From f64da1864f5e3d710c0f9d1c531d1f59bf80b3d2 Mon Sep 17 00:00:00 2001
From: Lior Halphon <LIJI32@gmail.com>
Date: Sat, 9 Jun 2018 15:11:20 +0300
Subject: [PATCH] APU glitch: When turning the APU on while DIV's bit 4 (or 5
 in double speed mode), the first DIV/APU event is skipped.

---
 Core/apu.c    | 9 +++++++++
 Core/apu.h    | 2 ++
 Core/gb.h     | 2 +-
 Core/timing.c | 2 ++
 4 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/Core/apu.c b/Core/apu.c
index 703355a..cb462d7 100644
--- a/Core/apu.c
+++ b/Core/apu.c
@@ -181,6 +181,10 @@ static void nrx2_glitch(uint8_t *volume, uint8_t value, uint8_t old_value)
 void GB_apu_div_event(GB_gameboy_t *gb)
 {
     if (!gb->apu.global_enable) return;
+    if (gb->apu.skip_div_event) {
+        gb->apu.skip_div_event = false;
+        return;
+    }
     gb->apu.div_divider++;
 
     if ((gb->apu.div_divider & 7) == 0) {
@@ -426,6 +430,11 @@ void GB_apu_init(GB_gameboy_t *gb)
 {
     memset(&gb->apu, 0, sizeof(gb->apu));
     gb->apu.lf_div = 1;
+    /* APU glitch: When turning the APU on while DIV's bit 4 (or 5 in double speed mode), the
+       first DIV/APU event is skipped. */
+    if (gb->div_counter & (gb->cgb_double_speed? 0x2000 : 0x1000)) {
+        gb->apu.skip_div_event = true;
+    }
 }
 
 uint8_t GB_apu_read(GB_gameboy_t *gb, uint8_t reg)
diff --git a/Core/apu.h b/Core/apu.h
index f75d25e..59d0e36 100644
--- a/Core/apu.h
+++ b/Core/apu.h
@@ -108,6 +108,8 @@ typedef struct
         
     } noise_channel;
     
+    bool skip_div_event;
+    
 } GB_apu_t;
 
 typedef enum {
diff --git a/Core/gb.h b/Core/gb.h
index aa16ede..afd5c27 100644
--- a/Core/gb.h
+++ b/Core/gb.h
@@ -350,7 +350,7 @@ struct GB_gameboy_internal_s {
     GB_SECTION(timing,
         GB_UNIT(display);
         GB_UNIT(div);
-        uint32_t div_counter;
+        uint16_t div_counter;
         uint8_t tima_reload_state; /* After TIMA overflows, it becomes 0 for 4 cycles before actually reloading. */
         uint16_t serial_cycles;
         uint16_t serial_length;
diff --git a/Core/timing.c b/Core/timing.c
index 6132f40..58177d5 100644
--- a/Core/timing.c
+++ b/Core/timing.c
@@ -130,6 +130,8 @@ static void GB_set_internal_div_counter(GB_gameboy_t *gb, uint32_t value)
         counter_overflow_check(gb->div_counter, value, GB_TAC_RATIOS[gb->io_registers[GB_IO_TAC] & 3])) {
         increase_tima(gb);
     }
+    
+    /* TODO: Can switching to double speed mode trigger an event? */
     if (counter_overflow_check(gb->div_counter, value, gb->cgb_double_speed? 0x4000 : 0x2000)) {
         GB_apu_run(gb);
         GB_apu_div_event(gb);