bus-main-m68k.c

#include "bus-main-m68k.h"

#include <assert.h>

#include "bus-sub-m68k.h"
#include "bus-z80.h"
#include "io-port.h"
#include "log.h"

/* The Z80 can trigger 68k bus errors by using the 68k address space window, so print its program counter here too. */
#define LOG_MAIN_CPU_BUS_ERROR_MESSAGE_PREFIX "[M68K PC: 0x%06" CC_PRIXLEAST32 ", Z80 PC: 0x%04" CC_PRIXLEAST16 "] "
#define LOG_MAIN_CPU_BUS_ERROR_ARGUMENTS clownmdemu->state->m68k.state.program_counter, clownmdemu->state->z80.state.program_counter
#define LOG_MAIN_CPU_BUS_ERROR_0(MESSAGE)                   LogMessage(LOG_MAIN_CPU_BUS_ERROR_MESSAGE_PREFIX MESSAGE, LOG_MAIN_CPU_BUS_ERROR_ARGUMENTS);
#define LOG_MAIN_CPU_BUS_ERROR_1(MESSAGE, ARG1)             LogMessage(LOG_MAIN_CPU_BUS_ERROR_MESSAGE_PREFIX MESSAGE, LOG_MAIN_CPU_BUS_ERROR_ARGUMENTS, ARG1);
#define LOG_MAIN_CPU_BUS_ERROR_2(MESSAGE, ARG1, ARG2)       LogMessage(LOG_MAIN_CPU_BUS_ERROR_MESSAGE_PREFIX MESSAGE, LOG_MAIN_CPU_BUS_ERROR_ARGUMENTS, ARG1, ARG2);
#define LOG_MAIN_CPU_BUS_ERROR_3(MESSAGE, ARG1, ARG2, ARG3) LogMessage(LOG_MAIN_CPU_BUS_ERROR_MESSAGE_PREFIX MESSAGE, LOG_MAIN_CPU_BUS_ERROR_ARGUMENTS, ARG1, ARG2, ARG3);

/* https://github.com/devon-artmeier/clownmdemu-mcd-boot */
static const cc_u16l megacd_boot_rom[] = {
#include "mega-cd-boot-rom.c"
};

static cc_u16f GetHCounterValue(const ClownMDEmu* const clownmdemu, const CycleMegaDrive target_cycle)
{
	/* TODO: V30 and PAL and H32. */

	/* TODO: This entire thing is a disgusting hack. */
	/* Once the VDP emulator becames slot-based, this junk should be erased. */
	const cc_u16f cycles_per_scanline = GetMegaDriveCyclesPerFrame(clownmdemu).cycle / GetTelevisionVerticalResolution(clownmdemu);

	/* Sourced from https://gendev.spritesmind.net/forum/viewtopic.php?t=3058. */
	const cc_u16f maximum_value = 0x100 - 0x30;

	return (target_cycle.cycle % cycles_per_scanline) * maximum_value / cycles_per_scanline;
}

static cc_bool GetHBlankBit(const ClownMDEmu* const clownmdemu, const CycleMegaDrive target_cycle)
{
	/* TODO: V30 and PAL and H32. */
	/* Sourced from https://plutiedev.com/mirror/kabuto-hardware-notes. */
	return GetHCounterValue(clownmdemu, target_cycle) > 0xB2;
}

static cc_u16f VDPReadCallback(void *user_data, cc_u32f address)
{
	return M68kReadCallbackWithDMA(user_data, address / 2, cc_true, cc_true, cc_true);
}

static void VDPKDebugCallback(void* const user_data, const char* const string)
{
	(void)user_data;

	LogMessage("KDEBUG: %s", string);
}

static cc_u16f SyncM68kCallback(const ClownMDEmu* const clownmdemu, void* const user_data)
{
	return CLOWNMDEMU_M68K_CLOCK_DIVIDER * Clown68000_DoCycle(clownmdemu->m68k, (const Clown68000_ReadWriteCallbacks*)user_data);
}

void SyncM68k(const ClownMDEmu* const clownmdemu, CPUCallbackUserData* const other_state, const CycleMegaDrive target_cycle)
{
	Clown68000_ReadWriteCallbacks m68k_read_write_callbacks;

	m68k_read_write_callbacks.read_callback = M68kReadCallback;
	m68k_read_write_callbacks.write_callback = M68kWriteCallback;
	m68k_read_write_callbacks.user_data = other_state;

	SyncCPUCommon(clownmdemu, &other_state->sync.m68k, target_cycle.cycle, cc_false, SyncM68kCallback, &m68k_read_write_callbacks);
}

cc_u16f M68kReadCallbackWithCycleWithDMA(const void* const user_data, const cc_u32f address_word, const cc_bool do_high_byte, const cc_bool do_low_byte, const CycleMegaDrive target_cycle, const cc_bool is_vdp_dma)
{
	CPUCallbackUserData* const callback_user_data = (CPUCallbackUserData*)user_data;
	const ClownMDEmu* const clownmdemu = callback_user_data->clownmdemu;
	const ClownMDEmu_Callbacks* const frontend_callbacks = clownmdemu->callbacks;
	const cc_u32f address = address_word * 2;

	cc_u16f value = 0;

	if (address < 0x800000)
	{
		if (((address & 0x400000) == 0) != clownmdemu->state->mega_cd.boot_from_cd)
		{
			if ((address & 0x200000) != 0 && clownmdemu->state->external_ram.mapped_in)
			{
				/* External RAM */
				const cc_u32f index = address & 0x1FFFFF;

				if (index >= clownmdemu->state->external_ram.size)
				{
					value = 0xFFFF;
					LOG_MAIN_CPU_BUS_ERROR_2("Attempted to read past the end of external RAM (0x%" CC_PRIXFAST32 " when the external RAM ends at 0x%" CC_PRIXLEAST16 ")", index, clownmdemu->state->external_ram.size);
				}
				else
				{
					value |= clownmdemu->state->external_ram.buffer[index + 0] << 8;
					value |= clownmdemu->state->external_ram.buffer[index + 1] << 0;
				}
			}
			else
			{
				/* Cartridge */
				if (do_high_byte)
					value |= frontend_callbacks->cartridge_read((void*)frontend_callbacks->user_data, (address & 0x3FFFFF) + 0) << 8;
				if (do_low_byte)
					value |= frontend_callbacks->cartridge_read((void*)frontend_callbacks->user_data, (address & 0x3FFFFF) + 1) << 0;
			}
		}
		else
		{
			if ((address & 0x200000) != 0)
			{
				/* WORD-RAM */
				if (clownmdemu->state->mega_cd.word_ram.in_1m_mode)
				{
					if ((address & 0x20000) != 0)
					{
						/* TODO */
						LOG_MAIN_CPU_BUS_ERROR_0("MAIN-CPU attempted to read from that weird half of 1M WORD-RAM");
					}
					else
					{
						value = clownmdemu->state->mega_cd.word_ram.buffer[(address_word & 0xFFFF) * 2 + clownmdemu->state->mega_cd.word_ram.ret];
					}
				}
				else
				{
					if (clownmdemu->state->mega_cd.word_ram.dmna)
					{
						LOG_MAIN_CPU_BUS_ERROR_0("MAIN-CPU attempted to read from WORD-RAM while SUB-CPU has it");
					}
					else
					{
						value = clownmdemu->state->mega_cd.word_ram.buffer[address_word & 0x1FFFF];
					}
				}

				if (is_vdp_dma)
				{
					/* Delay WORD-RAM DMA transfers. This is a real bug on the Mega CD that games have to work around. */
					/* This can easily be seen in Sonic CD's FMVs. */
					const cc_u16f delayed_value = value;

					value = clownmdemu->state->mega_cd.delayed_dma_word;
					clownmdemu->state->mega_cd.delayed_dma_word = delayed_value;
				}
			}
			else if ((address & 0x20000) == 0)
			{
				/* Mega CD BIOS */
				if ((address & 0x1FFFF) == 0x72)
				{
					/* The Mega CD has this strange hack in its bug logic, which allows
					   the H-Int interrupt address to be overridden with a register. */
					value = clownmdemu->state->mega_cd.hblank_address;
				}
				else
				{
					value = megacd_boot_rom[address_word & 0xFFFF];
				}
			}
			else
			{
				/* PRG-RAM */
				if (!clownmdemu->state->mega_cd.m68k.bus_requested)
				{
					LOG_MAIN_CPU_BUS_ERROR_0("Attempted to read from PRG-RAM while SUB-CPU has it");
				}
				else
				{
					value = clownmdemu->state->mega_cd.prg_ram.buffer[0x10000 * clownmdemu->state->mega_cd.prg_ram.bank + (address_word & 0xFFFF)];
				}
			}
		}
	}
	else if ((address >= 0xA00000 && address <= 0xA01FFF) || address == 0xA04000 || address == 0xA04002)
	{
		/* Z80 RAM and YM2612 */
		if (!clownmdemu->state->z80.bus_requested)
		{
			LOG_MAIN_CPU_BUS_ERROR_0("68k attempted to read Z80 memory/YM2612 ports without Z80 bus");
		}
		else if (clownmdemu->state->z80.reset_held)
		{
			/* TODO: Does this actually bother real hardware? */
			/* TODO: According to Devon, yes it does. */
			LOG_MAIN_CPU_BUS_ERROR_0("68k attempted to read Z80 memory/YM2612 ports while Z80 reset request was active");
		}
		else
		{
			/* This is unnecessary, as the Z80 bus will have to have been requested, causing a sync. */
			/*SyncZ80(clownmdemu, callback_user_data, target_cycle);*/

			if (do_high_byte && do_low_byte)
				LOG_MAIN_CPU_BUS_ERROR_0("68k attempted to perform word-sized read of Z80 memory/YM2612 ports; the read word will only contain the first byte repeated");

			value = Z80ReadCallbackWithCycle(user_data, (address + (do_high_byte ? 0 : 1)) & 0xFFFF, target_cycle);
			value = value << 8 | value;
		}
	}
	else if (address >= 0xA10000 && address <= 0xA1001F)
	{
		/* I/O AREA */
		/* TODO: The missing ports. */
		/* TODO: Detect when this is accessed without obtaining the Z80 bus and log a warning. */
		switch (address)
		{
			case 0xA10000:
				if (do_low_byte)
					value |= ((clownmdemu->configuration->general.region == CLOWNMDEMU_REGION_OVERSEAS) << 7) | ((clownmdemu->configuration->general.tv_standard == CLOWNMDEMU_TV_STANDARD_PAL) << 6) | (0 << 5);	/* Bit 5 clear = Mega CD attached */

				break;

			case 0xA10002:
			case 0xA10004:
				if (do_low_byte)
				{
					IOPortToController_Parameters parameters;

					const cc_u16f joypad_index = (address - 0xA10002) / 2;

					parameters.controller = &clownmdemu->state->controllers[joypad_index];
					parameters.frontend_callbacks = frontend_callbacks;

					value = IOPort_ReadData(&clownmdemu->state->io_ports[joypad_index], SyncCommon(&callback_user_data->sync.io_ports[joypad_index], target_cycle.cycle, CLOWNMDEMU_MASTER_CLOCK_NTSC / 1000000), &parameters);
				}

				break;

			case 0xA10006:
				value = 0xFF;
				break;

			case 0xA10008:
			case 0xA1000A:
			case 0xA1000C:
				if (do_low_byte)
				{
					const cc_u16f joypad_index = (address - 0xA10008) / 2;

					value = IOPort_ReadControl(&clownmdemu->state->io_ports[joypad_index]);
				}

				break;
		}
	}
	else if (address == 0xA11000)
	{
		/* MEMORY MODE */
		/* TODO */
		/* https://gendev.spritesmind.net/forum/viewtopic.php?p=28843&sid=65d8f210be331ff257a43b4e3dddb7c3#p28843 */
		/* According to this, this flag is only functional on earlier models, and effectively halves the 68k's speed when running from cartridge. */
	}
	else if (address == 0xA11100)
	{
		/* Z80 BUSREQ */
		/* On real hardware, bus requests do not complete if a reset is being held. */
		/* http://gendev.spritesmind.net/forum/viewtopic.php?f=2&t=2195 */
		const cc_bool z80_bus_obtained = clownmdemu->state->z80.bus_requested && !clownmdemu->state->z80.reset_held;

		if (clownmdemu->state->z80.reset_held)
			LOG_MAIN_CPU_BUS_ERROR_0("Z80 bus request will never end as long as the reset is asserted");

		/* TODO: According to Charles MacDonald's gen-hw.txt, the upper byte is actually the upper byte
		   of the next instruction and the lower byte is just 0 (and the flag bit, of course). */
		value = 0xFF ^ z80_bus_obtained;
		value = value << 8 | value;
	}
	else if (address == 0xA11200)
	{
		/* Z80 RESET */
		/* TODO: According to Charles MacDonald's gen-hw.txt, the upper byte is actually the upper byte
		   of the next instruction and the lower byte is just 0 (and the flag bit, of course). */
		value = 0xFF ^ clownmdemu->state->z80.reset_held;
		value = value << 8 | value;
	}
	else if (address == 0xA12000)
	{
		/* RESET, HALT */
		value = ((cc_u16f)clownmdemu->state->mega_cd.irq.enabled[1] << 15) |
			((cc_u16f)clownmdemu->state->mega_cd.m68k.bus_requested << 1) |
			((cc_u16f)!clownmdemu->state->mega_cd.m68k.reset_held << 0);
	}
	else if (address == 0xA12002)
	{
		/* Memory mode / Write protect */
		value = ((cc_u16f)clownmdemu->state->mega_cd.prg_ram.write_protect << 8) | ((cc_u16f)clownmdemu->state->mega_cd.prg_ram.bank << 6) | ((cc_u16f)clownmdemu->state->mega_cd.word_ram.in_1m_mode << 2) | ((cc_u16f)clownmdemu->state->mega_cd.word_ram.dmna << 1) | ((cc_u16f)clownmdemu->state->mega_cd.word_ram.ret << 0);
	}
	else if (address == 0xA12004)
	{
		/* CDC mode */
		value = CDC_Mode(&clownmdemu->state->mega_cd.cdc, cc_false);
	}
	else if (address == 0xA12006)
	{
		/* H-INT vector */
		value = clownmdemu->state->mega_cd.hblank_address;
	}
	else if (address == 0xA12008)
	{
		/* CDC host data */
		value = CDC_HostData(&clownmdemu->state->mega_cd.cdc, cc_false);
	}
	else if (address == 0xA1200C)
	{
		/* Stop watch */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to read from stop watch register");
	}
	else if (address == 0xA1200E)
	{
		/* Communication flag */
		SyncMCDM68k(clownmdemu, callback_user_data, CycleMegaDriveToMegaCD(clownmdemu, target_cycle));
		value = clownmdemu->state->mega_cd.communication.flag;
	}
	else if (address >= 0xA12010 && address < 0xA12020)
	{
		/* Communication command */
		SyncMCDM68k(clownmdemu, callback_user_data, CycleMegaDriveToMegaCD(clownmdemu, target_cycle));
		value = clownmdemu->state->mega_cd.communication.command[(address - 0xA12010) / 2];
	}
	else if (address >= 0xA12020 && address < 0xA12030)
	{
		/* Communication status */
		SyncMCDM68k(clownmdemu, callback_user_data, CycleMegaDriveToMegaCD(clownmdemu, target_cycle));
		value = clownmdemu->state->mega_cd.communication.status[(address - 0xA12020) / 2];
	}
	else if (address == 0xA12030)
	{
		/* Timer W/INT3 */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to read from Timer W/INT3 register");
	}
	else if (address == 0xA12032)
	{
		/* Interrupt mask control */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to read from interrupt mask control register");
	}
	else if (address == 0xA130F0)
	{
		/* External RAM control */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to read from external RAM control register");
	}
	/* TODO: According to Charles MacDonald's gen-hw.txt, the VDP stuff is mirrored in the following pattern:
	MSB                       LSB
	110n n000 nnnn nnnn 000m mmmm

	'1' - This bit must be 1.
	'0' - This bit must be 0.
	'n' - This bit can have any value.
	'm' - VDP addresses (00-1Fh) */
	else if (address == 0xC00000 || address == 0xC00002)
	{
		/* VDP data port */
		/* TODO - Reading from the data port causes real Mega Drives to crash (if the VDP isn't in read mode) */
		value = VDP_ReadData(&clownmdemu->vdp);
	}
	else if (address == 0xC00004 || address == 0xC00006)
	{
		/* VDP control port */
		value = VDP_ReadControl(&clownmdemu->vdp);

		/* Temporary stupid hack: shove the PAL bit in here. */
		/* TODO: This should be moved to the VDP core once it becomes sensitive to PAL mode differences. */
		value |= (clownmdemu->configuration->general.tv_standard == CLOWNMDEMU_TV_STANDARD_PAL);

		/* Temporary stupid hack: approximate the H-blank bit timing. */
		/* TODO: This should be moved to the VDP core once it becomes slot-based. */
		value |= GetHBlankBit(clownmdemu, target_cycle) << 2;
	}
	else if (address == 0xC00008)
	{
		/* H/V COUNTER */
		/* TODO: The V counter emulation is incredibly inaccurate: the timing is likely wrong, and it should be incremented while in the blanking areas too. */
		const cc_u8f h_counter = GetHCounterValue(clownmdemu, target_cycle);
		const cc_u8f v_counter = clownmdemu->state->vdp.double_resolution_enabled
			? ((clownmdemu->state->current_scanline & 0x7F) << 1) | ((clownmdemu->state->current_scanline & 0x80) >> 7)
			: clownmdemu->state->current_scanline;
		value = v_counter << 8 | h_counter;
	}
	else if (address >= 0xC00010 && address <= 0xC00016)
	{
		/* PSG */
		/* TODO - What's supposed to happen here, if you read from the PSG? */
		/* TODO: It freezes the 68k, that's what:
		   https://forums.sonicretro.org/index.php?posts/1066059/ */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to read from PSG; this will freeze a real Mega Drive");
	}
	else if (address >= 0xE00000 && address <= 0xFFFFFF)
	{
		/* 68k RAM */
		value = clownmdemu->state->m68k.ram[address_word % CC_COUNT_OF(clownmdemu->state->m68k.ram)];
	}
	else
	{
		LOG_MAIN_CPU_BUS_ERROR_1("Attempted to read invalid 68k address 0x%" CC_PRIXFAST32, address);
	}

	return value;
}

cc_u16f M68kReadCallbackWithCycle(const void* const user_data, const cc_u32f address, const cc_bool do_high_byte, const cc_bool do_low_byte, const CycleMegaDrive target_cycle)
{
	return M68kReadCallbackWithCycleWithDMA(user_data, address, do_high_byte, do_low_byte, target_cycle, cc_false);
}

cc_u16f M68kReadCallbackWithDMA(const void* const user_data, const cc_u32f address, const cc_bool do_high_byte, const cc_bool do_low_byte, const cc_bool is_vdp_dma)
{
	CPUCallbackUserData* const callback_user_data = (CPUCallbackUserData*)user_data;

	return M68kReadCallbackWithCycleWithDMA(user_data, address, do_high_byte, do_low_byte, MakeCycleMegaDrive(callback_user_data->sync.m68k.current_cycle), is_vdp_dma);
}

cc_u16f M68kReadCallback(const void* const user_data, const cc_u32f address, const cc_bool do_high_byte, const cc_bool do_low_byte)
{
	return M68kReadCallbackWithDMA(user_data, address, do_high_byte, do_low_byte, cc_false);
}

void M68kWriteCallbackWithCycle(const void* const user_data, const cc_u32f address_word, const cc_bool do_high_byte, const cc_bool do_low_byte, const cc_u16f value, const CycleMegaDrive target_cycle)
{
	CPUCallbackUserData* const callback_user_data = (CPUCallbackUserData*)user_data;
	const ClownMDEmu* const clownmdemu = callback_user_data->clownmdemu;
	const ClownMDEmu_Callbacks* const frontend_callbacks = clownmdemu->callbacks;
	const cc_u32f address = address_word * 2;

	const cc_u16f high_byte = (value >> 8) & 0xFF;
	const cc_u16f low_byte = (value >> 0) & 0xFF;

	cc_u16f mask = 0;

	if (do_high_byte)
		mask |= 0xFF00;
	if (do_low_byte)
		mask |= 0x00FF;

	if (address < 0x800000)
	{
		if (((address & 0x400000) == 0) != clownmdemu->state->mega_cd.boot_from_cd)
		{
			if ((address & 0x200000) != 0 && clownmdemu->state->external_ram.mapped_in)
			{
				/* External RAM */
				const cc_u32f index = address & 0x1FFFFF;

				if (index >= clownmdemu->state->external_ram.size)
				{
					LOG_MAIN_CPU_BUS_ERROR_2("Attempted to write past the end of external RAM (0x%" CC_PRIXFAST32 " when the external RAM ends at 0x%" CC_PRIXLEAST16 ")", index, clownmdemu->state->external_ram.size);
				}
				else
				{
					switch (clownmdemu->state->external_ram.data_size)
					{
						case 0:
						case 2:
							clownmdemu->state->external_ram.buffer[index + 0] = high_byte;
							break;
					}

					switch (clownmdemu->state->external_ram.data_size)
					{
						case 0:
						case 3:
							clownmdemu->state->external_ram.buffer[index + 1] = low_byte;
							break;
					}
				}
			}
			else
			{
				/* Cartridge */
				if (do_high_byte)
					frontend_callbacks->cartridge_written((void*)frontend_callbacks->user_data, (address & 0x3FFFFF) + 0, high_byte);
				if (do_low_byte)
					frontend_callbacks->cartridge_written((void*)frontend_callbacks->user_data, (address & 0x3FFFFF) + 1, low_byte);

				/* TODO - This is temporary, just to catch possible bugs in the 68k emulator */
				LOG_MAIN_CPU_BUS_ERROR_1("Attempted to write to ROM address 0x%" CC_PRIXFAST32, address);
			}
		}
		else
		{
			if ((address & 0x200000) != 0)
			{
				/* WORD-RAM */
				if (clownmdemu->state->mega_cd.word_ram.in_1m_mode)
				{
					if ((address & 0x20000) != 0)
					{
						/* TODO */
						LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to that weird half of 1M WORD-RAM");
					}
					else
					{
						clownmdemu->state->mega_cd.word_ram.buffer[(address_word & 0xFFFF) * 2 + clownmdemu->state->mega_cd.word_ram.ret] &= ~mask;
						clownmdemu->state->mega_cd.word_ram.buffer[(address_word & 0xFFFF) * 2 + clownmdemu->state->mega_cd.word_ram.ret] |= value & mask;
					}
				}
				else
				{
					if (clownmdemu->state->mega_cd.word_ram.dmna)
					{
						LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to WORD-RAM while SUB-CPU has it");
					}
					else
					{
						clownmdemu->state->mega_cd.word_ram.buffer[address_word & 0x1FFFF] &= ~mask;
						clownmdemu->state->mega_cd.word_ram.buffer[address_word & 0x1FFFF] |= value & mask;
					}
				}
			}
			else if ((address & 0x20000) == 0)
			{
				/* Mega CD BIOS */
				LOG_MAIN_CPU_BUS_ERROR_1("Attempted to write to BIOS (0x%" CC_PRIXFAST32 ")", address);
			}
			else
			{
				/* PRG-RAM */
				const cc_u32f prg_ram_index = 0x10000 * clownmdemu->state->mega_cd.prg_ram.bank + (address_word & 0xFFFF);

				if (!clownmdemu->state->mega_cd.m68k.bus_requested)
				{
					LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to PRG-RAM while SUB-CPU has it");
				}
				else if (prg_ram_index < (cc_u32f)clownmdemu->state->mega_cd.prg_ram.write_protect * 0x200)
				{
					LOG_MAIN_CPU_BUS_ERROR_1("Attempted to write to write-protected portion of PRG-RAM (0x%" CC_PRIXFAST32 ")", prg_ram_index);
				}
				else
				{
					clownmdemu->state->mega_cd.prg_ram.buffer[prg_ram_index] &= ~mask;
					clownmdemu->state->mega_cd.prg_ram.buffer[prg_ram_index] |= value & mask;
				}
			}
		}
	}
	else if ((address >= 0xA00000 && address <= 0xA01FFF) || address == 0xA04000 || address == 0xA04002)
	{
		/* Z80 RAM and YM2612 */
		if (!clownmdemu->state->z80.bus_requested)
		{
			LOG_MAIN_CPU_BUS_ERROR_0("68k attempted to write Z80 memory/YM2612 ports without Z80 bus");
		}
		else if (clownmdemu->state->z80.reset_held)
		{
			/* TODO: Does this actually bother real hardware? */
			/* TODO: According to Devon, yes it does. */
			LOG_MAIN_CPU_BUS_ERROR_0("68k attempted to write Z80 memory/YM2612 ports while Z80 reset request was active");
		}
		else
		{
			/* This is unnecessary, as the Z80 bus will have to have been requested, causing a sync. */
			/*SyncZ80(clownmdemu, callback_user_data, target_cycle);*/

			if (do_high_byte && do_low_byte)
				LOG_MAIN_CPU_BUS_ERROR_0("68k attempted to perform word-sized write of Z80 memory/YM2612 ports; only the top byte will be written");

			if (do_high_byte)
				Z80WriteCallbackWithCycle(user_data, (address + 0) & 0xFFFF, high_byte, target_cycle);
			else /*if (do_low_byte)*/
				Z80WriteCallbackWithCycle(user_data, (address + 1) & 0xFFFF, low_byte, target_cycle);
		}
	}
	else if (address >= 0xA10000 && address <= 0xA1001F)
	{
		/* I/O AREA */
		/* TODO */
		switch (address)
		{
			case 0xA10002:
			case 0xA10004:
			case 0xA10006:
				if (do_low_byte)
				{
					IOPortToController_Parameters parameters;

					const cc_u16f joypad_index = (address - 0xA10002) / 2;

					parameters.controller = &clownmdemu->state->controllers[joypad_index];
					parameters.frontend_callbacks = frontend_callbacks;

					IOPort_WriteData(&clownmdemu->state->io_ports[joypad_index], low_byte, CLOWNMDEMU_MASTER_CLOCK_NTSC / 1000000, &parameters);
				}

				break;

			case 0xA10008:
			case 0xA1000A:
			case 0xA1000C:
				if (do_low_byte)
				{
					const cc_u16f joypad_index = (address - 0xA10008) / 2;

					IOPort_WriteControl(&clownmdemu->state->io_ports[joypad_index], low_byte);
				}

				break;
		}
	}
	else if (address == 0xA11000)
	{
		/* MEMORY MODE */
		/* TODO: Make setting this to DRAM mode make the cartridge writeable. */
	}
	else if (address == 0xA11100)
	{
		/* Z80 BUSREQ */
		if (do_high_byte)
		{
			const cc_bool bus_request = (high_byte & 1) != 0;

			if (clownmdemu->state->z80.bus_requested != bus_request)
				SyncZ80(clownmdemu, callback_user_data, target_cycle);

			clownmdemu->state->z80.bus_requested = bus_request;
		}
	}
	else if (address == 0xA11200)
	{
		/* Z80 RESET */
		if (do_high_byte)
		{
			const cc_bool new_reset_held = (high_byte & 1) == 0;

			if (clownmdemu->state->z80.reset_held && !new_reset_held)
			{
				SyncZ80(clownmdemu, callback_user_data, target_cycle);
				Z80_Reset(&clownmdemu->z80);
				FM_State_Initialise(&clownmdemu->state->fm);
			}

			clownmdemu->state->z80.reset_held = new_reset_held;
		}
	}
	else if (address == 0xA12000)
	{
		/* RESET, HALT */
		Clown68000_ReadWriteCallbacks m68k_read_write_callbacks;

		const cc_bool interrupt = (high_byte & (1 << 0)) != 0;
		const cc_bool bus_request = (low_byte & (1 << 1)) != 0;
		const cc_bool reset = (low_byte & (1 << 0)) == 0;

		m68k_read_write_callbacks.read_callback = MCDM68kReadCallback;
		m68k_read_write_callbacks.write_callback = MCDM68kWriteCallback;
		m68k_read_write_callbacks.user_data = callback_user_data;

		if (clownmdemu->state->mega_cd.m68k.bus_requested != bus_request)
			SyncMCDM68k(clownmdemu, callback_user_data, CycleMegaDriveToMegaCD(clownmdemu, target_cycle));

		if (clownmdemu->state->mega_cd.m68k.reset_held && !reset)
		{
			SyncMCDM68k(clownmdemu, callback_user_data, CycleMegaDriveToMegaCD(clownmdemu, target_cycle));
			Clown68000_Reset(clownmdemu->mcd_m68k, &m68k_read_write_callbacks);
		}

		if (interrupt && clownmdemu->state->mega_cd.irq.enabled[1])
		{
			SyncMCDM68k(clownmdemu, callback_user_data, CycleMegaDriveToMegaCD(clownmdemu, target_cycle));
			Clown68000_Interrupt(clownmdemu->mcd_m68k, 2);
		}

		clownmdemu->state->mega_cd.m68k.bus_requested = bus_request;
		clownmdemu->state->mega_cd.m68k.reset_held = reset;
	}
	else if (address == 0xA12002)
	{
		/* Memory mode / Write protect */
		if (do_high_byte)
			clownmdemu->state->mega_cd.prg_ram.write_protect = high_byte;

		if (do_low_byte)
		{
			if ((low_byte & (1 << 1)) != 0)
			{
				SyncMCDM68k(clownmdemu, callback_user_data, CycleMegaDriveToMegaCD(clownmdemu, target_cycle));

				clownmdemu->state->mega_cd.word_ram.dmna = cc_true;

				if (!clownmdemu->state->mega_cd.word_ram.in_1m_mode)
					clownmdemu->state->mega_cd.word_ram.ret = cc_false;
			}

			clownmdemu->state->mega_cd.prg_ram.bank = (low_byte >> 6) & 3;
		}
	}
	else if (address == 0xA12004)
	{
		/* CDC mode */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to CDC mode register");
	}
	else if (address == 0xA12006)
	{
		/* H-INT vector */
		clownmdemu->state->mega_cd.hblank_address &= ~mask;
		clownmdemu->state->mega_cd.hblank_address |= value & mask;
	}
	else if (address == 0xA12008)
	{
		/* CDC host data */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to CDC host data register");
	}
	else if (address == 0xA1200C)
	{
		/* Stop watch */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to stop watch register");
	}
	else if (address == 0xA1200E)
	{
		/* Communication flag */
		if (do_high_byte)
		{
			SyncMCDM68k(clownmdemu, callback_user_data, CycleMegaDriveToMegaCD(clownmdemu, target_cycle));
			clownmdemu->state->mega_cd.communication.flag = (clownmdemu->state->mega_cd.communication.flag & 0x00FF) | (value & 0xFF00);
		}

		if (do_low_byte)
			LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to SUB-CPU's communication flag");
	}
	else if (address >= 0xA12010 && address < 0xA12020)
	{
		/* Communication command */
		SyncMCDM68k(clownmdemu, callback_user_data, CycleMegaDriveToMegaCD(clownmdemu, target_cycle));
		clownmdemu->state->mega_cd.communication.command[(address - 0xA12010) / 2] &= ~mask;
		clownmdemu->state->mega_cd.communication.command[(address - 0xA12010) / 2] |= value & mask;
	}
	else if (address >= 0xA12020 && address < 0xA12030)
	{
		/* Communication status */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to SUB-CPU's communication status");
	}
	else if (address == 0xA12030)
	{
		/* Timer W/INT3 */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to Timer W/INT3 register");
	}
	else if (address == 0xA12032)
	{
		/* Interrupt mask control */
		LOG_MAIN_CPU_BUS_ERROR_0("Attempted to write to interrupt mask control register");
	}
	else if (address == 0xA130F0)
	{
		/* External RAM control */
		if (do_low_byte)
			clownmdemu->state->external_ram.mapped_in = low_byte != 0;
	}
	else if (address == 0xC00000 || address == 0xC00002)
	{
		/* VDP data port */
		VDP_WriteData(&clownmdemu->vdp, value, frontend_callbacks->colour_updated, frontend_callbacks->user_data);
	}
	else if (address == 0xC00004 || address == 0xC00006)
	{
		/* VDP control port */
		VDP_WriteControl(&clownmdemu->vdp, value, frontend_callbacks->colour_updated, frontend_callbacks->user_data, VDPReadCallback, callback_user_data, VDPKDebugCallback, NULL);
	}
	else if (address == 0xC00008)
	{
		/* H/V COUNTER */
		/* TODO */
	}
	else if (address >= 0xC00010 && address <= 0xC00016)
	{
		/* PSG */
		if (do_low_byte)
		{
			SyncZ80(clownmdemu, callback_user_data, target_cycle);
			SyncPSG(callback_user_data, target_cycle);

			/* Alter the PSG's state */
			PSG_DoCommand(&clownmdemu->psg, low_byte);
		}
	}
	else if (address >= 0xE00000 && address <= 0xFFFFFF)
	{
		/* 68k RAM */
		clownmdemu->state->m68k.ram[address_word & 0x7FFF] &= ~mask;
		clownmdemu->state->m68k.ram[address_word & 0x7FFF] |= value & mask;
	}
	else
	{
		LOG_MAIN_CPU_BUS_ERROR_1("Attempted to write invalid 68k address 0x%" CC_PRIXFAST32, address);
	}
}

void M68kWriteCallback(const void* const user_data, const cc_u32f address, const cc_bool do_high_byte, const cc_bool do_low_byte, const cc_u16f value)
{
	CPUCallbackUserData* const callback_user_data = (CPUCallbackUserData*)user_data;

	M68kWriteCallbackWithCycle(user_data, address, do_high_byte, do_low_byte, value, MakeCycleMegaDrive(callback_user_data->sync.m68k.current_cycle));
}