This patch fixes the video corruption in Nethack for DOS when
no video mode is specified in defaults.nh and the F3 key is
hit.

This patch provided by Patrick Clot

More bug fixes and patches can be found at 
http://www.csh.rit.edu/~topher/nethack

diff -aur orig-nethack-3.4.3/sys/msdos/pckeys.c nethack-3.4.3/sys/msdos/pckeys.c
--- orig-nethack-3.4.3/sys/msdos/pckeys.c	2003-12-07 18:39:14.000000000 -0500
+++ nethack-3.4.3/sys/msdos/pckeys.c	2004-01-05 23:00:16.000000000 -0500
@@ -38,10 +38,12 @@
 #  ifdef SIMULATE_CURSOR
     switch(scancode) {
 	case 0x3d:	/* F3 = toggle cursor type */
-		HideCursor();
-		cursor_type += 1;
-		if (cursor_type >= NUM_CURSOR_TYPES) cursor_type = 0;
-		DrawCursor();
+		if (iflags.usevga) {
+            HideCursor();
+            cursor_type += 1;
+            if (cursor_type >= NUM_CURSOR_TYPES) cursor_type = 0;
+            DrawCursor();
+        }
 		break;
 #  endif
 	case 0x74:	/* Control-right_arrow = scroll horizontal to right */
diff -aur orig-nethack-3.4.3/sys/msdos/video.c nethack-3.4.3/sys/msdos/video.c
--- orig-nethack-3.4.3/sys/msdos/video.c	2003-12-07 18:39:14.000000000 -0500
+++ nethack-3.4.3/sys/msdos/video.c	2004-01-05 23:01:31.000000000 -0500
@@ -930,15 +930,15 @@
 #  endif
 #  ifdef SCREEN_VGA
 		if (vga_detect()) {
-			iflags.hasvga  = 1;
+			iflags.hasvga = 1;
+            iflags.usevga = 1;
 		}
 #  endif
 	/*
 	 * Auto-detect Priorities (arbitrary for now):
 	 *	VGA
 	 */
-		if (iflags.hasvga)	{
-			iflags.usevga  = 1;
+		if (iflags.hasvga && iflags.usevga)      {
 			/* VGA depends on BIOS to enable function keys*/
 			iflags.BIOS = 1;
 			iflags.rawio = 1;