machine/ncr53c90.cpp: Fix NCR53CF94 ID read sequence

mamedev · Jan 7, 2025 · fd93123 · fd93123
1 parent 2a9cfda
commit fd93123
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Showing 2 changed files with 22 additions and 9 deletions.
diff --git a/src/devices/machine/ncr53c90.cpp b/src/devices/machine/ncr53c90.cpp
@@ -918,6 +918,15 @@ void ncr53c90_device::command_pop_and_chain()
 	}
 }
 
+void ncr53c90_device::load_tcounter()
+{
+	LOGMASKED(LOG_COMMAND, "DMA command: tcounter reloaded to %d\n", tcount & tcounter_mask);
+	tcounter = tcount & tcounter_mask;
+
+	// clear transfer count zero flag when counter is reloaded
+	status &= ~S_TC0;
+}
+
 void ncr53c90_device::start_command()
 {
 	uint8_t c = command[0] & 0x7f;
@@ -932,11 +941,7 @@ void ncr53c90_device::start_command()
 	dma_command = command[0] & 0x80;
 	if (dma_command)
 	{
-		LOGMASKED(LOG_COMMAND, "DMA command: tcounter reloaded to %d\n", tcount);
-		tcounter = tcount;
-
-		// clear transfer count zero flag when counter is reloaded
-		status &= ~S_TC0;
+		load_tcounter();
 	}
 	else
 	{
@@ -1417,6 +1422,15 @@ void ncr53cf94_device::device_reset()
 	ncr53c94_device::device_reset();
 }
 
+void ncr53cf94_device::load_tcounter()
+{
+	ncr53c94_device::load_tcounter();
+
+	// ID may be read by executing DMA NOP command twice, first with the features bit clear and then with it set
+	if ((config2 & S2FE) == 0)
+		tcount = (1 << 23) | (family_id << 19) | (revision_level << 16) | (tcount & 0xffff);
+}
+
 void ncr53cf94_device::conf2_w(uint8_t data)
 {
 	tcounter_mask = (data & S2FE) ? 0xffffff : 0xffff;
@@ -1425,10 +1439,6 @@ void ncr53cf94_device::conf2_w(uint8_t data)
 
 uint8_t ncr53cf94_device::tcounter_hi2_r()
 {
-	// tcounter is 24-bit when the features bit is set, otherwise it returns the ID
-	if ((config2 & S2FE) == 0)
-		return (1 << 7) | (family_id << 3) | revision_level;
-
 	LOG("tcounter_hi2_r %02x (%s)\n", (tcounter >> 16) & 0xff, machine().describe_context());
 	return tcounter >> 16;
 }

diff --git a/src/devices/machine/ncr53c90.h b/src/devices/machine/ncr53c90.h
@@ -232,6 +232,7 @@ class ncr53c90_device : public nscsi_device, public nscsi_slot_card_interface
 	void delay_cycles(int cycles);
 
 	void decrement_tcounter(int count = 1);
+	virtual void load_tcounter();
 
 	devcb_write_line m_irq_handler;
 	devcb_write_line m_drq_handler;
@@ -361,6 +362,8 @@ class ncr53cf94_device : public ncr53c94_device
 	virtual void device_start() override ATTR_COLD;
 	virtual void device_reset() override ATTR_COLD;
 
+	virtual void load_tcounter() override;
+
 private:
 	u8 config4;
 	u8 family_id;