//todo: does data mirroring occur for all VDP addresses; or just data/control ports?

auto VDP::readByte(uint24 addr) -> uint8 {
  auto data = readWord(addr & ~1);
  return data << 8 | data << 0;
}

auto VDP::writeByte(uint24 addr, uint8 data) -> void {
  return writeWord(addr & ~1, data << 8 | data << 0);
}

//

auto VDP::readWord(uint24 addr) -> uint16 {
  switch(addr & 0xc0001f) {

  //data port
  case 0xc00000: case 0xc00002: {
    return readDataPort();
  }

  //control port
  case 0xc00004: case 0xc00006: {
    return readControlPort();
  }

  }

  return 0x0000;
}

auto VDP::writeWord(uint24 addr, uint16 data) -> void {
//print("[VDP] ", hex(addr, 6L), "=", hex(data, 4L), "\n");

  switch(addr & 0xc0001f) {

  //data port
  case 0xc00000: case 0xc00002: {
    return writeDataPort(data);
  }

  //control port
  case 0xc00004: case 0xc00006: {
    return writeControlPort(data);
  }

  }
}

//

auto VDP::readDataPort() -> uint16 {
  io.commandPending = false;

  //VRAM read
  if(io.command.bits(0,3) == 0) {
    return 0x0000;
  }

  //VSRAM read
  if(io.command.bits(0,3) == 4) {
    return 0x0000;
  }

  //CRAM read
  if(io.command.bits(0,3) == 8) {
    return 0x0000;
  }
}

auto VDP::writeDataPort(uint16 data) -> void {
  io.commandPending = false;

  //DMA VRAM fill
  if(io.command.bits(4,5) == 2) {
    io.dmaActive = true;
    io.dmaFillWord = data;
    return;
  }

  //VRAM write
  if(io.command.bits(0,3) == 1) {
    auto address = io.address.bits(1,15);
    if(io.address.bit(0)) data = data >> 8 | data << 8;
    vram[address] = data;
    io.address += io.dataIncrement;
    return;
  }

  //VSRAM write
  if(io.command.bits(0,3) == 5) {
    auto address = io.address.bits(1,6);
    if(address >= 40) return;
    //data format: ---- --yy yyyy yyyy
    vsram[address] = data.bits(0,9);
    io.address += io.dataIncrement;
    return;
  }

  //CRAM write
  if(io.command.bits(0,3) == 3) {
    auto address = io.address.bits(1,6);
    //data format: ---- bbb- ggg- rrr-
    cram[address] = data.bits(1,3) << 0 | data.bits(5,7) << 3 | data.bits(9,11) << 6;
    io.address += io.dataIncrement;
    return;
  }
}

//

auto VDP::readControlPort() -> uint16 {
  io.commandPending = false;

  uint16 result = 0b0011'0100'0000'0000;
  result |= io.dmaActive << 1;
  return result;
}

auto VDP::writeControlPort(uint16 data) -> void {
//print("[VDPC] ", hex(data, 4L), "\n");

  //command write (lo)
  if(io.commandPending) {
    io.commandPending = false;

    io.command.bits(2,5) = data.bits(4,7);
    io.address.bits(14,15) = data.bits(0,1);

    return;
  }

  //command write (hi)
  if(data.bits(14,15) != 2) {
    io.commandPending = true;

    io.command.bits(0,1) = data.bits(14,15);
    io.address.bits(0,13) = data.bits(0,13);
    return;
  }

  //register write (d13 is ignored)
  if(data.bits(14,15) == 2)
  switch(data.bits(8,12)) {

  //mode register 1
  case 0x00: {
    io.displayOverlayEnable = data.bit(0);
    io.counterLatch = data.bit(1);
    io.horizontalInterruptEnable = data.bit(4);
    io.leftColumnBlank = data.bit(5);
    return;
  }

  //mode register 2
  case 0x01: {
    io.videoMode = data.bit(2);
    io.overscan = data.bit(3);
    io.dmaEnable = data.bit(4);
    io.verticalBlankInterruptEnable = data.bit(5);
    io.displayEnable = data.bit(6);
    io.externalVRAM = data.bit(7);

    if(!io.dmaEnable) io.command.bit(5) = 0;

    return;
  }

  //plane A name table location
  case 0x02: {
    planeA.io.nametableAddress = data.bits(3,6) << 12;
    return;
  }

  //window name table location
  case 0x03: {
    window.io.nametableAddress = data.bits(1,6) << 10;
    return;
  }

  //plane B name table location
  case 0x04: {
    planeB.io.nametableAddress = data.bits(0,3) << 12;
    return;
  }

  //sprite attribute table location
  case 0x05: {
    io.attrtableSprite = data.bits(0,7);
    return;
  }

  //sprite pattern base address
  case 0x06: {
    io.nametableBaseSprite = data.bit(5);
    return;
  }

  //background color
  case 0x07: {
    io.backgroundColor = data.bits(0,5);
    return;
  }

  //horizontal interrupt counter
  case 0x0a: {
    io.horizontalInterruptCounter = data.bits(0,7);
    return;
  }

  //mode register 3
  case 0x0b: {
    io.horizontalScrollMode = data.bits(0,1);
    io.verticalScrollMode = data.bit(2);
    io.externalInterruptEnable = data.bit(3);
    return;
  }

  //mode register 4
  case 0x0c: {
    io.tileWidth = data.bit(0) | data.bit(7) << 1;
    io.interlaceMode = data.bits(1,2);
    io.shadowHighlightEnable = data.bit(3);
    io.externalColorEnable = data.bit(4);
    io.horizontalSync = data.bit(5);
    io.verticalSync = data.bit(6);
    return;
  }

  //horizontal scroll data location
  case 0x0d: {
    io.horizontalScrollTable = data.bits(1,6);
    return;
  }

  //nametable pattern base address
  case 0x0e: {
    io.nametableBasePatternA = data.bit(0);
    io.nametableBasePatternB = data.bit(1);
    return;
  }

  //data port auto-increment value
  case 0x0f: {
    io.dataIncrement = data.bits(0,7);
    return;
  }

  //plane size
  case 0x10: {
    //0 = 32 tiles
    //1 = 64 tiles
    //2 = invalid (repeats first row for every scanline)
    //3 = 128 tiles
    static const uint shift[] = {5, 6, 0, 7};

    planeA.io.nametableWidth = shift[data.bits(0,1)];
    window.io.nametableWidth = shift[data.bits(0,1)];
    planeB.io.nametableWidth = shift[data.bits(0,1)];

    planeA.io.nametableHeight = shift[data.bits(4,5)];
    window.io.nametableHeight = shift[data.bits(4,5)];
    planeB.io.nametableHeight = shift[data.bits(4,5)];

    return;
  }

  //window plane horizontal position
  case 0x11: {
    if(!data) {
      //disable
      io.windowHorizontalLo = ~0;
      io.windowHorizontalHi = ~0;
    } else if(data.bit(7) == 0) {
      //left
      io.windowHorizontalLo = 0;
      io.windowHorizontalHi = (data.bits(0,4) << 4) - 1;
    } else {
      //right
      io.windowHorizontalLo = (data.bits(0,4) << 4) - 1;
      io.windowHorizontalHi = ~0;
    }
    return;
  }

  //window plane vertical position
  case 0x12: {
    if(!data) {
      //disable
      io.windowVerticalLo = ~0;
      io.windowVerticalHi = ~0;
    } else if(data.bit(7) == 0) {
      //up
      io.windowVerticalLo = 0;
      io.windowVerticalHi = (data.bits(0,4) << 3) - 1;
    } else {
      //down
      io.windowVerticalLo = (data.bits(0,4) << 3) - 1;
      io.windowVerticalHi = ~0;
    }
    return;
  }

  //DMA length
  case 0x13: {
    io.dmaLength.bits(0,7) = data.bits(0,7);
    return;
  }

  //DMA length
  case 0x14: {
    io.dmaLength.bits(8,15) = data.bits(0,7);
    return;
  }

  //DMA source
  case 0x15: {
    io.dmaSource.bits(0,7) = data.bits(0,7);
    return;
  }

  //DMA source
  case 0x16: {
    io.dmaSource.bits(8,15) = data.bits(0,7);
    return;
  }

  //DMA source
  case 0x17: {
    io.dmaSource.bits(16,21) = data.bits(0,5);
    io.dmaMode = data.bits(6,7);
    return;
  }

  //unused
  default: {
    return;
  }

  }
}