auto CPU::keypadRead() -> uint4 {
  uint4 data = 0;

  if(r.ypadEnable) {
    data |= system.keypad.y1 << 0;
    data |= system.keypad.y2 << 1;
    data |= system.keypad.y3 << 2;
    data |= system.keypad.y4 << 3;
  }

  if(r.xpadEnable) {
    data |= system.keypad.x1 << 0;
    data |= system.keypad.x2 << 1;
    data |= system.keypad.x3 << 2;
    data |= system.keypad.x4 << 3;
  }

  if(r.buttonEnable) {
    data |= system.keypad.start << 1;
    data |= system.keypad.a     << 2;
    data |= system.keypad.b     << 3;
  }

  return data;
}

auto CPU::portRead(uint16 addr) -> uint8 {
  //DMA_SRC
  if(addr == 0x0040) return r.dmaSource.byte(0);
  if(addr == 0x0041) return r.dmaSource.byte(1);
  if(addr == 0x0042) return r.dmaSource.byte(2);

  //DMA_DST
  if(addr == 0x0044) return r.dmaTarget.byte(0);
  if(addr == 0x0045) return r.dmaTarget.byte(1);

  //DMA_LEN
  if(addr == 0x0046) return r.dmaLength.byte(0);
  if(addr == 0x0047) return r.dmaLength.byte(1);

  //DMA_CTRL
  if(addr == 0x0048) return r.dmaEnable << 7 | r.dmaMode << 0;

  //WSC_SYSTEM
  if(addr == 0x0062) {
    return (system.model() == Model::SwanCrystal) << 7;
  }

  //HW_FLAGS
  if(addr == 0x00a0) {
    bool model = system.model() != Model::WonderSwan;
    return (
      1 << 7      //1 = built-in self-test passed
    | 1 << 2      //0 = 8-bit bus width; 1 = 16-bit bus width
    | model << 1  //0 = WonderSwan; 1 = WonderSwan Color or SwanCrystal
    | 1 << 0      //0 = BIOS mapped; 1 = cartridge mapped
    );
  }

  //INT_BASE
  if(addr == 0x00b0) {
    if(system.model() == Model::WonderSwan) return r.interruptBase | 3;
    return r.interruptBase;
  }

  //SER_DATA
  if(addr == 0x00b1) return r.serialData;

  //INT_ENABLE
  if(addr == 0x00b2) return r.interruptEnable;

  //SER_STATUS
  if(addr == 0x00b3) return (
    1                << 2  //hack: always report send buffer as empty
  | r.serialBaudRate << 6
  | r.serialEnable   << 7
  );

  //INT_STATUS
  if(addr == 0x00b4) return r.interruptStatus;

  //KEYPAD
  if(addr == 0x00b5) return (
    keypadRead()   << 0
  | r.ypadEnable   << 4
  | r.xpadEnable   << 5
  | r.buttonEnable << 6
  );

  return 0x00;
}

auto CPU::portWrite(uint16 addr, uint8 data) -> void {
  //DMA_SRC
  if(addr == 0x0040) { r.dmaSource.byte(0) = data & ~1; return; }
  if(addr == 0x0041) { r.dmaSource.byte(1) = data; return; }
  if(addr == 0x0042) { r.dmaSource.byte(2) = data; return; }

  //DMA_DST
  if(addr == 0x0044) { r.dmaTarget.byte(0) = data & ~1; return; }
  if(addr == 0x0045) { r.dmaTarget.byte(1) = data; return; }

  //DMA_LEN
  if(addr == 0x0046) { r.dmaLength.byte(0) = data & ~1; return; }
  if(addr == 0x0047) { r.dmaLength.byte(1) = data; return; }

  //DMA_CTRL
  if(addr == 0x0048) {
    r.dmaEnable = data.bit(7);
    r.dmaMode   = data.bit(0);
    if(r.dmaEnable) dmaTransfer();
    return;
  }

  //WSC_SYSTEM
  if(addr == 0x0062) {
    //todo: d0 = 1 powers off system
    return;
  }

  //HW_FLAGS
  if(addr == 0x00a0) {
    //todo: d2 (bus width) bit is writable; but ... it will do very bad things
    return;
  }

  //INT_BASE
  if(addr == 0x00b0) {
    r.interruptBase = (system.model() == Model::WonderSwan) ? data & ~7 : data & ~1;
    return;
  }

  //SER_DATA
  if(addr == 0x00b1) r.serialData = data;

  //INT_ENABLE
  if(addr == 0x00b2) {
    r.interruptEnable = data;
    return;
  }

  //SER_STATUS
  if(addr == 0x00b3) {
    r.serialBaudRate = data.bit(6);
    r.serialEnable   = data.bit(7);
  }

  //KEYPAD
  if(addr == 0x00b5) {
    r.ypadEnable   = data.bit(4);
    r.xpadEnable   = data.bit(5);
    r.buttonEnable = data.bit(6);
  }

  //INT_ACK
  if(addr == 0x00b6) {
    //acknowledge only edge-sensitive interrupts
    r.interruptStatus &= ~(data & 0b11110010);
    return;
  }
}