MusicalMemoryMapper.cc

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#include "MusicalMemoryMapper.hh"
#include "enumerate.hh"
#include "narrow.hh"
#include "serialize.hh"
#include "xrange.hh"
 
namespace openmsx {
 
static constexpr byte MEM_ACCESS_ENABLED = 1 << 7;
static constexpr byte SOUND_PORT_ENABLED = 1 << 6;
static constexpr byte PORT_ACCESS_DISABLED = 1 << 5;
static constexpr byte UNUSED = 1 << 4;
static constexpr byte WRITE_PROTECT = 0x0F;
 
MusicalMemoryMapper::MusicalMemoryMapper(const DeviceConfig& config)
        : MSXMemoryMapperBase(config)
        , sn76489(config)
{
}
 
void MusicalMemoryMapper::reset(EmuTime::param time)
{
        controlReg = 0x00;
 
        // MMM inits the page registers to 3, 2, 1, 0 instead of zeroes, so we
        // don't call the superclass implementation.
        for (auto [page, reg] : enumerate(registers)) {
                reg = byte(3 - page);
        }
 
        // Note: The actual SN76489AN chip does not have a reset pin. I assume
        //       MMM either powers off the chip on reset or suppresses its audio
        //       output. We instead keep the chip in a silent state until it is
        //       activated.
        sn76489.reset(time);
}
 
byte MusicalMemoryMapper::readIO(word port, EmuTime::param time)
{
        if (controlReg & PORT_ACCESS_DISABLED) {
                return 0xFF;
        } else {
                return MSXMemoryMapperBase::readIO(port, time);
        }
}
 
byte MusicalMemoryMapper::peekIO(word port, EmuTime::param time) const
{
        if (controlReg & PORT_ACCESS_DISABLED) {
                return 0xFF;
        } else {
                return MSXMemoryMapperBase::peekIO(port, time);
        }
}
 
void MusicalMemoryMapper::writeIO(word port, byte value, EmuTime::param time)
{
        if ((port & 0xFC) == 0xFC) {
                // Mapper port.
                if (!(controlReg & PORT_ACCESS_DISABLED)) {
                        MSXMemoryMapperBase::writeIOImpl(port, value, time);
                        invalidateDeviceRWCache(0x4000 * (port & 0x03), 0x4000);
                }
        } else if (port & 1) {
                // Sound chip.
                if (controlReg & SOUND_PORT_ENABLED) {
                        sn76489.write(value, time);
                }
        } else {
                // Control port.
                // Only bit 7 of the control register is accessible through this port.
                // The documentation explicitly states that the sound chip port is still
                // accessible when port access is disabled, but this control port must
                // also remain accessible or the ROM2MMM loader won't work.
                updateControlReg((value & MEM_ACCESS_ENABLED) |
                                 (controlReg & ~MEM_ACCESS_ENABLED));
        }
}
 
bool MusicalMemoryMapper::registerAccessAt(word address) const
{
        if (controlReg & MEM_ACCESS_ENABLED) {
                if (controlReg & PORT_ACCESS_DISABLED) {
                        // Note: I'm assuming the mirroring is still active when the
                        //       address range is restricted, but this is unclear in
                        //       the documentation and not tested on real hardware.
                        if (controlReg & 0x08) {
                                return 0x4000 <= address && address < 0x8000;
                        } else {
                                return 0x8000 <= address && address < 0xC000;
                        }
                } else {
                        return 0x4000 <= address && address < 0xC000;
                }
        } else {
                return false;
        }
}
 
int MusicalMemoryMapper::readReg(word address) const
{
        if (registerAccessAt(address)) {
                switch (address & 0xFF) {
                case 0x3C:
                        return controlReg;
                case 0xFC:
                case 0xFD:
                case 0xFE:
                case 0xFF:
                        return 0xC0 | getSelectedSegment(address & 0x03);
                }
        }
        return -1;
}
 
void MusicalMemoryMapper::updateControlReg(byte value)
{
        // Note: Bit 4 is unused, but I don't know its value when read.
        //       For now, I'll force it to 0.
        value &= ~UNUSED;
 
        byte change = value ^ controlReg;
        if (change) {
                // Pages for which write protect is toggled must be invalidated.
                byte invalidate = change & WRITE_PROTECT;
 
                // Invalidate pages for which register access changes.
                byte regAccessBefore = 0;
                for (auto page : xrange(4)) {
                        regAccessBefore |= byte(registerAccessAt(narrow_cast<word>(0x4000 * page)) << page);
                }
                controlReg = value;
                byte regAccessAfter = 0;
                for (auto page : xrange(4)) {
                        regAccessAfter |= byte(registerAccessAt(narrow_cast<word>(0x4000 * page)) << page);
                }
                invalidate |= regAccessBefore ^ regAccessAfter;
 
                for (auto page : xrange(4)) {
                        if ((invalidate >> page) & 1) {
                                invalidateDeviceRWCache(0x4000 * page, 0x4000);
                        }
                }
        }
}
 
byte MusicalMemoryMapper::peekMem(word address, EmuTime::param time) const
{
        int reg = readReg(address);
        return reg >= 0 ? narrow<byte>(reg) : MSXMemoryMapperBase::peekMem(address, time);
}
 
byte MusicalMemoryMapper::readMem(word address, EmuTime::param time)
{
        int reg = readReg(address);
        return reg >= 0 ? narrow<byte>(reg) : MSXMemoryMapperBase::readMem(address, time);
}
 
void MusicalMemoryMapper::writeMem(word address, byte value, EmuTime::param time)
{
        if (registerAccessAt(address)) {
                switch (address & 0xFF) {
                case 0x3C:
                        // When port access is disabled, memory access is automatically
                        // enabled, according to the manual.
                        if (value & PORT_ACCESS_DISABLED) {
                                value |= MEM_ACCESS_ENABLED;
                        }
                        updateControlReg(value);
                        return;
                case 0xFC:
                case 0xFD:
                case 0xFE:
                case 0xFF:
                        MSXMemoryMapperBase::writeIOImpl(address & 0xFF, value, time);
                        invalidateDeviceRWCache(0x4000 * (address & 0x03), 0x4000);
                        return;
                }
        }
        if (!writeProtected(address)) {
                MSXMemoryMapperBase::writeMem(address, value, time);
        }
}
 
const byte* MusicalMemoryMapper::getReadCacheLine(word start) const
{
        if (controlReg & MEM_ACCESS_ENABLED) {
                if (0x4000 <= start && start < 0xC000) {
                        return nullptr;
                }
        }
        return MSXMemoryMapperBase::getReadCacheLine(start);
}
 
byte* MusicalMemoryMapper::getWriteCacheLine(word start) const
{
        if (controlReg & MEM_ACCESS_ENABLED) {
                if (0x4000 <= start && start < 0xC000) {
                        return nullptr;
                }
        }
        if (writeProtected(start)) {
                return unmappedWrite.data();
        } else {
                return MSXMemoryMapperBase::getWriteCacheLine(start);
        }
}
 
template<typename Archive>
void MusicalMemoryMapper::serialize(Archive& ar, unsigned /*version*/)
{
        ar.template serializeBase<MSXMemoryMapperBase>(*this);
        ar.serialize("ctrl",    controlReg,
                     "sn76489", sn76489);
}
INSTANTIATE_SERIALIZE_METHODS(MusicalMemoryMapper);
REGISTER_MSXDEVICE(MusicalMemoryMapper, "MusicalMemoryMapper");
 
} // namespace openmsx