From 6a8db89ae5f6ed01c7f977c8c7037a0ff6fa1325 Mon Sep 17 00:00:00 2001
From: Lior Halphon <LIJI32@gmail.com>
Date: Sat, 5 Feb 2022 14:52:09 +0200
Subject: [PATCH] Emulate PPU OAM reads while both DMA and GDMA are active

---
 Core/display.c | 3 +++
 Core/memory.c  | 4 ++--
 Core/memory.h  | 1 +
 3 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/Core/display.c b/Core/display.c
index 5057fcb..09991d3 100644
--- a/Core/display.c
+++ b/Core/display.c
@@ -469,6 +469,9 @@ static inline uint8_t oam_read(GB_gameboy_t *gb, uint8_t addr)
         return 0xFF;
     }
     if (unlikely(gb->dma_current_dest <= 0xa0 && gb->dma_current_dest > 0)) { // TODO: what happens in the last and first M cycles?
+        if (gb->hdma_in_progress) {
+            return GB_read_oam(gb, (gb->hdma_current_src & ~1) | (addr & 1));
+        }
         return gb->oam[((gb->dma_current_dest - 1 + (gb->halted || gb->stopped)) & ~1) | (addr & 1)];
     }
     return gb->oam[addr];
diff --git a/Core/memory.c b/Core/memory.c
index 12e9bd3..0ad9c84 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -473,7 +473,7 @@ static inline void sync_ppu_if_needed(GB_gameboy_t *gb, uint8_t register_accesse
     }
 }
 
-static uint8_t read_oam(GB_gameboy_t *gb, uint8_t addr)
+internal uint8_t GB_read_oam(GB_gameboy_t *gb, uint8_t addr)
 {
     if (addr < 0xa0) {
         return gb->oam[addr];
@@ -588,7 +588,7 @@ static uint8_t read_high_memory(GB_gameboy_t *gb, uint16_t addr)
             return 0xff;
         }
         
-        return read_oam(gb, addr);
+        return GB_read_oam(gb, addr);
     }
 
     if (addr < 0xFF80) {
diff --git a/Core/memory.h b/Core/memory.h
index 7a78283..9a5824d 100644
--- a/Core/memory.h
+++ b/Core/memory.h
@@ -16,6 +16,7 @@ internal void GB_dma_run(GB_gameboy_t *gb);
 internal bool GB_is_dma_active(GB_gameboy_t *gb);
 internal void GB_hdma_run(GB_gameboy_t *gb);
 internal void GB_trigger_oam_bug(GB_gameboy_t *gb, uint16_t address);
+internal uint8_t GB_read_oam(GB_gameboy_t *gb, uint8_t addr);
 #endif
 
 #endif /* memory_h */