *ops,
         if (ops->post_insn(s, &i, ops_data))
             return 0;
 
+    // return  2 means "successful step, updated P register"
     // return  1 means "successful step, can continue as-is"
     // return  0 means "stopped because a hook told us to stop"
     // return -1 means "stopped because some error occurred"
-    return 1;
+    // TODO use an enumeration
+    return updates_P(i.insn) ? 2 : 1;
+}
+
+int interp_run_sim_block(struct sim_state *s, const struct run_ops *ops,
+        void **run_data, void *ops_data)
+{
+    int rc = 0;
+
+    *run_data = NULL; // this runner needs no data yet
+    do {
+        rc = interp_step_sim(s, ops, run_data, ops_data);
+    } while (rc > 0 && rc != 2);
+
+    return rc;
 }
 
 int interp_run_sim(struct sim_state *s, const struct run_ops *ops,
diff --git a/src/sim.h b/src/sim.h
index 39ef2819..0c3123bb 100644
--- a/src/sim.h
+++ b/src/sim.h
@@ -63,7 +63,7 @@ struct run_ops {
 };
 
 typedef int sim_runner(struct sim_state *s, const struct run_ops *ops, void **run_data, void *ops_data);
-extern sim_runner interp_run_sim, interp_step_sim;
+extern sim_runner interp_run_sim, interp_step_sim, interp_run_sim_block;
 int load_sim(op_dispatcher *dispatch_op, void *sud, const struct format *f,
         void *fud, FILE *in, int load_address);