/// <summary>
/// Skips the next instruction (two bytes) if V[x] != V[y].
/// </summary>
void SkipIfXNotEqualY(OpCodeData data)
{
if (V[data.X] != V[data.Y])
{
PC += 2;
}
}
/// <summary>
/// Skips the next instruction (two bytes) if V[x] == nn.
/// </summary>
void SkipIfXEqual(OpCodeData data)
{
if (V[data.X] == data.NN)
{
PC += 2;
}
}
/// <summary>
/// Skips the next instruction (two bytes) if V[x] != nn.
/// </summary>
void SkipIfXNotEqual(OpCodeData data)
{
if (V[data.X] != data.NN)
{
PC += 2;
}
}
// Misc has its own dictionary because it's full of random stuff.
void Misc(OpCodeData data)
{
if (opCodesMisc.ContainsKey(data.NN))
{
opCodesMisc[data.NN](data);
}
}
/// <summary>
/// Draws an n-byte sprite from register I at V[x], V[y]. Sets V[0xF] if it collides.
/// </summary>
void DrawSprite(OpCodeData data)
{
var startX = V[data.X];
var startY = V[data.Y];
Debug.WriteLine(string.Format("Drawing {0}-line sprite from {1} at {2}, {3}", data.N, I, startX, startY));
V[0xF] = 0;
for (var i = 0; i < data.N; i++)
{
var spriteLine = RAM[I + i]; // A line of the sprite to render
for (var bit = 0; bit < 8; bit++)
{
var x = (startX + bit) % ScreenWidth;
var y = (startY + i) % ScreenHeight;
var spriteBit = ((spriteLine >> (7 - bit)) & 1);
var oldBit = buffer[x, y] ? 1 : 0;
// New bit is XOR of existing and new.
var newBit = oldBit ^ spriteBit;
buffer[x, y] = newBit != 0;
// If we wiped out a pixel, set flag for collission.
if (oldBit != 0 && newBit == 0)
{
V[0xF] = 1;
}
}
}
}
/// <summary>
/// Saves all registers to the address in register I.
/// </summary>
void SaveX(OpCodeData data)
{
for (var i = 0; i <= data.X; i++)
{
RAM[I + i] = V[i];
}
}
/// <summary>
/// Loads all registers from the address in register I.
/// </summary>
void LoadX(OpCodeData data)
{
for (var i = 0; i <= data.X; i++)
{
V[i] = RAM[I + i];
}
}
/// <summary>
/// Skips the next instruction based on the key at V[x] being pressed/not pressed.
/// </summary>
void SkipOnKey(OpCodeData data)
{
if (
(data.NN == 0x9E && pressedKeys.Contains(V[data.X])) || // 9E = IfKeyPressed
(data.NN == 0xA1 && !pressedKeys.Contains(V[data.X])) // A1 = IfKeyNotPressed
)
{
PC += 2;
}
}
/// <summary>
/// Waits for a key to be pressed by looping at the current instruction.
/// </summary>
void WaitForKey(OpCodeData data)
{
// If we have a key pressed, store it and more on.
if (pressedKeys.Count != 0)
{
V[data.X] = pressedKeys.First();
}
else
{
// Otherwise, wind the PC back so we will keep executing this instruction.
PC -= 2;
}
}
// http://devernay.free.fr/hacks/chip8/C8TECH10.HTM#3.1
/// <summary>
/// Handles 0x0... which either clears the screen or returns from a subroutine.
/// </summary>
void ClearOrReturn(OpCodeData data)
{
if (data.NN == 0xE0)
{
for (var x = 0; x < ScreenWidth; x++)
{
for (var y = 0; y < ScreenHeight; y++)
{
buffer[x, y] = false;
}
}
}
else if (data.NN == 0xEE)
{
PC = Pop();
}
}
/// <summary>
/// Sets V[x] to V[y].
/// </summary>
void Arithmetic(OpCodeData data)
{
switch (data.N)
{
case 0x0:
V[data.X] = V[data.Y];
break;
case 0x1:
V[data.X] |= V[data.Y];
break;
case 0x2:
V[data.X] &= V[data.Y];
break;
case 0x3:
V[data.X] ^= V[data.Y];
break;
case 0x4:
V[0xF] = (byte)(V[data.X] + V[data.Y] > 0xFF ? 1 : 0); // Set flag if we overflowed.
V[data.X] += V[data.Y];
break;
case 0x5:
V[0xF] = (byte)(V[data.X] > V[data.Y] ? 1 : 0); // Set flag if we underflowed.
V[data.X] -= V[data.Y];
break;
case 0x6:
V[0xF] = (byte)((V[data.X] & 0x1) != 0 ? 1 : 0); // Set flag if we shifted a 1 off the end.
V[data.X] /= 2; // Shift right.
break;
case 0x7: // Note: This is Y-X, 5 was X-Y.
V[0xF] = (byte)(V[data.Y] > V[data.X] ? 1 : 0); // Set flag if we underflowed.
V[data.Y] -= V[data.X];
break;
case 0xE:
V[0xF] = (byte)((V[data.X] & 0xF) != 0 ? 1 : 0); // Set flag if we shifted a 1 off the end.
V[data.X] *= 2; // Shift left.
break;
}
}
public void Tick()
{
// Read the two bytes of OpCode (big endian).
var opCode = (ushort)(RAM[PC++] << 8 | RAM[PC++]);
Debug.WriteLine((PC - 2).ToString("X4") + ": " + opCode.ToString("X4"));
// Split data into the possible formats the instruction might need.
// https://en.wikipedia.org/wiki/CHIP-8#Opcode_table
var op = new OpCodeData()
{
OpCode = opCode,
NNN = (ushort)(opCode & 0x0FFF),
NN = (byte)(opCode & 0x00FF),
N = (byte)(opCode & 0x000F),
X = (byte)((opCode & 0x0F00) >> 8),
Y = (byte)((opCode & 0x00F0) >> 4),
};
// Loop up the OpCode using the first nibble and execute.
opCodes[(byte)(opCode >> 12)](op);
}
/// <summary>
/// Takes the decimal representation of V[x] and puts each character into memory locations
/// starting at I (with a maximum of 3).
/// </summary>
void BinaryCodedDecimal(OpCodeData data)
{
RAM[I + 0] = (byte)((V[data.X] / 100) % 10);
RAM[I + 1] = (byte)((V[data.X] / 10) % 10);
RAM[I + 2] = (byte)(V[data.X] % 10);
}
/// <summary>
/// Sets I to the correct location of the font sprite V[x].
/// Each font sprite is 5 bytes long.
/// </summary>
void SetIForChar(OpCodeData data)
{
Debug.WriteLine(string.Format("Setting I to {0} to render a {1}", V[data.X] * 5, V[data.X].ToString("X")));
I = (ushort)(V[data.X] * 5); // 0 is at 0x0, 1 is at 0x5, ...
}
/// <summary>
/// Adds V[x] to register I.
/// </summary>
void AddXToI(OpCodeData data)
{
I += V[data.X];
}
/// <summary>
/// Play sound for V[x] 60ths of a second.
/// </summary>
void SetSound(OpCodeData data)
{
beep((int)(V[data.X] * (1000f / 60)));
}
/// <summary>
/// Sets the delay register to V[x].
/// </summary>
void SetDelay(OpCodeData data)
{
Delay = V[data.X];
}
/// <summary>
/// Sets V[x] to equal the Delay register.
/// </summary>
void SetXToDelay(OpCodeData data)
{
V[data.X] = Delay;
}
/// <summary>
/// Jumps to location nnn (not a subroutine, so old PC is not pushed to the stack).
/// </summary>
void Jump(OpCodeData data)
{
PC = data.NNN;
}
/// <summary>
/// Jumps to location nnn + v[0] (not a subroutine, so old PC is not pushed to the stack).
/// </summary>
void JumpWithOffset(OpCodeData data)
{
PC = (ushort)(data.NNN + V[0]);
}
/// <summary>
/// Jumps to subroutine nnn (unlike Jump, this pushes the previous PC to the stack to allow return).
/// </summary>
void CallSubroutine(OpCodeData data)
{
Push(PC);
PC = data.NNN;
}
/// <summary>
/// ANDs a random number with nn and stores in V[x].
/// </summary>
void Rnd(OpCodeData data)
{
V[data.X] = (byte)(rnd.Next(0, 256) & data.NN);
}
/// <summary>
/// Sets V[x] == nn.
/// </summary>
void SetX(OpCodeData data)
{
V[data.X] = data.NN;
}
/// <summary>
/// Adds nn to V[x].
/// </summary>
void AddX(OpCodeData data)
{
V[data.X] += data.NN; // TODO: Do we need to handle overflow?
}
/// <summary>
/// Sets the I register.
/// </summary>
void SetI(OpCodeData data)
{
I = data.NNN;
}
/// <summary>
/// Draws an n-byte sprite from register I at V[x], V[y]. Sets V[0xF] if it collides.
/// </summary>
void DrawSprite(OpCodeData data)
{
var startX = V[data.X];
var startY = V[data.Y];
//Debug.WriteLine(string.Format("Drawing {0}-line sprite from {1} at {2}, {3}", data.N, I, startX, startY));
// Write any pending clears
for (var x = 0; x < ScreenWidth; x++)
{
for (var y = 0; y < ScreenHeight; y++)
{
if (pendingClearBuffer[x, y])
{
if (buffer[x, y])
{
needsRedraw = true;
}
pendingClearBuffer[x, y] = false;
buffer[x, y] = false;
}
}
}
V[0xF] = 0;
for (var i = 0; i < data.N; i++)
{
var spriteLine = RAM[I + i]; // A line of the sprite to render
for (var bit = 0; bit < 8; bit++)
{
var x = (startX + bit) % ScreenWidth;
var y = (startY + i) % ScreenHeight;
var spriteBit = ((spriteLine >> (7 - bit)) & 1);
var oldBit = buffer[x, y] ? 1 : 0;
if (oldBit != spriteBit)
{
needsRedraw = true;
}
// New bit is XOR of existing and new.
var newBit = oldBit ^ spriteBit;
if (newBit != 0)
{
buffer[x, y] = true;
}
else // Otherwise write a pending clear
{
pendingClearBuffer[x, y] = true;
}
// If we wiped out a pixel, set flag for collission.
if (oldBit != 0 && newBit == 0)
{
V[0xF] = 1;
}
}
}
}