Correct emulation of FF6C (Turns out it controls object priority)

Core/display.c

@@ -939,7 +939,7 @@ void GB_display_run(GB_gameboy_t *gb, uint8_t cycles)
                                             gb->vram[line_address + 1],
                                             palette,
                                             object->flags & 0x80,
-                                            gb->cgb_mode? gb->visible_objs[gb->n_visible_objs - 1] : 0,
+                                            gb->object_priority == GB_OBJECT_PRIORITY_INDEX? gb->visible_objs[gb->n_visible_objs - 1] : 0,
                                             object->flags & 0x20);
                     
                     gb->n_visible_objs--;

Core/display.h

@@ -11,6 +11,13 @@ void GB_palette_changed(GB_gameboy_t *gb, bool background_palette, uint8_t index
 void GB_window_related_write(GB_gameboy_t *gb, uint8_t addr, uint8_t value);
 void GB_STAT_update(GB_gameboy_t *gb);
 void GB_lcd_off(GB_gameboy_t *gb);
+
+enum {
+  GB_OBJECT_PRIORITY_UNDEFINED, // For save state compatibility
+  GB_OBJECT_PRIORITY_X,
+  GB_OBJECT_PRIORITY_INDEX,
+};
+
 #endif
 
 typedef enum {

Core/gb.c

@@ -1008,11 +1008,13 @@ void GB_reset(GB_gameboy_t *gb)
         gb->vram_size = 0x2000 * 2;
         memset(gb->vram, 0, gb->vram_size);
         gb->cgb_mode = true;
+        gb->object_priority = GB_OBJECT_PRIORITY_INDEX;
     }
     else {
         gb->ram_size = 0x2000;
         gb->vram_size = 0x2000;
         memset(gb->vram, 0, gb->vram_size);
+        gb->object_priority = GB_OBJECT_PRIORITY_X;
         
         update_dmg_palette(gb);
     }

Core/gb.h

@@ -185,7 +185,7 @@ enum {
     // 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,
+    GB_IO_MODE = 0x4c,
 
     /* General CGB features */
     GB_IO_KEY1       = 0x4d, // CGB Mode Only - Prepare Speed Switch
@@ -212,9 +212,7 @@ enum {
     GB_IO_BGPD       = 0x69, // CGB Mode Only - Background Palette Data
     GB_IO_OBPI       = 0x6a, // CGB Mode Only - Sprite Palette Index
     GB_IO_OBPD       = 0x6b, // CGB Mode Only - Sprite Palette Data
-
+    GB_IO_OBJECT_PRIORITY = 0x6c, // Affects object priority (X based or index based)
-    // 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 */
 
@@ -516,6 +514,7 @@ struct GB_gameboy_internal_s {
         bool cgb_palettes_blocked;
         uint8_t current_lcd_line; // The LCD can go out of sync since the vsync signal is skipped in some cases.
         uint32_t cycles_in_stop_mode;
+        uint8_t object_priority;
     );
 
     /* Unsaved data. This includes all pointers, as well as everything that shouldn't be on a save state */

Core/memory.c

@@ -295,11 +295,11 @@ static uint8_t read_high_memory(GB_gameboy_t *gb, uint16_t 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:
+            case GB_IO_OBJECT_PRIORITY:
-                if (!gb->cgb_mode) {
+                if (!GB_is_cgb(gb)) {
                     return 0xFF;
                 }
-                return gb->io_registers[GB_IO_DMG_EMULATION_INDICATION] | 0xFE;
+                return gb->io_registers[GB_IO_OBJECT_PRIORITY] | 0xFE;
 
             case GB_IO_PCM_12:
                 if (!GB_is_cgb(gb)) return 0xFF;
@@ -656,13 +656,22 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
             case GB_IO_OBP1:
             case GB_IO_WY:
             case GB_IO_SB:
-            case GB_IO_DMG_EMULATION_INDICATION:
             case GB_IO_UNKNOWN2:
             case GB_IO_UNKNOWN3:
             case GB_IO_UNKNOWN4:
             case GB_IO_UNKNOWN5:
                 gb->io_registers[addr & 0xFF] = value;
                 return;
+            case GB_IO_OBJECT_PRIORITY:
+                if ((!gb->boot_rom_finished || (gb->io_registers[GB_IO_MODE] & 8)) && GB_is_cgb(gb)) {
+                    gb->io_registers[addr & 0xFF] = value;
+                    gb->object_priority = (value & 1) ? GB_OBJECT_PRIORITY_X : GB_OBJECT_PRIORITY_INDEX;
+                }
+                else if (gb->cgb_mode) {
+                    gb->io_registers[addr & 0xFF] = value;
+
+                }
+                return;
             case GB_IO_LYC:
                 
                 /* TODO: Probably completely wrong in double speed mode */
@@ -768,9 +777,10 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 gb->boot_rom_finished = true;
                 return;
 
-            case GB_IO_DMG_EMULATION:
+            case GB_IO_MODE:
                 if (GB_is_cgb(gb) && !gb->boot_rom_finished) {
                     gb->cgb_mode = !(value & 0xC); /* The real "contents" of this register aren't quite known yet. */
+                    gb->io_registers[GB_IO_MODE] = value;
                 }
                 return;
 

Core/save_state.c

@@ -266,6 +266,10 @@ int GB_load_state(GB_gameboy_t *gb, const char *path)
     gb->oam_fifo.read_end &= 0xF;
     gb->oam_fifo.write_end &= 0xF;
     
+    if (gb->object_priority == GB_OBJECT_PRIORITY_UNDEFINED) {
+        gb->object_priority = gb->cgb_mode? GB_OBJECT_PRIORITY_INDEX : GB_OBJECT_PRIORITY_X;
+    }
+    
 error:
     fclose(f);
     return errno;
@@ -371,6 +375,10 @@ int GB_load_state_from_buffer(GB_gameboy_t *gb, const uint8_t *buffer, size_t le
     gb->oam_fifo.read_end &= 0xF;
     gb->oam_fifo.write_end &= 0xF;
     
+    if (gb->object_priority == GB_OBJECT_PRIORITY_UNDEFINED) {
+        gb->object_priority = gb->cgb_mode? GB_OBJECT_PRIORITY_INDEX : GB_OBJECT_PRIORITY_X;
+    }
+    
     return 0;
 }
 

Misc/registers.sym

@@ -55,7 +55,7 @@
 00:FF69 IO_BGPD
 00:FF6A IO_OBPI
 00:FF6B IO_OBPD
-00:FF6C IO_DMG_EMULATION_INDICATION
+00:FF6C IO_OBJECT_PRIORITY
 00:FF70 IO_SVBK
 00:FF72 IO_UNKNOWN2
 00:FF73 IO_UNKNOWN3