diff --git a/Core/gb.h b/Core/gb.h
index ca810ba..f5061bb 100644
--- a/Core/gb.h
+++ b/Core/gb.h
@@ -88,8 +88,11 @@ enum {
     GB_IO_OBP1       = 0x49, // Object Palette 1 Data (R/W) - Non CGB Mode Only
     GB_IO_WY         = 0x4a, // Window Y Position (R/W)
     GB_IO_WX         = 0x4b, // Window X Position minus 7 (R/W)
-
-    /* Missing */
+    // Has some undocumented compatibility flags written at boot.
+    // Unfortunately it is not readable or writable after boot has finished, so research of this
+    // register is quite limited. The value written to this register, however, can be controlled
+    // in some cases.
+    GB_IO_DMG_EMULATION = 0x4c,
 
     /* General CGB features */
     GB_IO_KEY1       = 0x4d, // CGB Mode Only - Prepare Speed Switch
@@ -117,7 +120,8 @@ enum {
     GB_IO_OBPI       = 0x6a, // CGB Mode Only - Sprite Palette Index
     GB_IO_OBPD       = 0x6b, // CGB Mode Only - Sprite Palette Data
 
-    GB_IO_DMG_EMULATION   = 0x6c, // (FEh) Bit 0 (Read/Write) - CGB Mode Only
+    // 1 is written for DMG ROMs on a CGB. Does not appear to have an effect.
+    GB_IO_DMG_EMULATION_INDICATION   = 0x6c, // (FEh) Bit 0 (Read/Write)
 
     /* Missing */
 
diff --git a/Core/memory.c b/Core/memory.c
index 3e8c154..9ac5a1f 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -100,6 +100,8 @@ static unsigned char read_high_memory(GB_gameboy_t *gb, unsigned short addr)
                 return gb->io_registers[GB_IO_TAC] | 0xF8;
             case GB_IO_STAT:
                 return gb->io_registers[GB_IO_STAT] | 0x80;
+            case GB_IO_DMG_EMULATION_INDICATION:
+                return gb->io_registers[GB_IO_DMG_EMULATION_INDICATION] | 0xFE;
             case GB_IO_JOYP:
             case GB_IO_DIV:
             case GB_IO_TIMA:
@@ -155,7 +157,7 @@ static unsigned char read_high_memory(GB_gameboy_t *gb, unsigned short addr)
             }
 
             case GB_IO_KEY1:
-                if (!gb->is_cgb) {
+                if (!gb->cgb_mode) {
                     return 0xFF;
                 }
                 return (gb->io_registers[GB_IO_KEY1] & 0x7F) | (gb->cgb_double_speed? 0xFE : 0x7E);
@@ -349,6 +351,7 @@ static void write_high_memory(GB_gameboy_t *gb, unsigned short addr, unsigned ch
             case GB_IO_HDMA3:
             case GB_IO_HDMA4:
             case GB_IO_SB:
+            case GB_IO_DMG_EMULATION_INDICATION:
                 gb->io_registers[addr & 0xFF] = value;
                 return;
 
@@ -382,8 +385,9 @@ static void write_high_memory(GB_gameboy_t *gb, unsigned short addr, unsigned ch
                 return;
 
             case GB_IO_DMG_EMULATION:
-                // Todo: Can it be disabled? What about values other than 1?
-                gb->cgb_mode = false;
+                if (gb->is_cgb && !gb->bios_finished) {
+                    gb->cgb_mode = value != 4; /* The real "contents" of this register aren't quite known yet. */
+                }
                 return;
 
             case GB_IO_DMA: