From 72bb930435a5d1a2de7d23b02fafeeeef33714d4 Mon Sep 17 00:00:00 2001
From: cebix <>
Date: Thu, 25 Apr 2002 13:14:11 +0000
Subject: [PATCH] added some safety checks

---
 BasiliskII/src/video.cpp | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/BasiliskII/src/video.cpp b/BasiliskII/src/video.cpp
index 43d1f1a3..2cc6addd 100644
--- a/BasiliskII/src/video.cpp
+++ b/BasiliskII/src/video.cpp
@@ -416,10 +416,18 @@ void monitor_desc::switch_mode(vector<video_mode>::const_iterator it, uint32 par
 		WriteMacInt32(0x824, mac_frame_base);		// ScrnBase
 		WriteMacInt32(0x898, mac_frame_base);		// CrsrBase
 		uint32 gdev = ReadMacInt32(0x8a4);			// MainDevice
-		gdev = ReadMacInt32(gdev);
-		uint32 pmap = ReadMacInt32(gdev + 0x16);	// gdPMap
-		pmap = ReadMacInt32(pmap);
-		WriteMacInt32(pmap, mac_frame_base);		// baseAddr
+		D(bug("MainDevice handle at %08lx\n", gdev));
+		if (gdev != 0 && gdev != 0xffffffff) {
+			gdev = ReadMacInt32(gdev);
+			D(bug("          pointer at %08lx\n", gdev));
+			uint32 pmap = ReadMacInt32(gdev + 0x16);	// gdPMap
+			D(bug("    PixMap handle at %08lx\n", pmap));
+			pmap = ReadMacInt32(pmap);
+			D(bug("          pointer at %08lx\n", pmap));
+			WriteMacInt32(pmap, mac_frame_base);		// baseAddr
+		}
+		gdev = ReadMacInt32(0xcc8);			// TheGDevice
+		D(bug("TheGDevice handle at %08lx\n", gdev));
 	}
 }