diff --git a/src/BizHawk.Emulation.Cores/CPUs/LR35902/Interrupts.cs b/src/BizHawk.Emulation.Cores/CPUs/LR35902/Interrupts.cs
index 8adeb00472..bfc13572c3 100644
--- a/src/BizHawk.Emulation.Cores/CPUs/LR35902/Interrupts.cs
+++ b/src/BizHawk.Emulation.Cores/CPUs/LR35902/Interrupts.cs
@@ -21,8 +21,8 @@ namespace BizHawk.Emulation.Cores.Components.LR35902
 						DEC16, SPl, SPh,
 						INT_GET, 1, W,
 						WR, SPl, SPh, PCl,
-						IRQ_CLEAR,
 						IDLE,
+						IRQ_CLEAR,
 						TR, PCl, W,
 						OP };
 		}
@@ -50,8 +50,8 @@ namespace BizHawk.Emulation.Cores.Components.LR35902
 						DEC16, SPl, SPh,
 						INT_GET, 1, W,
 						WR, SPl, SPh, PCl,
-						IRQ_CLEAR,
 						IDLE,
+						IRQ_CLEAR,
 						TR, PCl, W,
 						OP };
 		}
diff --git a/src/BizHawk.Emulation.Cores/CPUs/LR35902/LR35902.cs b/src/BizHawk.Emulation.Cores/CPUs/LR35902/LR35902.cs
index 1f322d5e56..767a3e99a1 100644
--- a/src/BizHawk.Emulation.Cores/CPUs/LR35902/LR35902.cs
+++ b/src/BizHawk.Emulation.Cores/CPUs/LR35902/LR35902.cs
@@ -600,7 +600,7 @@ namespace BizHawk.Emulation.Cores.Components.LR35902
 					// interrupt src = 5 sets the PC to zero as observed
 					// also the triggering interrupt seems like it is held low (i.e. cannot trigger I flag) until the interrupt is serviced
 					ushort bit_check = instr_table[instr_pntr++];
-					//Console.WriteLine(interrupt_src + " " + interrupt_enable + " " + TotalExecutedCycles);
+					//Console.WriteLine("int " + TotalExecutedCycles);
 
 					interrupt_src_reg = GetIntRegs(0);
 					interrupt_enable_reg = GetIntRegs(1);
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IDebuggable.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IDebuggable.cs
index 97e7456582..c252c3a04a 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IDebuggable.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IDebuggable.cs
@@ -20,5 +20,6 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 		public void Step(StepType type) => throw new NotImplementedException();
 
 		public long TotalExecutedCycles => (long)cpu.TotalExecutedCycles;
+		//public long TotalExecutedCycles => CycleCount;
 	}
 }
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IEmulator.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IEmulator.cs
index e197d5b967..6e792d20b4 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IEmulator.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IEmulator.cs
@@ -20,6 +20,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 		public bool controller_was_checked;
 		public bool delays_to_process;
 		public int controller_delay_cd;
+		//public long CycleCount;
 
 		public bool FrameAdvance(IController controller, bool render, bool rendersound)
 		{
@@ -112,8 +113,10 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 						cpu.ExecuteOne();
 						timer.divider_reg++;
 						if (delays_to_process) { process_delays(); }
+
+						REG_FF0F_OLD = REG_FF0F;
 					}
-				
+
 					if (ppu.DMA_start && !cpu.halted && !cpu.stopped) { ppu.DMA_tick(); }
 					serialport.serial_transfer_tick();
 					timer.tick();
@@ -130,6 +133,8 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 						cpu.TotalExecutedCycles++;
 						timer.divider_reg++;
 						if (delays_to_process) { process_delays(); }
+
+						REG_FF0F_OLD = REG_FF0F;
 					}
 
 					timer.tick();
@@ -139,6 +144,8 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 					if (delays_to_process) { process_delays(); }
 				}
 
+				//CycleCount++;
+
 				if (in_vblank && !in_vblank_old)
 				{
 					_islag = false;
@@ -194,14 +201,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			if (!HDMA_transfer)
 			{
 				// These things all tick twice as fast in GBC double speed mode
-				if (ppu.DMA_start && !cpu.halted && !cpu.stopped) { ppu.DMA_tick(); }
-				serialport.serial_transfer_tick();
-				timer.tick();
-				cpu.ExecuteOne();
-				timer.divider_reg++;
-
-				if (delays_to_process) { process_delays(); }
-
+				// Note that DMA is halted when the CPU is halted
 				if (double_speed)
 				{
 					if (ppu.DMA_start && !cpu.halted && !cpu.stopped) { ppu.DMA_tick(); }
@@ -209,26 +209,36 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 					timer.tick();
 					cpu.ExecuteOne();
 					timer.divider_reg++;
-
 					if (delays_to_process) { process_delays(); }
+
+					REG_FF0F_OLD = REG_FF0F;
 				}
-			}
-			else
-			{
+
+				if (ppu.DMA_start && !cpu.halted && !cpu.stopped) { ppu.DMA_tick(); }
+				serialport.serial_transfer_tick();
 				timer.tick();
-				cpu.TotalExecutedCycles++;
+				cpu.ExecuteOne();
 				timer.divider_reg++;
 
 				if (delays_to_process) { process_delays(); }
-
+			}
+			else
+			{
 				if (double_speed)
 				{
 					timer.tick();
 					cpu.TotalExecutedCycles++;
 					timer.divider_reg++;
-
 					if (delays_to_process) { process_delays(); }
+
+					REG_FF0F_OLD = REG_FF0F;
 				}
+
+				timer.tick();
+				cpu.TotalExecutedCycles++;
+				timer.divider_reg++;
+
+				if (delays_to_process) { process_delays(); }
 			}
 
 			if (in_vblank && !in_vblank_old)
@@ -314,7 +324,12 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			else
 			{
 				// reset the divider (only way for speed_change_timing_fine.gbc and speed_change_cancel.gbc to both work)
-				timer.divider_reg = 1;
+				//Console.WriteLine("at stop " + timer.divider_reg);
+				timer.divider_reg = 0x1;
+
+				// TODO: resetting the divider causes an increment, but exact timing unclear
+				//timer.tick();
+
 				double_speed = !double_speed;
 				return 0;
 			}
@@ -343,7 +358,10 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 
 		public void SetIntRegs(byte r)
 		{
+			// For timer interrupts or serial interrupts that occur on the same cycle as the IRQ clear
+			// the clear wins on GB but the set wins on GBC
 			if (((REG_FF0F & 4) == 4) && ((r & 4) == 0) && timer.IRQ_block && !is_GBC) { r |= 4; }
+			if (((REG_FF0F & 8) == 8) && ((r & 8) == 0) && serialport.IRQ_block && !is_GBC) { r |= 8; }
 			REG_FF0F = r;
 		}
 
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.cs
index 7489d4110d..3cb1a10762 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.cs
@@ -220,6 +220,18 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			iptr3 = Marshal.AllocHGlobal(ppu.color_palette.Length * 8 * 8 + 1);
 
 			_scanlineCallback = null;
+
+			/*
+			for (int i = 0; i < ZP_RAM.Length; i++)
+			{
+				ZP_RAM[i] = 0xFF;
+			}
+
+			for (int i = 0; i < RAM.Length; i++)
+			{
+				RAM[i] = 0xFF;
+			}
+			*/
 		}
 
 		public bool IsCGBMode() => is_GBC;
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GB_PPU.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GB_PPU.cs
index dc13349b27..701cdefde1 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GB_PPU.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GB_PPU.cs
@@ -50,6 +50,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 						blank_frame = true;
 					}				
 					LCDC = value;
+					//Console.WriteLine(LY + " " + cycle);
 					break; 
 				case 0xFF41: // STAT
 					// writing to STAT during mode 0 or 1 causes a STAT IRQ
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/HW_Registers.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/HW_Registers.cs
index 328fd3006f..3bfa0b06cd 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/HW_Registers.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/HW_Registers.cs
@@ -38,6 +38,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 				// Interrupt flags
 				case 0xFF0F:
 					// TODO: Maybe some PPU bits are immediately visible, see 10spritesPrLine_10xposA7_m0irq_2_dmg08_cgb04c_out2.gbc
+					//Console.WriteLine("FF0F " + cpu.TotalExecutedCycles);
 					ret = REG_FF0F_OLD;
 					break;
 
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/MemoryMap.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/MemoryMap.cs
index bcd271f47b..508e58cfd8 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/MemoryMap.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/MemoryMap.cs
@@ -25,6 +25,9 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 {
 	public partial class GBHawk
 	{
+		//public byte[] RAM_read = new byte[0x8000];
+		//public byte[] ZP_RAM_read = new byte[0x80];
+
 		public byte ReadMemory(ushort addr)
 		{
 			if (MemoryCallbacks.HasReads)
@@ -61,6 +64,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 
 				if (addr >= 0xF000 && addr < 0xFE00)
 				{
+					//if (RAM_read[(RAM_Bank * 0x1000) + (addr - 0xF000)] == 0) { Console.WriteLine("RAM: " + addr + " " + cpu.TotalExecutedCycles); }
 					return RAM[(RAM_Bank * 0x1000) + (addr - 0xF000)];
 				}
 
@@ -85,6 +89,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 				{
 					if (addr != 0xFFFF)
 					{
+						//if (ZP_RAM_read[addr - 0xFF80] == 0) { Console.WriteLine("ZP: " + (addr - 0xFF80) + " " + cpu.TotalExecutedCycles); }
 						return ZP_RAM[addr - 0xFF80];
 					}
 					else
@@ -158,6 +163,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			if (addr < 0xFE00)
 			{
 				addr = (ushort)(RAM_Bank * (addr & 0x1000) + (addr & 0xFFF));
+				//if (RAM_read[addr] == 0) { Console.WriteLine("RAM: " + addr + " " + cpu.TotalExecutedCycles); }
 				return RAM[addr];
 			}
 
@@ -215,6 +221,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 
 			if (addr < 0xFFFF)
 			{
+				//if (ZP_RAM_read[addr - 0xFF80] == 0) { Console.WriteLine("ZP: " + (addr - 0xFF80) + " " + cpu.TotalExecutedCycles); }
 				return ZP_RAM[addr - 0xFF80];
 			}
 
@@ -234,7 +241,6 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			if (ppu.DMA_start)
 			{
 				// some of gekkio's tests require this to be accessible during DMA
-
 				if (addr >= 0xA000 && addr < 0xC000 && is_GBC)
 				{
 					// on GBC only, cart is accessible during DMA
@@ -243,10 +249,12 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 
 				if (addr >= 0xE000 && addr < 0xF000)
 				{
+					//RAM_read[addr - 0xE000] = 1;
 					RAM[addr - 0xE000] = value;
 				}
 				else if (addr >= 0xF000 && addr < 0xFE00)
 				{
+					//RAM_read[RAM_Bank * 0x1000 + (addr - 0xF000)] = 1;
 					RAM[RAM_Bank * 0x1000 + (addr - 0xF000)] = value;
 				}
 				else if (addr >= 0xFE00 && addr < 0xFEA0 && ppu.DMA_OAM_access)
@@ -261,6 +269,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 				{
 					if (addr != 0xFFFF)
 					{
+						//ZP_RAM_read[addr - 0xFF80] = 1;
 						ZP_RAM[addr - 0xFF80] = value;
 					}
 					else
@@ -281,6 +290,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 				}
 				else if (addr < 0xFFFF)
 				{
+					//ZP_RAM_read[addr - 0xFF80] = 1;
 					ZP_RAM[addr - 0xFF80] = value;
 				}
 				else
@@ -294,7 +304,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 				{
 					if (ppu.OAM_access_write) { OAM[addr - 0xFE00] = value; }
 				}
-				// unmapped memory writes depend on console
+				// unmapped memory writes depend on console		
 				else
 				{
 					if (is_GBC)
@@ -318,6 +328,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			else if (addr >= 0xC000)
 			{
 				addr = (ushort)(RAM_Bank * (addr & 0x1000) + (addr & 0xFFF));
+				//RAM_read[addr] = 1;
 				RAM[addr] = value;
 			}
 			else if (addr >= 0xA000)
@@ -326,7 +337,10 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			}
 			else if (addr >= 0x8000)
 			{
-				if (ppu.VRAM_access_write) { VRAM[(VRAM_Bank * 0x2000) + (addr - 0x8000)] = value; }				
+				if (ppu.VRAM_access_write) 
+				{ 
+					VRAM[(VRAM_Bank * 0x2000) + (addr - 0x8000)] = value; 
+				}
 			}
 			else
 			{
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/SerialPort.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/SerialPort.cs
index d65d562a00..327ed9e68d 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/SerialPort.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/SerialPort.cs
@@ -1,5 +1,6 @@
 using BizHawk.Common.NumberExtensions;
 using BizHawk.Common;
+using System;
 
 namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 {
@@ -15,6 +16,8 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 		public int clk_rate;
 		public byte going_out;
 		public byte coming_in;
+		public int serial_bits_old;
+		public bool IRQ_block;
 
 		public byte ReadReg(int addr)
 		{
@@ -40,20 +43,24 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 				case 0xFF02:
 					if ((value & 0x1) == 1)
 					{
+						
 						serial_bits = 8;
 
 						if (((value & 2) > 0) && Core.GBC_compat)
 						{
 							clk_rate = 16;
-							serial_clock = (16 - (int)(Core.cpu.TotalExecutedCycles % 8)) + 1;
+							serial_clock = 16 - (int)(Core.timer.divider_reg % 8) - 1;
+							
+							// if the clock rate is changing and it's on a GBA/C, the parity of (cpu.totalexecutedcycles & 512) effects the first bit
+							// Not sure exactly how yet
 						}
 						else
 						{
 							clk_rate = 512;
-							serial_clock = 512 - (int)(Core.cpu.TotalExecutedCycles % 256) + 1;
+							serial_clock = 512 - (int)(Core.timer.divider_reg % 256) - 1;
 
 							// there seems to be some clock inverting happening on some transfers
-							// not sure of the exact nature of it, here is one methor that gives correct result on one test rom but not others
+							// not sure of the exact nature of it, here is one method that gives correct result on one test rom but not others
 							/*
 							if (Core._syncSettings.GBACGB && Core.is_GBC)
 							{
@@ -68,10 +75,12 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 					}
 					else
 					{
+						serial_bits_old = serial_bits;
 						serial_bits = 8;
 						clk_rate = -1;
 						serial_clock = clk_rate;
 						can_pulse = false;
+						
 					}
 
 					if (Core.GBC_compat)
@@ -90,6 +99,8 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 
 		public void serial_transfer_tick()
 		{
+			IRQ_block = false;
+
 			if (serial_clock > 0) 
 			{ 
 				serial_clock--;
@@ -108,6 +119,8 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 
 							if (Core.REG_FFFF.Bit(3)) { Core.cpu.FlagI = true; }
 							Core.REG_FF0F |= 0x08;
+							//Console.WriteLine("SIRQ " + Core.cpu.TotalExecutedCycles);
+							IRQ_block = true;
 						}
 						else
 						{
@@ -141,6 +154,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			ser.Sync(nameof(going_out), ref going_out);
 			ser.Sync(nameof(coming_in), ref coming_in);
 			ser.Sync(nameof(can_pulse), ref can_pulse);
+			ser.Sync(nameof(IRQ_block), ref IRQ_block);
 		}
 	}
 }
diff --git a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Timer.cs b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Timer.cs
index 13830c0b25..d6febab9e7 100644
--- a/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Timer.cs
+++ b/src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/Timer.cs
@@ -204,6 +204,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 
 					// set interrupts
 					if (Core.REG_FFFF.Bit(2)) { Core.cpu.FlagI = true; }
+					//Console.WriteLine("timer " + Core.cpu.TotalExecutedCycles);
 					Core.REG_FF0F |= 0x04;
 					IRQ_block = true;
 				}