From 2b17ce4f4042394fd19cd06f44b271a5d1e97e67 Mon Sep 17 00:00:00 2001
From: Michael Zapf <Michael.Zapf@mizapf.de>
Date: Sun, 14 Oct 2018 15:13:42 +0200
Subject: [PATCH] ti99: Added guards against selecting non-existing drives.

---
 src/devices/bus/ti99/peb/bwg.cpp    | 41 ++++++++++++++++++-----------
 src/devices/bus/ti99/peb/ti_fdc.cpp | 11 +++++++-
 2 files changed, 36 insertions(+), 16 deletions(-)

diff --git a/src/devices/bus/ti99/peb/bwg.cpp b/src/devices/bus/ti99/peb/bwg.cpp
index 877485174f2..c7aa2c903b7 100644
--- a/src/devices/bus/ti99/peb/bwg.cpp
+++ b/src/devices/bus/ti99/peb/bwg.cpp
@@ -482,30 +482,41 @@ void snug_bwg_device::set_drive()
 {
 	LOGMASKED(LOG_CRU, "new DSEL = %d\n", m_DSEL);
 
-	if ((m_DSEL != 0) && (m_DSEL != 1) && (m_DSEL != 2) && (m_DSEL != 4) && (m_DSEL != 8))
+	int bits = m_DSEL & 0x0f;
+	int num = -1;
+
+	// If the selected floppy drive is not attached, remove that line
+	if (m_floppy[3] == nullptr) bits &= 0x07;  // 0111
+	if (m_floppy[2] == nullptr) bits &= 0x0b;  // 1011
+	if (m_floppy[1] == nullptr) bits &= 0x0d;  // 1101
+	if (m_floppy[0] == nullptr) bits &= 0x0e;  // 1110
+
+	if ((bits != 0) && (bits != 1) && (bits != 2) && (bits != 4) && (bits != 8))
 		LOGMASKED(LOG_WARN, "Warning - multiple drives selected\n");
 
 	// The schematics do not reveal any countermeasures against multiple selection
 	// so we assume that the highest value wins.
 
-	int bits = m_DSEL & 0x0f;
-	int i = -1;
-
-	while (bits != 0)
-	{
-		bits >>= 1;
-		i++;
-	}
-	if (i != -1)
-	{
-		m_current_floppy = m_floppy[i];
-		LOGMASKED(LOG_CRU, "Selected floppy %d\n", i);
-	}
-	else
+	if (bits==0)
 	{
 		m_current_floppy = nullptr;
 		LOGMASKED(LOG_CRU, "All drives deselected\n");
 	}
+	else
+	{
+		if ((bits & 0x08)!=0) num = 3;
+		else
+		{
+			if ((bits & 0x04)!=0) num = 2;
+			else
+			{
+				if ((bits & 0x02)!=0) num = 1;
+				else num = 0;
+			}
+		}
+		LOGMASKED(LOG_CRU, "Selected floppy DSK%d\n", num+1);
+		m_current_floppy = m_floppy[num];
+	}
 	m_wd1773->set_floppy(m_current_floppy);
 }
 
diff --git a/src/devices/bus/ti99/peb/ti_fdc.cpp b/src/devices/bus/ti99/peb/ti_fdc.cpp
index 7fd4192757c..530dc71d987 100644
--- a/src/devices/bus/ti99/peb/ti_fdc.cpp
+++ b/src/devices/bus/ti99/peb/ti_fdc.cpp
@@ -273,7 +273,14 @@ WRITE_LINE_MEMBER(ti_fdc_device::sidsel_w)
 
 void ti_fdc_device::set_drive()
 {
-	switch (m_DSEL)
+	int dsel = m_DSEL;
+
+	// If the selected floppy drive is not attached, remove that line
+	if (m_floppy[2] == nullptr) dsel &= 0x03;  // 011
+	if (m_floppy[1] == nullptr) dsel &= 0x05;  // 101
+	if (m_floppy[0] == nullptr) dsel &= 0x06;  // 110
+
+	switch (dsel)
 	{
 	case 0:
 		m_current = NONE;
@@ -398,6 +405,8 @@ void ti_fdc_device::device_reset()
 void ti_fdc_device::device_config_complete()
 {
 	// Seems to be null when doing a "-listslots"
+	for (auto &elem : m_floppy)
+		elem = nullptr;
 	if (subdevice("0")!=nullptr) m_floppy[0] = static_cast<floppy_image_device*>(subdevice("0")->subdevices().first());
 	if (subdevice("1")!=nullptr) m_floppy[1] = static_cast<floppy_image_device*>(subdevice("1")->subdevices().first());
 	if (subdevice("2")!=nullptr) m_floppy[2] = static_cast<floppy_image_device*>(subdevice("2")->subdevices().first());