From 4bbd27735fb6d12ba95deb10437214b2172aeec3 Mon Sep 17 00:00:00 2001
From: Lior Halphon <LIJI32@gmail.com>
Date: Fri, 25 Dec 2020 20:40:39 +0200
Subject: [PATCH] Fix a regression in speed switch timing, reset DIV on speed
 switch, better odd-mode detection and avoidance

---
 Core/memory.c   |  1 +
 Core/sm83_cpu.c | 16 ++++++++--------
 Core/timing.c   |  8 ++++++--
 3 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/Core/memory.c b/Core/memory.c
index 40af0e5..cff31b6 100644
--- a/Core/memory.c
+++ b/Core/memory.c
@@ -895,6 +895,7 @@ static void write_high_memory(GB_gameboy_t *gb, uint16_t addr, uint8_t value)
                 if ((value & 0x80) && !(gb->io_registers[GB_IO_LCDC] & 0x80)) {
                     gb->display_cycles = 0;
                     gb->display_state = 0;
+                    gb->double_speed_alignment = 0;
                     if (GB_is_sgb(gb)) {
                         gb->frame_skip_state = GB_FRAMESKIP_SECOND_FRAME_RENDERED;
                     }
diff --git a/Core/sm83_cpu.c b/Core/sm83_cpu.c
index e423fba..e56040b 100644
--- a/Core/sm83_cpu.c
+++ b/Core/sm83_cpu.c
@@ -332,6 +332,7 @@ static void nop(GB_gameboy_t *gb, uint8_t opcode)
 
 static void enter_stop_mode(GB_gameboy_t *gb)
 {
+    GB_write_memory(gb, 0xFF00 + GB_IO_DIV, 0);
     gb->stopped = true;
     gb->oam_ppu_blocked = !gb->oam_read_blocked;
     gb->vram_ppu_blocked = !gb->vram_read_blocked;
@@ -340,30 +341,30 @@ static void enter_stop_mode(GB_gameboy_t *gb)
 
 static void leave_stop_mode(GB_gameboy_t *gb)
 {
-    /* The CPU takes more time to wake up then the other components */
-    for (unsigned i = 0x200; i--;) {
-        GB_advance_cycles(gb, 0x10);
-    }
     gb->stopped = false;
     gb->oam_ppu_blocked = false;
     gb->vram_ppu_blocked = false;
     gb->cgb_palettes_ppu_blocked = false;
+    /* The CPU takes more time to wake up then the other components */
+    for (unsigned i = 0x2000; i--;) {
+        GB_advance_cycles(gb, 0x10);
+    }
+    GB_write_memory(gb, 0xFF00 + GB_IO_DIV, 0);
 }
 
 static void stop(GB_gameboy_t *gb, uint8_t opcode)
 {
     if (gb->io_registers[GB_IO_KEY1] & 0x1) {
-        if (gb->cgb_double_speed && gb->io_registers[GB_IO_LCDC] & 0x80) {
-            GB_log(gb, "Returning from double speed mode while the PPU is on may trigger odd-mode\n");
-        }
         flush_pending_cycles(gb);
         bool needs_alignment = false;
         
         GB_advance_cycles(gb, 0x4);
         /* Make sure we keep the CPU ticks aligned correctly when returning from double speed mode */
+        
         if (gb->double_speed_alignment & 7) {
             GB_advance_cycles(gb, 0x4);
             needs_alignment = true;
+            GB_log(gb, "ROM triggered PPU odd mode, which is currently not supported. Reverting to even-mode.\n");
         }
 
         gb->cgb_double_speed ^= true;
@@ -388,7 +389,6 @@ static void stop(GB_gameboy_t *gb, uint8_t opcode)
             enter_stop_mode(gb);
         }
     }
-    
     /* Todo: is PC being actually read? */
     gb->pc++;
 }
diff --git a/Core/timing.c b/Core/timing.c
index 44ff8f7..7009d7b 100644
--- a/Core/timing.c
+++ b/Core/timing.c
@@ -157,7 +157,9 @@ static void GB_set_internal_div_counter(GB_gameboy_t *gb, uint32_t value)
 static void GB_timers_run(GB_gameboy_t *gb, uint8_t cycles)
 {
     if (gb->stopped) {
-        gb->apu.apu_cycles += 4 << !gb->cgb_double_speed;
+        if (GB_is_cgb(gb)) {
+            gb->apu.apu_cycles += 4 << !gb->cgb_double_speed;
+        }
         return;
     }
     
@@ -248,7 +250,9 @@ void GB_advance_cycles(GB_gameboy_t *gb, uint8_t cycles)
     }
     
     // Not affected by speed boost
-    gb->double_speed_alignment += cycles;
+    if (gb->io_registers[GB_IO_LCDC] & 0x80) {
+        gb->double_speed_alignment += cycles;
+    }
     gb->hdma_cycles += cycles;
     gb->apu_output.sample_cycles += cycles;
     gb->cycles_since_last_sync += cycles;