From 0afa2b7765ef8f4fea6cf708e40877538f28eed5 Mon Sep 17 00:00:00 2001
From: wilbertpol <wilbertpol@users.noreply.github.com>
Date: Wed, 31 Jan 2024 23:53:48 +0000
Subject: [PATCH] sega/segaai.cpp: Improve upd7759 banking. (#11991)

---
 src/mame/sega/segaai.cpp | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/src/mame/sega/segaai.cpp b/src/mame/sega/segaai.cpp
index 15dc91a972c..6f5b318762d 100644
--- a/src/mame/sega/segaai.cpp
+++ b/src/mame/sega/segaai.cpp
@@ -216,6 +216,7 @@ private:
 
 	u8 m_i8255_portb;
 	u8 m_upd7759_ctrl;
+	u8 m_upd7759_bank_ff;
 	u8 m_port_1c;
 	u8 m_port_1d;
 	u8 m_port_1e;
@@ -544,13 +545,25 @@ void segaai_state::upd7759_ctrl_w(u8 data)
 {
 	LOG("I/O Port $0b write: $%02x\n", data);
 
+	u8 prev_upd7759_ctrl = m_upd7759_ctrl;
 	m_upd7759_ctrl = data;
 
 	// bit0 is connected to /md line of the uPD7759
 	m_upd7759->md_w((m_upd7759_ctrl & UPD7759_MODE) ? 0 : 1);
 
-	// bit1 selects which ROM should be used?
-	m_upd7759->set_rom_bank((m_upd7759_ctrl & UPD7759_BANK) >> 1);
+	if (BIT(prev_upd7759_ctrl, 0))
+	{
+		if (!BIT(m_upd7759_ctrl, 0))
+		{
+			m_upd7759_bank_ff = 0;
+			m_upd7759->set_rom_bank(m_upd7759_bank_ff);
+		}
+	}
+	else
+	{
+		m_upd7759_bank_ff ^= 0x01;
+		m_upd7759->set_rom_bank(m_upd7759_bank_ff);
+	}
 }
 
 
@@ -632,6 +645,7 @@ void segaai_state::machine_start()
 {
 	m_i8255_portb = 0x7f;
 	m_upd7759_ctrl = 0;
+	m_upd7759_bank_ff = 0;
 	m_port_1c = 0;
 	m_port_1d = 0;
 	m_port_1e = 0;
@@ -645,6 +659,7 @@ void segaai_state::machine_start()
 
 	save_item(NAME(m_i8255_portb));
 	save_item(NAME(m_upd7759_ctrl));
+	save_item(NAME(m_upd7759_bank_ff));
 	save_item(NAME(m_port_1c));
 	save_item(NAME(m_port_1d));
 	save_item(NAME(m_port_1e));