cbus/pc9801_86.cpp: attempt to resolve DAC FIFO under/overflows

src/devices/bus/cbus/pc9801_86.cpp

@@ -20,6 +20,7 @@
 
 TODO:
 - Test all pcm modes;
+- Fix PCM overflow bug properly (CPUENB signal yield host CPU until DAC catches up?)
 - Make volume work;
 - Recording;
 - SpeakBoard: no idea about software that uses this, also board shows a single YM2608B?
@@ -368,14 +369,21 @@ void pc9801_86_device::io_map(address_map &map)
 			return 0;
 		}),
 		NAME([this] (u8 data) {
+			// HACK: on overflow make sure to single step the FIFO enough to claim some space back
+			// os2warp3 initializes the full buffer with 0x00 then quickly pretends
+			// that DAC already catched up by the time the actual startup/shutdown chimes are sent.
+			if (queue_count() == QUEUE_SIZE)
+			{
+				dac_transfer();
+				logerror("Warning: $a46c write with FIFO overflow %02x\n", m_pcm_mode);
+			}
+
 			if(queue_count() < QUEUE_SIZE)
 			{
 				m_queue[m_head++] = data;
 				m_head %= QUEUE_SIZE;
 				m_count++;
 			}
-			// TODO: what should happen on overflow?
-			// os2warp3 startup / shutdown chimes do this already
 		})
 	);
 }
@@ -424,12 +432,8 @@ u8 pc9801_86_device::queue_pop()
 	return ret;
 }
 
-TIMER_CALLBACK_MEMBER(pc9801_86_device::dac_tick)
+void pc9801_86_device::dac_transfer()
 {
-	m_pcm_clk = !m_pcm_clk;
-	if((m_pcm_ctrl & 0x40) || !(m_pcm_ctrl & 0x80))
-		return;
-
 	switch(m_pcm_mode & 0x70)
 	{
 		case 0x70: // 8bit stereo
@@ -461,6 +465,20 @@ TIMER_CALLBACK_MEMBER(pc9801_86_device::dac_tick)
 			m_rdac->write(rsample);
 		}   break;
 	}
+}
+
+TIMER_CALLBACK_MEMBER(pc9801_86_device::dac_tick)
+{
+	m_pcm_clk = !m_pcm_clk;
+	if((m_pcm_ctrl & 0x40) || !(m_pcm_ctrl & 0x80))
+		return;
+
+	// TODO: verify underflow
+	// should leave the DACs in whatever state they are or ...?
+	if (!queue_count())
+		return;
+
+	dac_transfer();
 	if((queue_count() < m_irq_rate) && (m_pcm_ctrl & 0x20))
 	{
 		m_pcmirq = true;

src/devices/bus/cbus/pc9801_86.h

@@ -31,6 +31,9 @@ public:
 	pc9801_86_device(const machine_config &mconfig, const char *tag, device_t *owner, uint32_t clock);
 	pc9801_86_device(const machine_config &mconfig, device_type type, const char *tag, device_t *owner, uint32_t clock);
 
+	static constexpr feature_type unemulated_features() { return feature::MICROPHONE; }
+	static constexpr feature_type imperfect_features() { return feature::SOUND; }
+
 protected:
 	void io_map(address_map &map) ATTR_COLD;
 	u8 pcm_control_r();
@@ -74,6 +77,8 @@ private:
 	required_device<dac_16bit_r2r_twos_complement_device> m_rdac;
 	std::vector<u8> m_queue;
 	emu_timer *m_dac_timer;
+
+	void dac_transfer();
 };
 
 class pc9801_speakboard_device : public pc9801_86_device