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◆ main openmsx / src / memory / RomGameMaster2.cc
3.5 KiB · C++ · last changed chore(): initial commit 1 month ago source blame history raw
e8555c89 Andy McCall 1 
// GAME MASTER 2
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//
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// This is a 1 megabit ROM cartridge with 8 Kb SRAM. Because of the SRAM,
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// the mappers have special features.
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//
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// Since the size of the mapper is 8Kb, the memory banks are:
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//   Bank 1: 4000h - 5FFFh
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//   Bank 2: 6000h - 7FFFh
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//   Bank 3: 8000h - 9FFFh
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//   Bank 4: A000h - BFFFh
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//
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// And the addresses to change banks:
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//   Bank 1: <none>
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//   Bank 2: 6000h - 6FFFh (6000h used)
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//   Bank 3: 8000h - 8FFFh (8000h used)
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//   Bank 4: A000h - AFFFh (A000h used)
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//   SRAM write: B000h - BFFFh
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//
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// If SRAM is selected in bank 4, you can write to it in the memory area
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// B000h - BFFFh.
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//
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// The value you write to change banks also determines whether you select ROM
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// or SRAM. SRAM can be in any memory bank (except bank 1 which can't be
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// modified) but it can only be written too in bank 4.
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//
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//   bit      |  0 |  1 |  2 |  3 |      4       |  5 | 6 | 7 |
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//   ----------------------------------------------------------
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//   function | R0 | R1 | R2 | R3 | 1=SRAM/0=ROM | S0 | X | X |
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//
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// If bit 4 is reset, bits 0 - 3 select the ROM page as you would expect them
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// to do. Bits 5 - 7 are ignored now. If bit 4 is set, bit 5 selects the SRAM
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// page (first or second 4Kb of the 8Kb). Bits 6 - 7 and bits 0 - 3 are
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// ignored now.
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//
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// Since you can only select 4Kb of the SRAM at once in a memory bank and a
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// memory bank is 8Kb in size, the first and second 4Kb of the memory bank
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// read the same 4Kb of SRAM if SRAM is selected.
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#include "RomGameMaster2.hh"
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#include "SRAM.hh"
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#include "serialize.hh"
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#include "xrange.hh"
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#include <memory>
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namespace openmsx {
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RomGameMaster2::RomGameMaster2(const DeviceConfig& config, Rom&& rom_)
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	: Rom4kBBlocks(config, std::move(rom_), 1)
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{
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	sram = std::make_unique<SRAM>(getName() + " SRAM", 0x2000, config);
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	reset(EmuTime::dummy());
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}
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void RomGameMaster2::reset(EmuTime /*time*/)
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{
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	for (auto i : xrange(4)) {
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		setUnmapped(i);
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	}
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	for (auto i : xrange(4, 12)) {
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		setRom(i, i - 4);
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	}
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	for (auto i : xrange(12, 16)) {
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		setUnmapped(i);
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	}
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	sramOffset = 0;
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	sramEnabled = false;
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}
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void RomGameMaster2::writeMem(uint16_t address, byte value, EmuTime /*time*/)
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{
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	if ((0x6000 <= address) && (address < 0xB000)) {
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		if (!(address & 0x1000)) {
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			auto region = address >> 12; // 0x6, 0x8 or 0xA
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			if (region == 0x0A) {
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				sramEnabled = (value & 0x10) != 0;
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				invalidateDeviceWCache(0xB000, 0x1000); // 'R' is handled below
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			}
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			if (value & 0x10) {
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				// switch SRAM
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				sramOffset = (value & 0x20) ? 0x1000: 0x0000;
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				setBank(region,     &(*sram)[sramOffset], value);
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				setBank(region + 1, &(*sram)[sramOffset], value);
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			} else {
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				// switch ROM
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				setRom(region,     2 * (value & 0x0F));
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				setRom(region + 1, 2 * (value & 0x0F) + 1);
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			}
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		}
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	} else if ((0xB000 <= address) && (address < 0xC000)) {
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		// write SRAM
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		if (sramEnabled) {
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			sram->write(sramOffset | (address & 0x0FFF), value);
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		}
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	}
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}
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byte* RomGameMaster2::getWriteCacheLine(uint16_t address)
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{
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	if ((0x6000 <= address) && (address < 0xB000)) {
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		if (!(address & 0x1000)) {
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			return nullptr;
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		} else {
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			return unmappedWrite.data();
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		}
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	} else if ((0xB000 <= address) && (address < 0xC000) && sramEnabled) {
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		// write SRAM
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		return nullptr;
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	} else {
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		return unmappedWrite.data();
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	}
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}
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template<typename Archive>
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void RomGameMaster2::serialize(Archive& ar, unsigned /*version*/)
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{
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	ar.template serializeBase<Rom4kBBlocks>(*this);
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	ar.serialize("sramOffset",  sramOffset,
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	             "sramEnabled", sramEnabled);
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}
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INSTANTIATE_SERIALIZE_METHODS(RomGameMaster2);
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REGISTER_MSXDEVICE(RomGameMaster2, "RomGameMaster2");
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} // namespace openmsx