diff --git a/Core/gb.c b/Core/gb.c
index b38715c..99d6c2c 100644
--- a/Core/gb.c
+++ b/Core/gb.c
@@ -648,6 +648,7 @@ void GB_reset(GB_gameboy_t *gb)
         if (!gb->sgb) {
             gb->sgb = malloc(sizeof(*gb->sgb));
         }
+        memset(gb->sgb, 0, sizeof(*gb->sgb));
         gb->sgb->player_count = 1;
 
     }
diff --git a/Core/sgb.c b/Core/sgb.c
index f23abfa..c60afff 100644
--- a/Core/sgb.c
+++ b/Core/sgb.c
@@ -2,8 +2,16 @@
 
 enum {
     MLT_REQ = 0x11,
+    MASK_EN = 0x17,
 };
 
+typedef enum {
+    MASK_DISABLED,
+    MASK_FREEZE,
+    MASK_COLOR_3,
+    MASK_COLOR_0,
+} mask_mode_t;
+
 #define SGB_PACKET_SIZE 16
 
 static void command_ready(GB_gameboy_t *gb)
@@ -46,6 +54,9 @@ static void command_ready(GB_gameboy_t *gb)
             gb->sgb->player_count = (uint8_t[]){1, 2, 1, 4}[gb->sgb->command[1] & 3];
             gb->sgb->current_player = gb->sgb->player_count - 1;
             break;
+        case MASK_EN:
+            gb->sgb->mask_mode = gb->sgb->command[1] & 3;
+            break;
         default:
             GB_log(gb, "Unimplemented SGB command %x: ", gb->sgb->command[0] >> 3);
             for (unsigned i = 0; i < gb->sgb->command_write_index / 8; i++) {
@@ -144,8 +155,28 @@ void GB_sgb_render(GB_gameboy_t *gb)
         gb->screen[i] = border;
     }
     
+    switch ((mask_mode_t) gb->sgb->mask_mode) {
+        case MASK_DISABLED:
+            memcpy(gb->sgb->effective_screen_buffer,
+                   gb->sgb->screen_buffer,
+                   sizeof(gb->sgb->effective_screen_buffer));
+            break;
+        case MASK_FREEZE:
+            break;
+        
+        case MASK_COLOR_3:
+            memset(gb->sgb->effective_screen_buffer,
+                   3,
+                   sizeof(gb->sgb->effective_screen_buffer));
+            break;
+        case MASK_COLOR_0:
+            memset(gb->sgb->effective_screen_buffer,
+                   0,
+                   sizeof(gb->sgb->effective_screen_buffer));
+    }
+    
     uint32_t *output = &gb->screen[48 + 39 * 256];
-    uint8_t *input = gb->sgb->screen_buffer;
+    uint8_t *input = gb->sgb->effective_screen_buffer;
     for (unsigned y = 0; y < 144; y++) {
         for (unsigned x = 0; x < 160; x++) {
             *(output++) = colors[*(input++) & 3];
diff --git a/Core/sgb.h b/Core/sgb.h
index 5074737..bafde35 100644
--- a/Core/sgb.h
+++ b/Core/sgb.h
@@ -14,9 +14,14 @@ typedef struct {
     bool disable_commands;
     
     /* Screen buffer */
-    uint8_t screen_buffer[160 * 144];
+    uint8_t screen_buffer[160 * 144]; // Live image from the Game Boy
+    uint8_t effective_screen_buffer[160 * 144]; // Image actually rendered to the screen
+    
     /* Multiplayer Input */
     uint8_t player_count, current_player;
+    
+    /* Mask */
+    uint8_t mask_mode;
 } GB_sgb_t;
 
 void GB_sgb_write(GB_gameboy_t *gb, uint8_t value);