diff --git a/src/vid_et4000.c b/src/vid_et4000.c
index dbeb681..882da42 100644
--- a/src/vid_et4000.c
+++ b/src/vid_et4000.c
@@ -21,6 +21,18 @@ 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;
@@ -32,7 +44,7 @@ void et4000_out(uint16_t addr, uint8_t val, void *p)
                 addr ^= 0x60;
 
 //        pclog("ET4000 out %04X %02X\n", addr, val);
-                
+
         switch (addr)
         {
                 case 0x3C6: case 0x3C7: case 0x3C8: case 0x3C9:
@@ -51,6 +63,7 @@ void et4000_out(uint16_t addr, uint8_t val, void *p)
                 case 0x3D5:
                 if (svga->crtcreg <= 7 && svga->crtc[0x11] & 0x80) return;
                 old = svga->crtc[svga->crtcreg];
+                val &= crtc_mask[svga->crtcreg];
                 svga->crtc[svga->crtcreg] = val;
                 if (old != val)
                 {