From fa321cc7cee09715d096e6a427ef4b1cb096732f Mon Sep 17 00:00:00 2001
From: AJR <ajrhacker@users.noreply.github.com>
Date: Sun, 30 Jan 2022 15:02:08 -0500
Subject: [PATCH] st2xxx: Fix regression by preventing IRQ level from changing
 between vector pulls

---
 src/devices/cpu/m6502/st2xxx.cpp | 34 ++++++++++++++++++++++----------
 src/devices/cpu/m6502/st2xxx.h   |  2 ++
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/src/devices/cpu/m6502/st2xxx.cpp b/src/devices/cpu/m6502/st2xxx.cpp
index bc73afc7750..d0428332878 100644
--- a/src/devices/cpu/m6502/st2xxx.cpp
+++ b/src/devices/cpu/m6502/st2xxx.cpp
@@ -65,6 +65,7 @@ st2xxx_device::st2xxx_device(const machine_config &mconfig, device_type type, co
 	, m_misc(0)
 	, m_ireq(0)
 	, m_iena(0)
+	, m_irq_level(0xff)
 	, m_lssa(0)
 	, m_lvpw(0)
 	, m_lxmax(0)
@@ -187,6 +188,7 @@ void st2xxx_device::save_common_registers()
 		save_item(NAME(m_misc));
 	save_item(NAME(m_ireq));
 	save_item(NAME(m_iena));
+	save_item(NAME(m_irq_level));
 	if (st2xxx_lctr_mask() != 0)
 	{
 		save_item(NAME(m_lssa));
@@ -282,20 +284,34 @@ void st2xxx_device::device_reset()
 	m_bctr = 0;
 }
 
+u8 st2xxx_device::active_irq_level() const
+{
+	// IREQH interrupts have priority over IREQL interrupts
+	u16 ireq_active = swapendian_int16(m_ireq & m_iena);
+	if (ireq_active != 0)
+		return 31 - (8 ^ count_leading_zeros_32(ireq_active & -ireq_active));
+	else
+		return 0xff;
+}
+
 u8 st2xxx_device::read_vector(u16 adr)
 {
 	if (adr >= 0xfffe)
 	{
-		u16 ireq_active = m_ireq & m_iena;
-		if (ireq_active != 0 && irq_taken)
+		if (adr == 0xfffe)
 		{
-			// IREQH interrupts have priority over IREQL interrupts
-			ireq_active = swapendian_int16(ireq_active);
-			int level = 31 - int(8 ^ count_leading_zeros_32(ireq_active & -ireq_active));
-			adr -= (level + 3) << 1;
+			set_irq_service(true);
+
+			// Make sure this doesn't change in between vector pull cycles
+			m_irq_level = irq_taken ? active_irq_level() : 0xff;
+		}
+
+		if (m_irq_level != 0xff)
+		{
+			adr -= (m_irq_level + 3) << 1;
 
 			LOGMASKED(LOG_IRQ, "Acknowledging %s interrupt (PC = $%04X, IREQ = $%04X, IENA = $%04X, vector pull from $%04X)\n",
-				st2xxx_irq_name(level),
+				st2xxx_irq_name(m_irq_level),
 				PPC,
 				m_ireq,
 				m_iena,
@@ -303,11 +319,9 @@ u8 st2xxx_device::read_vector(u16 adr)
 
 			if (BIT(adr, 0))
 			{
-				m_ireq &= ~(1 << level);
+				m_ireq &= ~(1 << m_irq_level);
 				update_irq_state();
 			}
-			else
-				set_irq_service(true);
 		}
 	}
 	return downcast<mi_st2xxx &>(*mintf).read_vector(adr);
diff --git a/src/devices/cpu/m6502/st2xxx.h b/src/devices/cpu/m6502/st2xxx.h
index 90f7c4d53c9..811c2008d49 100644
--- a/src/devices/cpu/m6502/st2xxx.h
+++ b/src/devices/cpu/m6502/st2xxx.h
@@ -135,6 +135,7 @@ protected:
 
 	void set_irq_service(bool state) { downcast<mi_st2xxx &>(*mintf).irq_service = state; }
 	void update_irq_state() { irq_state = (m_ireq & m_iena) != 0; }
+	u8 active_irq_level() const;
 
 	TIMER_CALLBACK_MEMBER(bt_interrupt);
 	TIMER_CALLBACK_MEMBER(lcd_interrupt);
@@ -268,6 +269,7 @@ protected:
 
 	u16 m_ireq;
 	u16 m_iena;
+	u8 m_irq_level;
 
 	u16 m_lssa;
 	u8 m_lvpw;