sinclair/specnext_dma.cpp: Fixed counter state returning natural value in ZXN mode. (#15179)

Author: holub

src/devices/machine/z80dma.cpp

@@ -601,7 +601,7 @@ TIMER_CALLBACK_MEMBER(z80dma_device::clock_w)
 			m_addressA = PORTA_ADDRESS;
 			m_addressB = PORTB_ADDRESS;
 			m_count = BLOCKLEN;
-			m_byte_counter = 0;
+			reset_byte_counter();
 			m_status |= 0x30;
 			enable();
 		}
@@ -792,7 +792,7 @@ void z80dma_device::write(u8 data)
 				m_addressA = PORTA_ADDRESS;
 				m_addressB = PORTB_ADDRESS;
 				m_count = BLOCKLEN;
-				m_byte_counter = 0;
+				reset_byte_counter();
 				m_status |= 0x30;
 
 				LOGREGS("CMD Load A: %x B: %x N: %x\n", m_addressA, m_addressB, m_count);
@@ -812,7 +812,7 @@ void z80dma_device::write(u8 data)
 			case COMMAND_CONTINUE:
 				LOGREGS("CMD Continue\n");
 				m_count = BLOCKLEN;
-				m_byte_counter = 0;
+				reset_byte_counter();
 				//enable(); //???m_dma_enabled = 1;
 				//"match not found" & "end of block" status flags zeroed here
 				m_status |= 0x30;

src/devices/machine/z80dma.h

@@ -118,14 +118,15 @@ class z80dma_device :   public device_t,
 	void enable();
 	void disable();
 	u8 num_follow() const noexcept { return m_num_follow; }
-	virtual int is_ready();
+	int is_ready();
 	void interrupt_check();
 	void trigger_interrupt(int level);
-	virtual void do_read();
+	void do_read();
 	void do_write();
 	void do_transfer_write();
 	void do_search();
 	void setup_next_read(int rr);
+	virtual void reset_byte_counter() { m_byte_counter = 0; };
 
 	virtual TIMER_CALLBACK_MEMBER(clock_w);
 

src/mame/sinclair/specnext_dma.cpp

@@ -42,6 +42,11 @@ specnext_dma_device::specnext_dma_device(const machine_config &mconfig, const ch
 {
 }
 
+void specnext_dma_device::reset_byte_counter()
+{
+	m_byte_counter = m_dma_mode ? 0 : 1;
+}
+
 void specnext_dma_device::write(u8 data)
 {
 	if (num_follow() == 0)
@@ -52,7 +57,7 @@ void specnext_dma_device::write(u8 data)
 			switch (data)
 			{
 			case COMMAND_ENABLE_DMA:
-				m_byte_counter = 0;
+				reset_byte_counter();
 				break;
 			case COMMAND_RESET:
 				m_r2_portB_preescaler_s = 0;
@@ -78,14 +83,6 @@ void specnext_dma_device::write(u8 data)
 	}
 }
 
-void specnext_dma_device::do_read()
-{
-	z80dma_device::do_read();
-	// zxnDMA?
-	if (m_dma_mode == 0 && (m_byte_counter + 1) == m_count)
-		m_byte_counter++;
-}
-
 TIMER_CALLBACK_MEMBER(specnext_dma_device::clock_w)
 {
 	if (m_dma_delay)
@@ -111,7 +108,10 @@ TIMER_CALLBACK_MEMBER(specnext_dma_device::clock_w)
 	}
 
 	const bool may_prescaled = m_dma_seq == SEQ_TRANS1_WRITE_DEST && m_r2_portB_preescaler_s;
-	z80dma_device::clock_w(param);
+	if (m_dma_mode == 0 && m_dma_seq == SEQ_WAIT_READY && is_ready() && m_byte_counter == 1)
+		m_dma_seq = SEQ_REQUEST_BUS;
+	else
+		z80dma_device::clock_w(param);
 
 	if (may_prescaled && m_dma_seq != SEQ_FINISH)
 	{

src/mame/sinclair/specnext_dma.h

@@ -21,7 +21,7 @@ class specnext_dma_device : public z80dma_device
 	virtual void device_start() override ATTR_COLD;
 	virtual void device_reset() override ATTR_COLD;
 
-	virtual void do_read() override;
+	virtual void reset_byte_counter() override;
 
 	virtual TIMER_CALLBACK_MEMBER(clock_w) override;