From 5ef3d3a3b59fe47934582f6c816fb92aa24c8459 Mon Sep 17 00:00:00 2001
From: Michael Zapf <github@mizapf.de>
Date: Mon, 6 Jan 2020 13:54:56 +0100
Subject: [PATCH] ti99: Block invalid accesses to VDP (like writing on read
 address); fixes Ambulance game.

---
 src/devices/bus/ti99/internal/datamux.cpp | 40 ++++++++++-------------
 1 file changed, 17 insertions(+), 23 deletions(-)

diff --git a/src/devices/bus/ti99/internal/datamux.cpp b/src/devices/bus/ti99/internal/datamux.cpp
index d0e7d05a6c1..91265ab6d5c 100644
--- a/src/devices/bus/ti99/internal/datamux.cpp
+++ b/src/devices/bus/ti99/internal/datamux.cpp
@@ -123,31 +123,25 @@ datamux_device::datamux_device(const machine_config &mconfig, const char *tag, d
 
 void datamux_device::read_all(uint16_t addr, uint8_t *value)
 {
-	// Valid access
-	bool validaccess = ((addr & 0x0400)==0);
-
-	if (validaccess)
+	// GROM access
+	if ((addr & 0xfc01)==0x9800)
 	{
-		// GROM access
-		if ((addr & 0xf801)==0x9800)
+		if (m_console_groms_present)
 		{
-			if (m_console_groms_present)
-			{
-				m_grom0->readz(value);
-				m_grom1->readz(value);
-				m_grom2->readz(value);
-			}
-			// GROMport (GROMs)
-			m_gromport->readz(addr, value);
-			m_grom_idle = false;
+			m_grom0->readz(value);
+			m_grom1->readz(value);
+			m_grom2->readz(value);
 		}
+		// GROMport (GROMs)
+		m_gromport->readz(addr, value);
+		m_grom_idle = false;
+	}
 
-		// Video
-		if ((addr & 0xf801)==0x8800)
-		{
-			// Forward to VDP unless we have an EVPC
-			if (m_video != nullptr) *value = m_video->read(addr>>1); // A14 determines data or register read
-		}
+	// Video
+	if ((addr & 0xfc01)==0x8800)
+	{
+		// Forward to VDP unless we have an EVPC
+		if (m_video != nullptr) *value = m_video->read((addr>>1)&1); // A14 determines data or register read
 	}
 
 	// GROMport (ROMs)
@@ -185,10 +179,10 @@ void datamux_device::write_all(uint16_t addr, uint8_t value)
 	}
 
 	// Video
-	if ((addr & 0xf801)==0x8800)
+	if ((addr & 0xfc01)==0x8c00)
 	{
 		// Forward to VDP unless we have an EVPC
-		if (m_video != nullptr) m_video->write(addr>>1, value);   // A14 determines data or register write
+		if (m_video != nullptr) m_video->write((addr>>1)&1, value);   // A14 determines data or register write
 	}
 
 	// I/O port gets all accesses