Initial emulation of "SCX banging" to prolong mode 3

Core/display.c

@@ -433,6 +433,7 @@ void GB_STAT_update(GB_gameboy_t *gb)
 
 void GB_lcd_off(GB_gameboy_t *gb)
 {
+    gb->cycles_for_line = 0;
     gb->display_state = 0;
     gb->display_cycles = 0;
     /* When the LCD is disabled, state is constant */
@@ -566,6 +567,17 @@ static void render_pixel_if_possible(GB_gameboy_t *gb)
             bg_priority |= oam_fifo_item->bg_priority;
         }
     }
+    
+    // (gb->position_in_line + 16 < 8) is (gb->position_in_line < -8) in unsigned logic
+    if (((uint8_t)(gb->position_in_line + 16) < 8)) {
+        if ((gb->position_in_line & 7) == (gb->io_registers[GB_IO_SCX] & 7)) {
+            gb->position_in_line = -8;
+        }
+        else if (gb->position_in_line == (uint8_t) -9) {
+            gb->position_in_line = -16;
+            return;
+        }
+    }
 
     /* Drop pixels for scrollings */
     if (gb->position_in_line >= 160 || (gb->disable_rendering && !gb->sgb)) {
@@ -771,6 +783,9 @@ static void advance_fetcher_state_machine(GB_gameboy_t *gb, unsigned *cycles)
             if (gb->wx_triggered) {
                 x = gb->window_tile_x;
             }
+            else if ((uint8_t)(gb->position_in_line + 16) < 8) {
+                x = gb->io_registers[GB_IO_SCX] >> 3;
+            }
             else {
                 /* TODO: There is some CGB timing error around here.
                    Adjusting SCX by 7 or less shouldn't have an effect on a CGB,
@@ -1295,6 +1310,13 @@ static inline uint16_t mode3_batching_length(GB_gameboy_t *gb)
  */
 void GB_display_run(GB_gameboy_t *gb, unsigned cycles, bool force)
 {
+    if (unlikely((gb->io_registers[GB_IO_LCDC] & 0x80) && (signed)(gb->cycles_for_line * 2 + cycles + gb->display_cycles) > LINE_LENGTH * 2)) {
+        unsigned first_batch = (LINE_LENGTH * 2 - gb->cycles_for_line * 2 + gb->display_cycles);
+        GB_display_run(gb, first_batch, force);
+        cycles -= first_batch;
+        gb->display_state = 9;
+        gb->display_cycles = 0;
+    }
     gb->cycles_since_vblank_callback += cycles / 2;
 
     /* The PPU does not advance while in STOP mode on the DMG */
@@ -1314,7 +1336,7 @@ void GB_display_run(GB_gameboy_t *gb, unsigned cycles, bool force)
         GB_STATE(gb, display, 6);
         GB_STATE(gb, display, 7);
         GB_STATE(gb, display, 8);
-        // GB_STATE(gb, display, 9);
+        GB_STATE(gb, display, 9);
         GB_STATE(gb, display, 10);
         GB_STATE(gb, display, 11);
         GB_STATE(gb, display, 12);
@@ -1346,6 +1368,7 @@ void GB_display_run(GB_gameboy_t *gb, unsigned cycles, bool force)
         GB_STATE(gb, display, 40);
         GB_STATE(gb, display, 41);
         GB_STATE(gb, display, 42);
+        GB_STATE(gb, display, 43);
     }
     
     if (!(gb->io_registers[GB_IO_LCDC] & 0x80)) {
@@ -1414,7 +1437,33 @@ void GB_display_run(GB_gameboy_t *gb, unsigned cycles, bool force)
     gb->wx_triggered = false;
     gb->wx166_glitch = false;
     goto mode_3_start;
-    
+
+    // Mode 3 abort, state 9
+    display9: {
+        // TODO: Timing of things in this scenario is almost completely untested
+        if (gb->current_line < 144 && !GB_is_sgb(gb) && !gb->disable_rendering) {
+            GB_log(gb, "The ROM is preventing line %d from fully rendering, this could damage a real device's LCD display.\n", gb->current_line);
+            uint32_t *dest = NULL;
+            if (gb->border_mode != GB_BORDER_ALWAYS) {
+                dest = gb->screen + gb->lcd_x + gb->current_line * WIDTH;
+            }
+            else {
+                dest = gb->screen + gb->lcd_x + gb->current_line * BORDERED_WIDTH + (BORDERED_WIDTH - WIDTH) / 2 + (BORDERED_HEIGHT - LINES) / 2 * BORDERED_WIDTH;
+            }
+            uint32_t color = GB_is_cgb(gb)? GB_convert_rgb15(gb, 0x7FFF, false) : gb->background_palettes_rgb[4];
+            while (gb->lcd_x < 160) {
+                *(dest++) = color;
+                gb->lcd_x++;
+            }
+        }
+        gb->current_line++;
+        gb->vram_read_blocked = false;
+        gb->vram_write_blocked = false;
+        gb->wx_triggered = false;
+        gb->wx166_glitch = false;
+        gb->cycles_for_line = 0;
+    }
+        
     while (true) {
         /* Lines 0 - 143 */
         gb->window_y = -1;
@@ -1514,7 +1563,7 @@ void GB_display_run(GB_gameboy_t *gb, unsigned cycles, bool force)
             /* Fill the FIFO with 8 pixels of "junk", it's going to be dropped anyway. */
             fifo_push_bg_row(&gb->bg_fifo, 0, 0, 0, false, false);
             /* Todo: find out actual access time of SCX */
-            gb->position_in_line = - (gb->io_registers[GB_IO_SCX] & 7) - 8;
+            gb->position_in_line = -16;
             gb->lcd_x = 0;
           
             gb->extra_penalty_for_object_at_0 = MIN((gb->io_registers[GB_IO_SCX] & 7), 5);
@@ -1785,7 +1834,17 @@ skip_slow_mode_3:
             GB_SLEEP(gb, display, 36, 2);
             gb->cgb_palettes_blocked = false;
             
-            GB_SLEEP(gb, display, 11, LINE_LENGTH - gb->cycles_for_line - 2);
+            if (gb->cycles_for_line > LINE_LENGTH - 2) {
+                gb->cycles_for_line = 0;
+                GB_SLEEP(gb, display, 43, LINE_LENGTH - gb->cycles_for_line);
+                goto display9;
+            }
+            
+            {
+                uint16_t cycles_for_line =  gb->cycles_for_line;
+                gb->cycles_for_line = 0;
+                GB_SLEEP(gb, display, 11, LINE_LENGTH - cycles_for_line - 2);
+            }
             /*
              TODO: Verify double speed timing
              TODO: Timing differs on a DMG
@@ -1794,6 +1853,7 @@ skip_slow_mode_3:
                 (gb->io_registers[GB_IO_WY] == gb->current_line)) {
                 gb->wy_triggered = true;
             }
+            gb->cycles_for_line = 0;
             GB_SLEEP(gb, display, 31, 2);
             if (gb->current_line != LINES - 1) {
                 gb->mode_for_interrupt = 2;

Core/memory.c

@@ -1432,6 +1432,7 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                     gb->display_cycles = 0;
                     gb->display_state = 0;
                     gb->double_speed_alignment = 0;
+                    gb->cycles_for_line = 0;
                     if (GB_is_sgb(gb)) {
                         gb->frame_skip_state = GB_FRAMESKIP_SECOND_FRAME_RENDERED;
                     }