diff --git a/src/emu/bay.c b/src/emu/bay.c
index 3a83882..2cf2c10 100644
--- a/src/emu/bay.c
+++ b/src/emu/bay.c
@@ -6,6 +6,11 @@
 #include "uthash.h"
 #include "utlist.h"
 
+static char *propname[BAY_CB_MAX] = {
+	[BAY_CB_DIRTY] = "dirty",
+	[BAY_CB_EMIT] = "emit"
+};
+
 /* Called from the channel when it becomes dirty */
 static int
 cb_chan_is_dirty(struct chan *chan, void *arg)
@@ -14,6 +19,12 @@ cb_chan_is_dirty(struct chan *chan, void *arg)
 	struct bay_chan *bchan = arg;
 	struct bay *bay = bchan->bay;
 
+	if (bay->state != BAY_READY && bay->state != BAY_PROPAGATING) {
+		err("cannot add dirty channel %s in current bay state\n",
+				chan->name);
+		return -1;
+	}
+
 	if (bchan->is_dirty) {
 		err("channel %s already on dirty list\n", chan->name);
 		return -1;
@@ -94,7 +105,8 @@ bay_remove(struct bay *bay, struct chan *chan)
 }
 
 int
-bay_add_cb(struct bay *bay, struct chan *chan, bay_cb_func_t func, void *arg)
+bay_add_cb(struct bay *bay, enum bay_cb_type type,
+		struct chan *chan, bay_cb_func_t func, void *arg)
 {
 	if (func == NULL) {
 		err("bay_add_cb: func is NULL\n");
@@ -117,8 +129,8 @@ bay_add_cb(struct bay *bay, struct chan *chan, bay_cb_func_t func, void *arg)
 	cb->func = func;
 	cb->arg = arg;
 
-	DL_APPEND(bchan->cb, cb);
-	bchan->ncallbacks++;
+	DL_APPEND(bchan->cb[type], cb);
+	bchan->ncallbacks[type]++;
 	return 0;
 }
 
@@ -130,13 +142,14 @@ bay_init(struct bay *bay)
 }
 
 static int
-propagate_chan(struct bay_chan *bchan)
+propagate_chan(struct bay_chan *bchan, enum bay_cb_type type)
 {
-	dbg("- propagating dirty channel %s\n", bchan->chan->name);
+	dbg("- propagating channel '%s' phase %s\n",
+			bchan->chan->name, propname[type]);
 
 	struct bay_cb *cur = NULL;
 	struct bay_cb *tmp = NULL;
-	DL_FOREACH_SAFE(bchan->cb, cur, tmp) {
+	DL_FOREACH_SAFE(bchan->cb[type], cur, tmp) {
 		if (cur->func(bchan->chan, cur->arg) != 0) {
 			err("propagate_chan: callback failed\n");
 			return -1;
@@ -149,20 +162,31 @@ propagate_chan(struct bay_chan *bchan)
 int
 bay_propagate(struct bay *bay)
 {
-	dbg("-- propagating channels begins\n");
-	struct bay_chan *cur = NULL;
-	struct bay_chan *tmp = NULL;
+	struct bay_chan *cur, *tmp;
+	bay->state = BAY_PROPAGATING;
 	DL_FOREACH_SAFE(bay->dirty, cur, tmp) {
 		/* May add more dirty channels */
-		if (propagate_chan(cur) != 0) {
+		if (propagate_chan(cur, BAY_CB_DIRTY) != 0) {
 			err("bay_propagate: propagate_chan failed\n");
 			return -1;
 		}
 	}
 
-	/* Flush channels after running all the dirty callbacks, so we
-	 * capture any potential double write when running the
-	 * callbacks */
+	/* Once the dirty callbacks have been propagated,
+	 * begin the emit stage */
+	bay->state = BAY_EMITTING;
+	DL_FOREACH_SAFE(bay->dirty, cur, tmp) {
+		/* May add more dirty channels */
+		if (propagate_chan(cur, BAY_CB_EMIT) != 0) {
+			err("bay_propagate: propagate_chan failed\n");
+			return -1;
+		}
+	}
+
+	/* Flush channels after running all the dirty and emit
+	 * callbacks, so we capture any potential double write when
+	 * running the callbacks */
+	bay->state = BAY_FLUSHING;
 	DL_FOREACH_SAFE(bay->dirty, cur, tmp) {
 		if (chan_flush(cur->chan) != 0) {
 			err("bay_propagate: chan_flush failed\n");
@@ -172,19 +196,7 @@ bay_propagate(struct bay *bay)
 	}
 
 	bay->dirty = NULL;
-
-	dbg("-- propagating channels ends\n");
+	bay->state = BAY_READY;
 
 	return 0;
 }
-
-//int
-//bay_emit(struct bay *bay)
-//{
-//	for (chan in dirty channels) {
-//		/* May add more dirty channels */
-//		emit_chan(chan);
-//	}
-//
-//	return 0;
-//}
diff --git a/src/emu/bay.h b/src/emu/bay.h
index fa1289e..9f9770a 100644
--- a/src/emu/bay.h
+++ b/src/emu/bay.h
@@ -10,6 +10,12 @@ struct bay;
 struct bay_cb;
 struct bay_chan;
 
+enum bay_cb_type {
+	BAY_CB_DIRTY = 0,
+	BAY_CB_EMIT,
+	BAY_CB_MAX,
+};
+
 typedef int (*bay_cb_func_t)(struct chan *chan, void *ptr);
 
 struct bay_cb {
@@ -25,8 +31,8 @@ struct bay_cb {
 
 struct bay_chan {
 	struct chan *chan;
-	int ncallbacks;
-	struct bay_cb *cb;
+	int ncallbacks[BAY_CB_MAX];
+	struct bay_cb *cb[BAY_CB_MAX];
 	struct bay *bay;
 	int is_dirty;
 
@@ -39,12 +45,11 @@ struct bay_chan {
 };
 
 enum bay_state {
-	BAY_CONFIG = 1,
+	BAY_UKNOWN = 0,
 	BAY_READY,
 	BAY_PROPAGATING,
-	BAY_PROPAGATED,
 	BAY_EMITTING,
-	BAY_EMITTED
+	BAY_FLUSHING
 };
 
 struct bay {
@@ -53,11 +58,11 @@ struct bay {
 	struct bay_chan *dirty;
 };
 
+void bay_init(struct bay *bay);
 int bay_register(struct bay *bay, struct chan *chan);
 int bay_remove(struct bay *bay, struct chan *chan);
 struct chan *bay_find(struct bay *bay, const char *name);
-int bay_add_cb(struct bay *bay, struct chan *chan, bay_cb_func_t func, void *arg);
-void bay_init(struct bay *bay);
+int bay_add_cb(struct bay *bay, enum bay_cb_type type, struct chan *chan, bay_cb_func_t func, void *arg);
 int bay_propagate(struct bay *bay);
 
 #endif /* BAY_H */
diff --git a/src/emu/mux.c b/src/emu/mux.c
index bd18502..cbdfe80 100644
--- a/src/emu/mux.c
+++ b/src/emu/mux.c
@@ -173,7 +173,7 @@ mux_init(struct mux *mux,
 	mux->select_func = select_func;
 	mux->bay = bay;
 
-	if (bay_add_cb(bay, select, cb_select, mux) != 0) {
+	if (bay_add_cb(bay, BAY_CB_DIRTY, select, cb_select, mux) != 0) {
 		err("mux_init: bay_add_cb failed\n");
 		return -1;
 	}
@@ -222,7 +222,7 @@ mux_add_input(struct mux *mux, struct value key, struct chan *chan)
 
 	HASH_ADD_KEYPTR(hh, mux->input, &input->key, sizeof(input->key), input);
 
-	if (bay_add_cb(mux->bay, chan, cb_input, mux) != 0) {
+	if (bay_add_cb(mux->bay, BAY_CB_DIRTY, chan, cb_input, mux) != 0) {
 		err("mux_add_input: bay_add_cb failed\n");
 		return -1;
 	}
diff --git a/src/emu/mux.h b/src/emu/mux.h
index 2d74c6e..f85597b 100644
--- a/src/emu/mux.h
+++ b/src/emu/mux.h
@@ -17,15 +17,6 @@ typedef int (* mux_select_func_t)(struct mux *mux,
 		struct value value,
 		struct mux_input **input);
 
-/* MUX logic:
- *
- * select		input	output
- * ---------------------------
- * null			-		null
- * input		x		x
- *
- */
-
 struct mux {
 	struct bay *bay;
 	int64_t ninputs;
diff --git a/src/include/common.h b/src/include/common.h
index 23d1eac..1caee4c 100644
--- a/src/include/common.h
+++ b/src/include/common.h
@@ -27,6 +27,8 @@
 /* Poison assert */
 #pragma GCC poison assert
 
+#define USE_RET __attribute__((warn_unused_result))
+
 /* clang-format on */
 
 #endif /* COMMON_H */
diff --git a/test/unit/bay.c b/test/unit/bay.c
index 7166418..56525f8 100644
--- a/test/unit/bay.c
+++ b/test/unit/bay.c
@@ -62,7 +62,7 @@ test_callback(struct bay *bay)
 		die("bay_register failed\n");
 
 	int64_t data = 0;
-	if (bay_add_cb(bay, &chan, callback, &data) != 0)
+	if (bay_add_cb(bay, BAY_CB_DIRTY, &chan, callback, &data) != 0)
 		die("bay_add_cb failed\n");
 
 	if (data != 0)