From 6532aef089dc7a434b982eb59248682e5a714db8 Mon Sep 17 00:00:00 2001
From: Lior Halphon <LIJI32@gmail.com>
Date: Sat, 26 May 2018 18:06:40 +0300
Subject: [PATCH] Correct emulation of the DMG stat write bug

---
 Core/memory.c  |  9 ---------
 Core/z80_cpu.c | 12 +++++++++++-
 2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/Core/memory.c b/Core/memory.c
index 1cb2eb9..f280c94 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -664,15 +664,6 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 return;
 
             case GB_IO_STAT:
-                /* A DMG bug: http://www.devrs.com/gb/files/faqs.html#GBBugs */
-                if (!gb->is_cgb && /* Only occurs on DMGs */
-                    (gb->io_registers[GB_IO_LCDC] & 0x80) && /* LCD must be on */
-                    !gb->stat_interrupt_line && /* The interrupt line must be off */
-                    gb->display_state != 2 && /* State 2 is line 0's faux Mode 2 */
-                    !gb->oam_read_blocked /* OAM must be readable (Modes 0 and 1) */
-                    ) {
-                    gb->io_registers[GB_IO_IF] |= 2;
-                }
                 /* Delete previous R/W bits */
                 gb->io_registers[GB_IO_STAT] &= 7;
                 /* Set them by value */
diff --git a/Core/z80_cpu.c b/Core/z80_cpu.c
index 5119ff7..90c50f5 100644
--- a/Core/z80_cpu.c
+++ b/Core/z80_cpu.c
@@ -17,6 +17,7 @@ typedef enum {
     GB_CONFLICT_WRITE_CPU,
     /* Register specific values */
     GB_CONFLICT_STAT_CGB,
+    GB_CONFLICT_STAT_DMG,
 } GB_conflict_t;
 
 /* Todo: How does double speed mode affect these? */
@@ -33,7 +34,7 @@ static const GB_conflict_t dmg_conflict_map[0x80] = {
     [GB_IO_LYC] = GB_CONFLICT_READ_OLD,
     [GB_IO_LCDC] = GB_CONFLICT_READ_NEW,
     [GB_IO_SCY] = GB_CONFLICT_READ_NEW,
-    [GB_IO_STAT] = GB_CONFLICT_READ_NEW,
+    [GB_IO_STAT] = GB_CONFLICT_STAT_DMG,
  
     /* Todo: these are GB_CONFLICT_READ_NEW on MGB/SGB2 */
     [GB_IO_BGP] = GB_CONFLICT_READ_OR,
@@ -112,6 +113,15 @@ static void cycle_write(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
             GB_write_memory(gb, addr, value);
             gb->pending_cycles = 3;
         }
+        
+        /* The DMG STAT-write bug is basically the STAT register being read as FF for a single T-cycle*/
+        case GB_CONFLICT_STAT_DMG:
+            GB_advance_cycles(gb, gb->pending_cycles);
+            GB_write_memory(gb, addr, 0xFF);
+            GB_advance_cycles(gb, 1);
+            GB_write_memory(gb, addr, value);
+            gb->pending_cycles = 3;
+            return;
     }
 }