harness: model the 0x5800000/0x5900000 DMA command/data pair

harness/mame/test_vt1200_board.cpp

@@ -86,6 +86,20 @@ static void test_switch_mix_only_when_configured(void)
     CHECK(b.read(VT1200_WIN3 + VT1200_OFF_SWITCH) == 0x0070);   // returned verbatim
 }
 
+static void test_dma_pair_command_completes(void)
+{
+    vt1200_board b;
+    b.set_model_handshake(true);
+    b.set_dma_ticks(2);
+
+    b.write(VT1200_WIN0, 0x0000);            // data port first (as the firmware does)
+    b.write(VT1200_WIN1, 0x0001);            // then a command
+    CHECK(b.read(VT1200_WIN1) == 0x0001);    // accepted
+
+    b.tick(); b.tick();
+    CHECK(b.read(VT1200_WIN1) == 0x0000);    // completed
+}
+
 int main(void)
 {
     test_default_read();
@@ -95,6 +109,7 @@ int main(void)
     test_windows_are_independent();
     test_monitor_switch_accepts_raw_bits_or_position();
     test_switch_mix_only_when_configured();
+    test_dma_pair_command_completes();
     std::printf(failures ? "%d FAILURE(S)\n" : "all board tests passed\n", failures);
     return failures ? 1 : 0;
 }

harness/mame/vt1200_board.cpp

@@ -18,6 +18,11 @@ uint16_t vt1200_board::read(uint32_t addr)
 	if (st != m_store.end())
 		value = st->second;
 
+	// DMA command port: while a command is in flight the port reads back the
+	// latched command; once the configured ticks elapse it reads back 0.
+	if (m_dma_ticks > 0 && (addr & 0xffff0000) == VT1200_WIN1)
+		return (m_dma_left > 0) ? m_dma_cmd : (uint16_t)0x0000;
+
 	if (m_switch_set && is_switch_port(addr))
 		return (uint16_t)((value & 0xff8f) | (m_switch_bits & 0x0070));
 
@@ -41,6 +46,15 @@ void vt1200_board::write(uint32_t addr, uint16_t data)
 		auto ticks = m_busy_ticks_by_win.find(win);
 		m_busy_left[win] = (ticks != m_busy_ticks_by_win.end()) ? ticks->second : 0;
 	}
+
+	// A command written to the DMA command port (WIN1) is latched and, when the
+	// DMA model is configured, starts its completion timer.
+	if ((addr & 0xffff0000) == VT1200_WIN1)
+	{
+		m_dma_cmd = data;
+		if (m_dma_ticks > 0)
+			m_dma_left = m_dma_ticks;
+	}
 }
 
 void vt1200_board::tick()
@@ -49,4 +63,8 @@ void vt1200_board::tick()
 	for (auto &entry : m_busy_left)
 		if (entry.second > 0)
 			entry.second--;
+
+	// Merge the DMA pair's timer into the same tick.
+	if (m_dma_left > 0)
+		m_dma_left--;
 }

harness/mame/vt1200_board.h

@@ -53,6 +53,12 @@ public:
 	}
 	void set_window_busy_ticks(uint32_t window_base, int n) { m_busy_ticks_by_win[window_base] = n; }
 
+	// DMA command/data pair at WIN0 (data) / WIN1 (command).  A write to the
+	// command port is accepted and reads back its value for n ticks; after the
+	// ticks elapse the command port reads back 0.  n == 0 (the default) leaves
+	// the pair with no DMA semantics, so the port just behaves as a latch.
+	void set_dma_ticks(int n) { m_dma_ticks = n; }
+
 	// Monitor switch (bits 4-6 of the switch port).  The argument is accepted
 	// unambiguously as EITHER the raw register bits (e.g. 0x70) OR a small
 	// position 0-7: a value above 0x07 is taken as the raw bit pattern, any
@@ -82,6 +88,13 @@ private:
 	// count for the command currently in flight.
 	std::map<uint32_t, int> m_busy_ticks_by_win;
 	std::map<uint32_t, int> m_busy_left;
+	// DMA command/data pair.  m_dma_ticks is the configured completion time;
+	// m_dma_left is how many ticks remain for the command in flight; m_dma_cmd
+	// is the value written to the command port.  All three only have meaning
+	// when m_dma_ticks > 0.
+	int      m_dma_ticks = 0;
+	int      m_dma_left = 0;
+	uint16_t m_dma_cmd = 0;
 	bool     m_switch_set = false;             // set_monitor_switch was called
 	uint16_t m_switch_bits = 0;
 };