From fd9f301339c23ba069e024fb28d8c36419e22de4 Mon Sep 17 00:00:00 2001
From: mariuszw1 <mariuszw1@users.noreply.github.com>
Date: Tue, 13 Oct 2009 19:14:14 +0000
Subject: [PATCH] Fixed TRIG3 register handling in GTIA for Atari XL/XE
 [Mariusz Wojcieszek]

(change for MESS).
---
 src/mame/video/atari.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/mame/video/atari.c b/src/mame/video/atari.c
index 63baecc5b8c..5e8438cc161 100644
--- a/src/mame/video/atari.c
+++ b/src/mame/video/atari.c
@@ -1512,9 +1512,12 @@ static void generic_atari_interrupt(running_machine *machine, void (*handle_keyb
 			gtia.r.but[i] &= (button_port >> i) & 1;
 		}
 
-		/* specify buttons not relevant to this Atari variant */
-		for (i = button_count; i < 4; i++)
-			gtia.r.but[i] = 1;
+		/* button registers for xl/xe */
+		if (button_count == 2)
+		{
+			gtia.r.but[2] = 1; /* not used on xl/xe */
+			gtia.r.but[3] = 0; /* 1 if external cartridge is inserted */
+		}
 
 		handle_keyboard(machine);