From 4f11821ee68740ec1fde29a64b826d27279caa1c Mon Sep 17 00:00:00 2001
From: TomW <TomW@localhost>
Date: Sun, 6 Jul 2014 21:40:28 +0100
Subject: [PATCH] Port 0x3da diagnostic bits now held static during blanking,
 fixes DOS 6.2/V hang.

---
 src/vid_svga.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/vid_svga.c b/src/vid_svga.c
index 36034f7..4e1f676 100644
--- a/src/vid_svga.c
+++ b/src/vid_svga.c
@@ -229,7 +229,10 @@ uint8_t svga_in(uint16_t addr, void *p)
                 return svga->gdcreg[svga->gdcaddr & 0xf];
                 case 0x3DA:
                 svga->attrff = 0;
-                svga->cgastat ^= 0x30;
+                if (svga->cgastat & 0x01)
+                        svga->cgastat &= ~0x30;
+                else
+                        svga->cgastat ^= 0x30;
                 return svga->cgastat;
         }
 //        printf("Bad EGA read %04X %04X:%04X\n",addr,cs>>4,pc);