Updated SameBoy to pass Mooneye-GB’s lcdon_timing test (on a DMG), as well as refined related CBG behaviors.

Core/display.c

@@ -266,6 +266,9 @@ static void update_display_state(GB_gameboy_t *gb, uint8_t cycles)
             gb->hdma_steps_left = 0xff;
         }
         
+        gb->oam_blocked = false;
+        gb->vram_blocked = false;
+        
         /* Keep sending vblanks to user even if the screen is off */
         gb->display_cycles += cycles;
         if (gb->display_cycles >= LCDC_PERIOD) {
@@ -281,14 +284,25 @@ static void update_display_state(GB_gameboy_t *gb, uint8_t cycles)
     
     uint8_t atomic_increase = gb->cgb_double_speed? 2 : 4;
     uint8_t stat_delay = gb->cgb_double_speed? 2 : (gb->cgb_mode? 0 : 4);
-    /* This is correct for DMG. Is it correct for the 3 CGB modes (DMG/single/double)?*/
+    /* Todo: This is correct for DMG. Is it correct for the 3 CGB modes (DMG/single/double)?*/
     uint8_t scx_delay = ((gb->effective_scx & 7) + atomic_increase - 1) & ~(atomic_increase - 1);
+    /* Todo: These are correct for DMG, DMG-mode CGB, and single speed CGB. Is is correct for double speed CGB? */
+    uint8_t oam_blocking_rush = gb->cgb_double_speed? 2 : 4;
+    uint8_t vram_blocking_rush = gb->is_cgb? 0 : 4;
     
     for (; cycles; cycles -= atomic_increase) {
+        
         gb->display_cycles += atomic_increase;
+        /* The very first line is 2 (4 from the CPU's perseptive) clocks shorter when the LCD turns on.
+           Todo: Verify on the 3 CGB modes, especially double speed mode. */
+        if (gb->first_scanline && gb->display_cycles >= LINE_LENGTH - atomic_increase) {
+            gb->first_scanline = false;
+            gb->display_cycles += atomic_increase;
+        }
         bool should_compare_ly = true;
         uint8_t ly_for_comparison = gb->io_registers[GB_IO_LY] = gb->display_cycles / LINE_LENGTH;
         
+        
         /* Handle cycle completion. STAT's initial value depends on model and mode */
         if (gb->display_cycles == LCDC_PERIOD) {
             /* VBlank! */
@@ -304,6 +318,10 @@ static void update_display_state(GB_gameboy_t *gb, uint8_t cycles)
             }
             ly_for_comparison = gb->io_registers[GB_IO_LY] = 0;
             
+            /* Todo: verify timing */
+            gb->oam_blocked = true;
+            gb->vram_blocked = false;
+            
             /* Reset window rendering state */
             gb->current_window_line = 0xFF;
         }
@@ -333,9 +351,42 @@ static void update_display_state(GB_gameboy_t *gb, uint8_t cycles)
             }
         }
         
+        /* Handle line 0 right after turning the LCD on  */
+        else if (gb->first_scanline) {
+            /* OAM and VRAM blocking is not delayed in the very first scanline */
+            if (gb->display_cycles == atomic_increase) {
+                gb->io_registers[GB_IO_STAT] &= ~3;
+                gb->oam_blocked = false;
+                gb->vram_blocked = false;
+            }
+            else if (gb->display_cycles == MODE2_LENGTH) {
+                gb->io_registers[GB_IO_STAT] &= ~3;
+                gb->io_registers[GB_IO_STAT] |= 3;
+                gb->oam_blocked = true;
+                gb->vram_blocked = true;
+            }
+            else if (gb->display_cycles == MODE2_LENGTH + MODE3_LENGTH) {
+                gb->io_registers[GB_IO_STAT] &= ~3;
+                gb->oam_blocked = false;
+                gb->vram_blocked = false;
+            }
+        }
+        
         /* Handle STAT changes for lines 0-143 */
-        else if (gb->display_cycles < LINES * LINE_LENGTH ) {
+        else if (gb->display_cycles < LINES * LINE_LENGTH) {
             unsigned position_in_line = gb->display_cycles % LINE_LENGTH;
+            
+            /* Handle OAM and VRAM blocking */
+            if (position_in_line == stat_delay - oam_blocking_rush ||
+                 // In case stat_delay is 0
+                (position_in_line == LINE_LENGTH + stat_delay - oam_blocking_rush && gb->io_registers[GB_IO_LY] != 143)) {
+                gb->oam_blocked = true;
+            }
+            else if (position_in_line == MODE2_LENGTH + stat_delay - vram_blocking_rush) {
+                gb->vram_blocked = true;
+            }
+            
+            /* Handle everything else */
             if (position_in_line == stat_delay) {
                 gb->io_registers[GB_IO_STAT] &= ~3;
                 gb->io_registers[GB_IO_STAT] |= 2;
@@ -360,6 +411,8 @@ static void update_display_state(GB_gameboy_t *gb, uint8_t cycles)
             }
             else if (position_in_line == MODE2_LENGTH + MODE3_LENGTH + stat_delay + scx_delay) {
                 gb->io_registers[GB_IO_STAT] &= ~3;
+                gb->vram_blocked = false;
+                gb->oam_blocked = false;
                 if (gb->hdma_on_hblank) {
                     gb->hdma_on = true;
                     gb->hdma_cycles = 0;
@@ -466,9 +519,6 @@ static void update_display_state(GB_gameboy_t *gb, uint8_t cycles)
                 gb->stat_interrupt_line = true;
             }
         }
-        
-        if (!gb->stat_interrupt_line) {
-        }
     }
     
     if (gb->stat_interrupt_line && !previous_stat_interrupt_line) {

Core/gb.h

@@ -375,6 +375,9 @@ struct GB_gameboy_internal_s {
             GB_FRAMESKIP_FIRST_FRAME_SKIPPED, // This state is 'skipped' when emulating a DMG
             GB_FRAMESKIP_SECOND_FRAME_RENDERED,
         } frame_skip_state;
+        bool first_scanline; // The very first scan line after turning the LCD behaves differently.
+        bool oam_blocked;
+        bool vram_blocked;
     );
 
     /* Unsaved data. This includes all pointers, as well as everything that shouldn't be on a save state */

Core/memory.c

@@ -60,7 +60,7 @@ static uint8_t read_mbc_rom(GB_gameboy_t *gb, uint16_t addr)
 
 static uint8_t read_vram(GB_gameboy_t *gb, uint16_t addr)
 {
-    if ((gb->io_registers[GB_IO_STAT] & 0x3) == 3) {
+    if (gb->vram_blocked) {
         return 0xFF;
     }
     return gb->vram[(addr & 0x1FFF) + (uint16_t) gb->cgb_vram_bank * 0x2000];
@@ -115,7 +115,7 @@ static uint8_t read_high_memory(GB_gameboy_t *gb, uint16_t addr)
     }
 
     if (addr < 0xFEA0) {
-        if ((gb->io_registers[GB_IO_STAT] & 0x3) >= 2 || (gb->dma_steps_left && (gb->dma_cycles > 0 || gb->is_dma_restarting))) {
+        if (gb->oam_blocked || (gb->dma_steps_left && (gb->dma_cycles > 0 || gb->is_dma_restarting))) {
             return 0xFF;
         }
         return gb->oam[addr & 0xFF];
@@ -338,7 +338,7 @@ static void write_mbc(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
 
 static void write_vram(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
 {
-    if ((gb->io_registers[GB_IO_STAT] & 0x3) == 3) {
+    if (gb->vram_blocked) {
         //GB_log(gb, "Wrote %02x to %04x (VRAM) during mode 3\n", value, addr);
         return;
     }
@@ -385,7 +385,7 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
     }
 
     if (addr < 0xFEA0) {
-        if ((gb->io_registers[GB_IO_STAT] & 0x3) >= 2 || (gb->dma_steps_left && (gb->dma_cycles > 0 || gb->is_dma_restarting))) {
+        if (gb->oam_blocked|| (gb->dma_steps_left && (gb->dma_cycles > 0 || gb->is_dma_restarting))) {
             return;
         }
         gb->oam[addr & 0xFF] = value;
@@ -438,7 +438,8 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 if ((value & 0x80) && !(gb->io_registers[GB_IO_LCDC] & 0x80)) {
                     /* It appears that there's a slight delay after enabling the screen? */
                     /* Todo: verify this. */
-                    gb->display_cycles = gb->cgb_double_speed? -2 : -4;
+                    gb->display_cycles = 0;
+                    gb->first_scanline = true;
                     if (gb->frame_skip_state == GB_FRAMESKIP_SECOND_FRAME_RENDERED) {
                         gb->frame_skip_state = GB_FRAMESKIP_LCD_TURNED_ON;
                     }