Index: emu/vid.c
===================================================================
--- emu/vid.c	(revision d021bbbcf6b7bcd36aead194c11a6c7c0148cf2d)
+++ emu/vid.c	(revision fffab34326fd1cd2d7a46707916f38e2d7db16a3)
@@ -50,4 +50,5 @@
 #define OD0_CB 0x0800
 
+#define AT_UND 0x0100
 #define AT_CG 0x8000
 
@@ -125,6 +126,14 @@
 				int32_t fg = (at & 0x00f0) >> 4;
 
-				uint16_t *cg = (at & AT_CG) != 0 ? cg0 : cg1;
-				int32_t bits = cg[256 * line + ch];
+				int32_t bits;
+
+				if ((at & AT_UND) != 0 && line == 1) {
+					bits = 0xff;
+				}
+				else {
+					uint16_t *cg = (at & AT_CG) != 0 ? cg0 : cg1;
+					bits = cg[256 * line + ch];
+				}
+
 				int32_t mask = 0x01;