diff --git a/src/video/vid_et4000.c b/src/video/vid_et4000.c
index d3aa90572..6e76d2ecb 100644
--- a/src/video/vid_et4000.c
+++ b/src/video/vid_et4000.c
@@ -8,13 +8,13 @@
  *
  *		Emulation of the Tseng Labs ET4000.
  *
- * Version:	@(#)vid_et4000.c	1.0.3	2017/11/04
+ * Version:	@(#)vid_et4000.c	1.0.4	2018/02/25
  *
  * Authors:	Sarah Walker, <http://pcem-emulator.co.uk/>
  *		Miran Grca, <mgrca8@gmail.com>
  *
- *		Copyright 2008-2017 Sarah Walker.
- *		Copyright 2016,2017 Miran Grca.
+ *		Copyright 2008-2018 Sarah Walker.
+ *		Copyright 2016-2018 Miran Grca.
  */
 #include <stdio.h>
 #include <stdint.h>
@@ -45,6 +45,18 @@ typedef struct et4000_t
         uint8_t banking;
 } et4000_t;
 
+static uint8_t crtc_mask[0x40] =
+{
+        0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+        0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+        0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+        0xff, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+        0xff, 0xff, 0xff, 0x0f, 0xff, 0xff, 0xff, 0xff,
+        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
+};
+
 void et4000_out(uint16_t addr, uint8_t val, void *p)
 {
         et4000_t *et4000 = (et4000_t *)p;
@@ -75,6 +87,7 @@ void et4000_out(uint16_t addr, uint8_t val, void *p)
                 if ((svga->crtcreg == 7) && (svga->crtc[0x11] & 0x80))
                         val = (svga->crtc[7] & ~0x10) | (val & 0x10);
                 old = svga->crtc[svga->crtcreg];
+                val &= crtc_mask[svga->crtcreg];
                 svga->crtc[svga->crtcreg] = val;
                 if (old != val)
                 {