Report dubious SN76489 write timings when write enable left on.

Chris Evans · Chris Evans · commit 416aa90c77d8 · 2025-07-10T11:03:39.000-07:00
Useful for developing an advanced sampled sound player that must
use cycle accurate timings.
diff --git a/bbc.c b/bbc.c
@@ -2658,15 +2658,18 @@ bbc_set_IC32(struct bbc_struct* p_bbc, uint8_t val) {
    * Changing this selection requires various notifications.
    */
 
-  /* Selecting or deselecting the keyboard may need to change interrupt and / or
-   * bus value status.
-   */
-  via_update_port_a(p_bbc->p_system_via);
-
   /* The video ULA needs to know about changes to the video wrap-around
    * address.
    */
   video_IC32_updated(p_bbc->p_video, val);
+
+  /* The SN76489 needs to see if its write enable line changed. */
+  sound_sn_IC32_updated(p_bbc->p_sound, val);
+
+  /* Selecting or deselecting the keyboard may need to change interrupt and / or
+   * bus value status.
+   */
+  via_update_port_a(p_bbc->p_system_via);
 }
 
 uint8_t*
diff --git a/sound.c b/sound.c
@@ -60,6 +60,9 @@ struct sound_struct {
   double iir_second_pass_output_history[2];
 
   /* sn76489 state. */
+  int is_write_enabled;
+  int had_write_disabled;
+  uint64_t prev_bus_change_ticks;
   uint16_t counter[k_sound_num_channels];
   uint8_t output[k_sound_num_channels];
   uint16_t noise_rng;
@@ -528,6 +531,11 @@ void
 sound_power_on_reset(struct sound_struct* p_sound) {
   uint32_t i;
 
+  /* Mirrors initial IC32 state. */
+  p_sound->is_write_enabled = 1;
+  p_sound->had_write_disabled = 0;
+  p_sound->prev_bus_change_ticks = 0;
+
   /* EMU: initial sn76489 state and behavior is something no two sources seem
    * to agree on. It doesn't matter a huge amount for BBC emulation because
    * MOS sets the sound channels up on boot. But the intial BBC power-on
@@ -674,11 +682,46 @@ sound_tick(struct sound_struct* p_sound, uint64_t curr_time_us) {
 }
 
 void
-sound_sn_write(struct sound_struct* p_sound, uint8_t value) {
+sound_sn_IC32_updated(struct sound_struct* p_sound, uint8_t value) {
+  int is_write_enabled = !(value & 1);
+  if (is_write_enabled == p_sound->is_write_enabled) {
+    return;
+  }
+
+  p_sound->is_write_enabled = is_write_enabled;
+  if (!is_write_enabled) {
+    p_sound->had_write_disabled = 1;
+    p_sound->prev_bus_change_ticks = 0;
+  }
+}
+
+void
+sound_sn_set_bus_value(struct sound_struct* p_sound, uint8_t value) {
   uint8_t command;
   uint8_t channel;
+  int32_t new_period;
+  uint64_t ticks;
+  uint64_t prev_bus_change_ticks;
+
+  if (!p_sound->is_write_enabled) {
+    return;
+  }
+
+  ticks = timing_get_scaled_total_timer_ticks(p_sound->p_timing);
+  prev_bus_change_ticks = p_sound->prev_bus_change_ticks;
+  if ((prev_bus_change_ticks != 0) && p_sound->had_write_disabled) {
+    uint64_t delta = (ticks - prev_bus_change_ticks);
+    if ((delta % 32) != 0) {
+      log_do_log(k_log_audio,
+                 k_log_warning,
+                 "bad bus timing for multiple writes: %"PRIu64", value $%.2X",
+                 delta,
+                 value);
+    }
+  }
+  p_sound->prev_bus_change_ticks = ticks;
 
-  int32_t new_period = -1;
+  new_period = -1;
 
   if (sound_is_active(p_sound) && p_sound->synchronous) {
     sound_advance_sn_timing(p_sound);
diff --git a/sound.h b/sound.h
@@ -43,6 +43,7 @@ void sound_set_state(struct sound_struct* p_sound,
                      uint8_t noise_frequency,
                      uint16_t noise_rng);
 
-void sound_sn_write(struct sound_struct* p_sound, uint8_t data);
+void sound_sn_IC32_updated(struct sound_struct* p_sound, uint8_t value);
+void sound_sn_set_bus_value(struct sound_struct* p_sound, uint8_t value);
 
 #endif /* BEEBJIT_SOUND_H */
diff --git a/via.c b/via.c
@@ -474,6 +474,7 @@ sysvia_update_port_a(struct via_struct* p_via) {
   struct bbc_struct* p_bbc = p_via->p_bbc;
   struct keyboard_struct* p_keyboard = bbc_get_keyboard(p_bbc);
   struct cmos_struct* p_cmos = bbc_get_cmos(p_bbc);
+  struct sound_struct* p_sound = bbc_get_sound(p_bbc);
   int fire = 0;
   uint8_t IC32 = bbc_get_IC32(p_bbc);
   uint8_t peripheral_a = 0xFF;
@@ -511,10 +512,7 @@ sysvia_update_port_a(struct via_struct* p_via) {
 
   via_set_CA2(p_via, fire);
 
-  if (!(IC32 & 1)) {
-    struct sound_struct* p_sound = bbc_get_sound(p_via->p_bbc);
-    sound_sn_write(p_sound, bus_val);
-  }
+  sound_sn_set_bus_value(p_sound, bus_val);
 
   p_via->bus_value_a = bus_val;
 }
