From fbfb6849c4acbf855400bd2369ed8463e98e9318 Mon Sep 17 00:00:00 2001
From: OBattler <oubattler@gmail.com>
Date: Wed, 7 Aug 2024 05:08:41 +0200
Subject: [PATCH] Compaq EGA: Implement C&T behavior where reads in mode 0
 return 0xFF if bit 2 of GDC register 4 is set, fixes video memory size
 detection, fixes #4571.

---
 src/video/vid_ega.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/video/vid_ega.c b/src/video/vid_ega.c
index 8f995117c..9a0ebfc50 100644
--- a/src/video/vid_ega.c
+++ b/src/video/vid_ega.c
@@ -1278,6 +1278,10 @@ ega_read(uint32_t addr, void *priv)
         temp4 &= (ega->colournocare & 8) ? 0xff : 0;
         return ~(temp | temp2 | temp3 | temp4);
     }
+
+    if ((ega_type == 2) && (ega->gdcreg[4] & 0x04))
+        return 0xff;
+
     return ega->vram[addr | readplane];
 }