ZeroRAMBank.h

/*
 * This file is part of libsidplayfp, a SID player engine.
 *
 * Copyright 2012-2015 Leandro Nini <drfiemost@users.sourceforge.net>
 * Copyright 2009-2014 VICE Project
 * Copyright 2010 Antti Lankila
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */
 
#ifndef ZERORAMBANK_H
#define ZERORAMBANK_H
 
#include <stdint.h>
 
#include "Bank.h"
#include "SystemRAMBank.h"
#include "pla.h"
 
#include "Event.h"
 
#include "sidcxx11.h"
 
namespace libsidplayfp
{
 
template <int Bit>
class dataBit
{
private:
    static constexpr event_clock_t C64_CPU6510_DATA_PORT_FALL_OFF_CYCLES =  350000;
    static constexpr event_clock_t C64_CPU8500_DATA_PORT_FALL_OFF_CYCLES = 1500000; // Curently unused
 
private:
    event_clock_t dataSetClk;
 
    bool isFallingOff;
 
    uint8_t dataSet;
 
public:
    void reset()
    {
        isFallingOff = false;
        dataSet = 0;
    }
 
    uint8_t readBit(event_clock_t phi2time)
    {
        if (isFallingOff && (dataSetClk < phi2time))
        {
            // discharge the "capacitor"
            reset();
        }
        return dataSet;
    }
 
    void writeBit(event_clock_t phi2time, uint8_t value)
    {
        dataSetClk = phi2time + C64_CPU6510_DATA_PORT_FALL_OFF_CYCLES;
        dataSet = value & (1 << Bit);
        isFallingOff = true;
    }
};
 
class ZeroRAMBank final : public Bank
{
private:
    // not emulated
    static bool const tape_sense = false;
 
private:
    PLA &pla;
 
    SystemRAMBank &ramBank;
 
 
    dataBit<6> dataBit6;
    dataBit<7> dataBit7;
 
 
    uint8_t dir;
    uint8_t data;
 
    uint8_t dataRead;
 
    uint8_t procPortPins;
 
private:
    void updateCpuPort()
    {
        // Update data pins for which direction is OUTPUT
        procPortPins = (procPortPins & ~dir) | (data & dir);
 
        dataRead = (data | ~dir) & (procPortPins | 0x17);
 
        pla.setCpuPort((data | ~dir) & 0x07);
 
        if ((dir & 0x20) == 0)
        {
            dataRead &= ~0x20;
        }
        if (tape_sense && (dir & 0x10) == 0)
        {
            dataRead &= ~0x10;
        }
    }
 
public:
    ZeroRAMBank(PLA &pla, SystemRAMBank &ramBank) :
        pla(pla),
        ramBank(ramBank)
    {}
 
    void reset()
    {
        dataBit6.reset();
        dataBit7.reset();
 
        dir = 0;
        data = 0x3f;
        dataRead = 0x3f;
        procPortPins = 0x3f;
 
        updateCpuPort();
    }
 
    uint8_t peek(uint_least16_t address) override
    {
        switch (address)
        {
        case 0:
            return dir;
        case 1:
        {
            uint8_t retval = dataRead;
 
            // for unused bits in input mode, the value comes from the "capacitor"
 
            // set real value of bit 6
            if (!(dir & 0x40))
            {
                retval &= ~0x40;
                retval |= dataBit6.readBit(pla.getPhi2Time());
            }
 
            // set real value of bit 7
            if (!(dir & 0x80))
            {
                retval &= ~0x80;
                retval |= dataBit7.readBit(pla.getPhi2Time());
            }
 
            return retval;
        }
        default:
            return ramBank.peek(address);
        }
    }
 
    void poke(uint_least16_t address, uint8_t value) override
    {
        switch (address)
        {
        case 0:
            // when switching an unused bit from output (where it contained a
            // stable value) to input mode (where the input is floating), some
            // of the charge is transferred to the floating input
 
            if (dir != value)
            {
                // check if bit 6 has flipped from 1 to 0
                if ((dir & 0x40) && !(value & 0x40))
                    dataBit6.writeBit(pla.getPhi2Time(), data);
 
                // check if bit 7 has flipped from 1 to 0
                if ((dir & 0x80) && !(value & 0x80))
                    dataBit7.writeBit(pla.getPhi2Time(), data);
 
                dir = value;
                updateCpuPort();
            }
 
            value = pla.getLastReadByte();
            break;
        case 1:
            // when writing to an unused bit that is output, charge the "capacitor",
            // otherwise don't touch it
 
            if (dir & 0x40)
                dataBit6.writeBit(pla.getPhi2Time(), value);
 
            if (dir & 0x80)
                dataBit7.writeBit(pla.getPhi2Time(), value);
 
            if (data != value)
            {
                data = value;
                updateCpuPort();
            }
 
            value = pla.getLastReadByte();
            break;
        default:
            break;
        }
 
        ramBank.poke(address, value);
    }
};
 
}
 
#endif