b8rk/src/chip8.cpp

#include "chip8.hpp"
#include "dbc.hpp"
#include <fmt/core.h>

Chip8::Chip8() :
  randGen(std::chrono::system_clock::now().time_since_epoch().count())
{
  pc = START_ADDRESS;

  for(size_t i = 0; i < FONTSET_SIZE; ++i) {
    memory[FONTSET_START_ADDRESS + i] = fontset[i];
  }

  randByte = std::uniform_int_distribution<uint8_t>(0, 255U);

  // Set up function pointer table
  table[0x0] = &Chip8::Table0;
  table[0x1] = &Chip8::OP_1nnn;
  table[0x2] = &Chip8::OP_2nnn;
  table[0x3] = &Chip8::OP_3xkk;
  table[0x4] = &Chip8::OP_4xkk;
  table[0x5] = &Chip8::OP_5xy0;
  table[0x6] = &Chip8::OP_6xkk;
  table[0x7] = &Chip8::OP_7xkk;
  table[0x8] = &Chip8::Table8;
  table[0x9] = &Chip8::OP_9xy0;
  table[0xA] = &Chip8::OP_Annn;
  table[0xB] = &Chip8::OP_Bnnn;
  table[0xC] = &Chip8::OP_Cxkk;
  table[0xD] = &Chip8::OP_Dxyn;
  table[0xE] = &Chip8::TableE;
  table[0xF] = &Chip8::TableF;

  for (size_t i = 0; i <= 0xE; i++)
  {
    table0[i] = &Chip8::OP_NULL;
    table8[i] = &Chip8::OP_NULL;
    tableE[i] = &Chip8::OP_NULL;
  }

  table0[0x0] = &Chip8::OP_00E0;
  table0[0xE] = &Chip8::OP_00EE;

  table8[0x0] = &Chip8::OP_8xy0;
  table8[0x1] = &Chip8::OP_8xy1;
  table8[0x2] = &Chip8::OP_8xy2;
  table8[0x3] = &Chip8::OP_8xy3;
  table8[0x4] = &Chip8::OP_8xy4;
  table8[0x5] = &Chip8::OP_8xy5;
  table8[0x6] = &Chip8::OP_8xy6;
  table8[0x7] = &Chip8::OP_8xy7;
  table8[0xE] = &Chip8::OP_8xyE;

  tableE[0x1] = &Chip8::OP_ExA1;
  tableE[0xE] = &Chip8::OP_Ex9E;

  for (size_t i = 0; i <= 0x65; i++)
  {
    tableF[i] = &Chip8::OP_NULL;
  }

  tableF[0x07] = &Chip8::OP_Fx07;
  tableF[0x0A] = &Chip8::OP_Fx0A;
  tableF[0x15] = &Chip8::OP_Fx15;
  tableF[0x18] = &Chip8::OP_Fx18;
  tableF[0x1E] = &Chip8::OP_Fx1E;
  tableF[0x29] = &Chip8::OP_Fx29;
  tableF[0x33] = &Chip8::OP_Fx33;
  tableF[0x55] = &Chip8::OP_Fx55;
  tableF[0x65] = &Chip8::OP_Fx65;
}

void Chip8::LoadROM(const std::string& filename) {
  std::ifstream file(filename, std::ios::binary | std::ios::ate);
  dbc::check(file.is_open(), $F("Can't open {}", filename));

  size_t size = file.tellg();
  char* buffer = new char[size];
  file.seekg(0, std::ios::beg);
  file.read(buffer, size);
  file.close();

  for(size_t i = 0; i < size; ++i) {
    memory[START_ADDRESS + i] = buffer[i];
  }

  delete[] buffer;
}

void Chip8::Cycle() {
  // Fetch
  opcode = (memory[pc] << 8u) | memory[pc + 1];

  // Increment the PC before we execute anything
  pc += 2;

  // decode and Execute
  ((*this).*(table[(opcode & 0xF000u) >> 12u]))();

  // Decrement the delay timer if it's been set
  if(delayTimer > 0) {
    --delayTimer;
  }

  // decrement the sound timer if it's been set
  if(soundTimer > 0) {
    --soundTimer;
  }
}


void Chip8::OP_00E0() {
  memset(video, 0, sizeof(video));
}

void Chip8::OP_00EE() {
  --sp;
  pc = stack[sp];
}

void Chip8::OP_1nnn() {
  uint16_t address = opcode & 0x0FFFu;
  pc = address;
}

void Chip8::OP_2nnn() {
  uint16_t address = opcode & 0x0FFFu;
  stack[sp] = pc;
  ++sp;
  pc = address;
}

void Chip8::OP_3xkk()
{
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t byte = opcode & 0x00FFu;

  if(registers[Vx] == byte) {
    pc += 2;
  }
}

void Chip8::OP_4xkk()
{
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t byte = opcode & 0x00FFu;

  if(registers[Vx] != byte) {
    pc += 2;
  }
}

void Chip8::OP_5xy0()
{
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;

  if(registers[Vx] == registers[Vy]) {
    pc += 2;
  }
}

void Chip8::OP_6xkk() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t byte = opcode & 0x00FFu;

  registers[Vx] = byte;
}

void Chip8::OP_7xkk() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t byte = opcode & 0x00FFu;

  registers[Vx] += byte;
}


void Chip8::OP_8xy0() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;
  registers[Vx] = registers[Vy];
}

void Chip8::OP_8xy1()
{
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;

  registers[Vx] |= registers[Vy];
}

void Chip8::OP_8xy2()
{
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;

  registers[Vx] &= registers[Vy];
}

void Chip8::OP_8xy3()
{
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;

  registers[Vx] ^= registers[Vy];
}

void Chip8::OP_8xy4() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;

  uint16_t sum = registers[Vx] + registers[Vy];
  registers[0xF] = sum > 255U ? 1 : 0;

  registers[Vx] = sum & 0xFFu;
}

void Chip8::OP_8xy5()
{
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;

  registers[0xF] = registers[Vx] > registers[Vy] ? 1 : 0;
  registers[Vx] -= registers[Vy];
}

void Chip8::OP_8xy6()
{
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;

  // Save LSB in VF
  registers[0xF] = (registers[Vx] & 0x1u);

  registers[Vx] >>= 1;
}

void Chip8::OP_8xy7() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;
  registers[0xF] = registers[Vy] > registers[Vx] ? 1 : 0;

  registers[Vx] = registers[Vy] - registers[Vx];
}

void Chip8::OP_8xyE() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;

  // Save MSB in VF
  registers[0xF] = (registers[Vx] & 0x80u) >> 7u;
  registers[Vx] <<= 1;
}

void Chip8::OP_9xy0() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;

  if(registers[Vx] != registers[Vy]) {
    pc += 2;
  }
}

void Chip8::OP_Annn() {
  uint16_t address = opcode & 0x0FFFu;
  index = address;
}

void Chip8::OP_Bnnn() {
  uint16_t address = opcode & 0x0FFFu;
  pc = registers[0] + address;
}

void Chip8::OP_Cxkk() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint16_t byte = opcode & 0x0FFFu;

  registers[Vx] = randByte(randGen) & byte;
}

void Chip8::OP_Dxyn() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t Vy = (opcode & 0x00F0u) >> 4u;
  uint8_t height = opcode & 0x00Fu;

  // wrap if going beyond the screen boundaries
  uint8_t xPos = registers[Vx] % VIDEO_WIDTH;
  uint8_t yPos = registers[Vy] % VIDEO_HEIGHT;

  registers[0xF] = 0;
  for(size_t row = 0; row < height; ++row) {
    uint8_t spriteByte = memory[index + row];
    for(size_t col = 0; col < 8; ++col) {
      uint8_t spritePixel = spriteByte & (0x80u >> col);
      uint32_t* screenPixel = &video[(yPos + row) * VIDEO_WIDTH + (xPos + col)];

      // Sprite pixel is on
      if(spritePixel) {
        // Screen pixel also on - collision
        if(*screenPixel == 0xFFFFFFFF) {
          registers[0xF] = 1;
        }

        *screenPixel ^= 0xFFFFFFFF;
      }
    }
  }
}

void Chip8::OP_Ex9E() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t key = registers[Vx];

  if(keypad[key]) {
    pc += 2;
  }
}

void Chip8::OP_ExA1() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t key = registers[Vx];

  if(!keypad[key]) {
    pc += 2;
  }
}

void Chip8::OP_Fx07() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;

  registers[Vx] = delayTimer;
}

void Chip8::OP_Fx0A() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;

  if (keypad[0])
  {
    registers[Vx] = 0;
  }
  else if (keypad[1])
  {
    registers[Vx] = 1;
  }
  else if (keypad[2])
  {
    registers[Vx] = 2;
  }
  else if (keypad[3])
  {
    registers[Vx] = 3;
  }
  else if (keypad[4])
  {
    registers[Vx] = 4;
  }
  else if (keypad[5])
  {
    registers[Vx] = 5;
  }
  else if (keypad[6])
  {
    registers[Vx] = 6;
  }
  else if (keypad[7])
  {
    registers[Vx] = 7;
  }
  else if (keypad[8])
  {
    registers[Vx] = 8;
  }
  else if (keypad[9])
  {
    registers[Vx] = 9;
  }
  else if (keypad[10])
  {
    registers[Vx] = 10;
  }
  else if (keypad[11])
  {
    registers[Vx] = 11;
  }
  else if (keypad[12])
  {
    registers[Vx] = 12;
  }
  else if (keypad[13])
  {
    registers[Vx] = 13;
  }
  else if (keypad[14])
  {
    registers[Vx] = 14;
  }
  else if (keypad[15])
  {
    registers[Vx] = 15;
  }
  else
  {
    pc -= 2;
  }
}

void Chip8::OP_Fx15() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  delayTimer = registers[Vx];
}

void Chip8::OP_Fx18() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  soundTimer = registers[Vx];
}

void Chip8::OP_Fx1E() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  index += registers[Vx];
}

void Chip8::OP_Fx29() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t digit = registers[Vx];

  index = FONTSET_START_ADDRESS + (5 * digit);
}

// WTF? binary coded decimal
void Chip8::OP_Fx33() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;
  uint8_t value = registers[Vx];

  memory[index + 2] = value % 10;
  value /= 10;

  memory[index + 1] = value % 10;
  value /= 10;

  memory[index] = value % 10;
}

void Chip8::OP_Fx55() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;

  for(uint8_t i = 0; i <= Vx; ++i) {
    memory[index + i] = registers[i];
  }
}

void Chip8::OP_Fx65() {
  uint8_t Vx = (opcode & 0x0F00u) >> 8u;

  for(uint8_t i = 0; i <= Vx; ++i) {
    registers[i] = memory[index + i];
  }
}