From c05c3c2abd42e9faf6404700cb7520fd7882b95a Mon Sep 17 00:00:00 2001
From: Lior Halphon <LIJI32@gmail.com>
Date: Fri, 8 Oct 2021 19:29:43 +0300
Subject: [PATCH] Improved accuracy of mid-line SCX writes

---
 Core/display.c    | 14 +++++---------
 Core/gb.h         |  2 +-
 Core/save_state.c |  1 -
 Core/sm83_cpu.c   |  3 ++-
 4 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/Core/display.c b/Core/display.c
index 2956cbc..ef1dde6 100644
--- a/Core/display.c
+++ b/Core/display.c
@@ -632,7 +632,7 @@ static void advance_fetcher_state_machine(GB_gameboy_t *gb)
         GB_FETCHER_SLEEP,
     } fetcher_step_t;
     
-    fetcher_step_t fetcher_state_machine [8] = {
+    static const fetcher_step_t fetcher_state_machine [8] = {
         GB_FETCHER_SLEEP,
         GB_FETCHER_GET_TILE,
         GB_FETCHER_SLEEP,
@@ -666,7 +666,10 @@ static void advance_fetcher_state_machine(GB_gameboy_t *gb)
                 x = gb->window_tile_x;
             }
             else {
-                x = ((gb->io_registers[GB_IO_SCX] / 8) + gb->fetcher_x) & 0x1F;
+                /* TODO: There is some CGB timing error around here.
+                   Adjusting SCX by 7 or less shouldn't have an effect on a CGB,
+                   but SameBoy is affected by a change of both 7 and 6 (but not less). */
+                x = ((gb->io_registers[GB_IO_SCX] + gb->position_in_line + 8) / 8) & 0x1F;
             }
             if (gb->model > GB_MODEL_CGB_C) {
                 /* This value is cached on the CGB-D and newer, so it cannot be used to mix tiles together */
@@ -777,12 +780,6 @@ static void advance_fetcher_state_machine(GB_gameboy_t *gb)
             
         // fallthrough
         case GB_FETCHER_PUSH: {
-            if (gb->fetcher_state == 6) {
-                /* The background map index increase at this specific point. If this state is not reached,
-                   it will simply not increase. */
-                gb->fetcher_x++;
-                gb->fetcher_x &= 0x1f;
-            }
             if (gb->fetcher_state < 7) {
                 gb->fetcher_state++;
             }
@@ -1049,7 +1046,6 @@ void GB_display_run(GB_gameboy_t *gb, uint8_t cycles)
             gb->position_in_line = - (gb->io_registers[GB_IO_SCX] & 7) - 8;
             gb->lcd_x = 0;
           
-            gb->fetcher_x = 0;
             gb->extra_penalty_for_sprite_at_0 = (gb->io_registers[GB_IO_SCX] & 7);
 
             
diff --git a/Core/gb.h b/Core/gb.h
index 80465df..f0b3b16 100644
--- a/Core/gb.h
+++ b/Core/gb.h
@@ -581,7 +581,7 @@ struct GB_gameboy_internal_s {
         uint8_t current_line;
         uint16_t ly_for_comparison;
         GB_fifo_t bg_fifo, oam_fifo;
-        uint8_t fetcher_x;
+        GB_PADDING(uint8_t, fetcher_x);
         uint8_t fetcher_y;
         uint16_t cycles_for_line;
         uint8_t current_tile;
diff --git a/Core/save_state.c b/Core/save_state.c
index 04ed4af..7461598 100644
--- a/Core/save_state.c
+++ b/Core/save_state.c
@@ -348,7 +348,6 @@ static void sanitize_state(GB_gameboy_t *gb)
     gb->oam_fifo.read_end &= 0xF;
     gb->oam_fifo.write_end &= 0xF;
     gb->object_low_line_address &= gb->vram_size & ~1;
-    gb->fetcher_x &= 0x1f;
     if (gb->lcd_x > gb->position_in_line) {
         gb->lcd_x = gb->position_in_line;
     }
diff --git a/Core/sm83_cpu.c b/Core/sm83_cpu.c
index 09499bc..1ddea1d 100644
--- a/Core/sm83_cpu.c
+++ b/Core/sm83_cpu.c
@@ -35,7 +35,8 @@ static const GB_conflict_t cgb_conflict_map[0x80] = {
     [GB_IO_OBP0] = GB_CONFLICT_PALETTE_CGB,
     [GB_IO_OBP1] = GB_CONFLICT_PALETTE_CGB,
     [GB_IO_NR10] = GB_CONFLICT_NR10,
-    
+    [GB_IO_SCX] = GB_CONFLICT_WRITE_CPU, // TODO: Similar to BGP, there's some time travelling involved
+
     /* Todo: most values not verified, and probably differ between revisions */
 };