ROMHunterMk2.cc

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#include "ROMHunterMk2.hh"
#include "ranges.hh"
#include "serialize.hh"
#include <cassert>
 
/*
 
As reverse engineered by BiFi:
 
At 0x3FFF is a configuration register.
bit 1: page 1 control:
       0: ROM hunter ROM in page 1 for READs, writes ignored (default)
       1: writes go to RAM in page 1, mapper switching disabled
bits 2 and 0 is a mapper selector:
  00 = ASCII16 (default),
  01 = ASCII8,
  11 = Konami,
  10 = unknown (seems to be some Konami 16K variant)
bit 3: RAM write protect:
       0: write enable (default)
       1: write protect; also sets reads in page 1 to RAM (and not ROM)
 
When the ROM loads a MegaROM, bit 1 is left on 0. But for plain ROMs, it is set
to 1.
*/
 
namespace openmsx {
 
ROMHunterMk2::ROMHunterMk2(const DeviceConfig& config, Rom&& rom_)
        : MSXRom(config, std::move(rom_))
        , ram(config, getName() + " RAM", "ROM Hunter Mk 2 RAM", 0x40000)
{
        reset(getCurrentTime());
}
 
void ROMHunterMk2::reset(EmuTime::param /*time*/)
{
        configReg = 0;
        ranges::fill(bankRegs, 0);
        invalidateDeviceRCache(); // flush all to be sure
}
 
unsigned ROMHunterMk2::getRamAddr(unsigned addr) const
{
        unsigned page = (addr >> 13) - 2;
        assert(page < 4);
        unsigned bank = bankRegs[page];
        return (bank * 0x2000) + (addr & 0x1FFF);
}
 
const byte* ROMHunterMk2::getReadCacheLine(word addr) const
{
        // reads outside [0x4000, 0xC000) return 0xFF
        if ((addr < 0x4000) || (0xC000 <= addr)) {
                return unmappedRead.data();
        }
 
        // if bit 3 is 0, reads from page 1 come from the ROM, else from the RAM
        if (((configReg & 0b1000) == 0) && (addr < 0x8000)) {
                return &rom[addr & 0x1FFF];
        } else {
                return &ram[getRamAddr(addr)];
        }
}
 
byte ROMHunterMk2::peekMem(word addr, EmuTime::param /*time*/) const
{
        return *getReadCacheLine(addr);
}
 
byte ROMHunterMk2::readMem(word addr, EmuTime::param time)
{
        return peekMem(addr, time); // reads have no side effects
}
 
void ROMHunterMk2::writeMem(word addr, byte value, EmuTime::param /*time*/)
{
        // config register at address 0x3FFF
        if (addr == 0x3FFF) {
                configReg = value;
                invalidateDeviceRCache(0x4000, 0x8000);
                return;
        }
 
        // ignore (other) writes outside [0x4000, 0xC000)
        if ((addr < 0x4000) || (0xC000 <= addr)) {
                return;
        }
 
        // address is calculated before writes to other regions take effect
        unsigned ramAddr = getRamAddr(addr);
 
        // only write mapper registers if bit 1 is not set
        if ((configReg & 0b10) == 0) {
                // (possibly) write to bank registers
                switch (configReg & 0b101) {
                case 0b000: {
                        // ASCII-16
                        // Implemented similarly as the (much later) MFR SCC+,
                        // TODO did we verify that ROMHunterMk2 behaves the same?
                        //
                        // ASCII-16 uses all 4 bank registers and one bank
                        // switch changes 2 registers at once. This matters
                        // when switching mapper mode, because the content of
                        // the bank registers is unchanged after a switch.
                        const byte maskedValue = value & 0xF;
                        if ((0x6000 <= addr) && (addr < 0x6800)) {
                                bankRegs[0] = 2 * maskedValue + 0;
                                bankRegs[1] = 2 * maskedValue + 1;
                                invalidateDeviceRCache(0x4000, 0x4000);
                        }
                        if ((0x7000 <= addr) && (addr < 0x7800)) {
                                bankRegs[2] = 2 * maskedValue + 0;
                                bankRegs[3] = 2 * maskedValue + 1;
                                invalidateDeviceRCache(0x8000, 0x4000);
                        }
                        break;
                }
                case 0b001:
                        // ASCII-8
                        if ((0x6000 <= addr) && (addr < 0x8000)) {
                                byte bank = (addr >> 11) & 0x03;
                                bankRegs[bank] = value & 0x1F;
                                invalidateDeviceRCache(0x4000 + 0x2000 * bank, 0x2000);
                        }
                        break;
                case 0b101:
                        // Konami
                        if ((0x6000 <= addr) && (addr < 0xC000)) {
                                unsigned bank = (addr >> 13) - 2;
                                bankRegs[bank] = value & 0x1F;
                                invalidateDeviceRCache(0x4000 + 0x2000 * bank, 0x2000);
                        }
                        break;
                case 0b100:
                        // TODO how does this configuration behave?
                        break;
                }
        }
 
        // write to RAM, if not write-protected
        if ((configReg & 0b1000) == 0) {
                // if write to [0x8000, 0xC000), just do it
                // if write to [0x4000, 0x8000), only do it if bit 1 is set
                if ((addr >= 0x8000) || ((configReg & 0b10) == 0b10)) {
                        ram[ramAddr] = value;
                }
        }
}
 
byte* ROMHunterMk2::getWriteCacheLine(word /*addr*/) const
{
        return nullptr;
}
 
template<typename Archive>
void ROMHunterMk2::serialize(Archive& ar, unsigned /*version*/)
{
        // skip MSXRom base class
        ar.template serializeBase<MSXDevice>(*this);
        ar.serialize("ram",       ram,
                     "configReg", configReg,
                     "bankRegs",  bankRegs);
}
INSTANTIATE_SERIALIZE_METHODS(ROMHunterMk2);
REGISTER_MSXDEVICE(ROMHunterMk2, "ROMHunterMk2");
 
} // namespace openmsx