private void StartInstruction(Instruction instruction)
{
if (instruction == Instruction.RESET)
{
// Instruction count should be zero after a RESET
instructionCounter = 0;
}
else
{
// Any other internal instruction increments the instruction counter
instructionCounter++;
}
if (instruction == Instruction.FetchNewInstruction)
{
instructionOrigin = new InstructionOrigin(InstructionSource.Memory, null, PC);
}
else
{
instructionOrigin = InstructionOrigin.Internal;
}
currentInstruction = instruction;
DecodeInstructionExecutionMethod();
}
// Reset CPU control section and instruction decoder internal state
private void ResetCPUControlState()
{
// Z80CPU.cs internal state
tStateCounter = 0;
machineCycleCounter = 0;
instructionCounter = 0;
instructionOrigin = null;
currentInstruction = null;
executeInstruction = null;
halfTStateIndex = 0;
machineCycleIndex = 0;
machineCycleCountAfterInstruction = 0;
currentMachineCycle = null;
executeMachineCycle = null;
// Z80CPU_Instruction_Decoder.cs internal state
opcodeTableIndex = 0;
// Z80CPU_ExternalControl.cs internal state
busRequestPending = false;
maskableInterruptPending = false;
nonMaskableInterruptPending = false;
}
// Read instruction register and decode instruction
private void DecodeInstruction()
{
if (instructionOrigin.Source != InstructionSource.Internal) // internal instruction already decoded, nothing to do
{
InstructionCode candidateInstructionCode = Z80OpCodes.Tables[opcodeTableIndex, IR];
// Ignore DD and FD prefixes if the second byte of the opcode does not lead to a valid instruction
if (candidateInstructionCode.FetchMoreBytes == 0 && candidateInstructionCode.SwitchToTableNumber == 0)
{
// Ignore the prefix and try to find the candidate instruction in the first opcode table
candidateInstructionCode = Z80OpCodes.Tables[0, IR];
// The previous machine cycle did not contribute to the new instruction
machineCycleIndex--;
instructionOrigin.Address++;
}
// An opcode was recognized, decode it and prepare its execution
if (candidateInstructionCode.FetchMoreBytes == 0 && candidateInstructionCode.SwitchToTableNumber == null)
{
opcodeTableIndex = 0;
InstructionCode instructionCode = candidateInstructionCode;
// Register decoded instruction
instructionOrigin.OpCode = instructionCode;
currentInstruction = new Instruction(instructionCode.InstructionTypeIndex, instructionCode.InstructionTypeParamVariant);
DecodeInstructionExecutionMethod();
// Correct the actual duration of the current opcode fetch cycle (depends on the instruction)
currentMachineCycle = currentInstruction.ExecutionTimings.MachineCycles[machineCycleIndex];
}
// We need to read more bytes to recognize a multi-byte opcode
else if (candidateInstructionCode.FetchMoreBytes > 0)
{
opcodeTableIndex = candidateInstructionCode.SwitchToTableNumber.Value;
}
}
if (TraceMicroInstructions)
{
TraceMicroInstruction(new MicroInstruction(Z80MicroInstructionTypes.CPUControlDecodeInstruction));
}
}
private void EndInstruction()
{
instructionOrigin = null;
currentInstruction = null;
executeInstruction = null;
}
private void DecodeInterruptingDeviceInstruction()
{
// The interrupting device inserts an instruction code on the data bus
InstructionCode candidateInstructionCode = Z80OpCodes.Tables[0, InternalDataBus];
// An opcode was recognized, decode it and prepare its execution
if (candidateInstructionCode.FetchMoreBytes == 0 && candidateInstructionCode.SwitchToTableNumber == null)
{
InstructionCode instructionCode = candidateInstructionCode;
// Register decoded instruction
instructionOrigin = new InstructionOrigin(InstructionSource.InterruptingDevice, instructionCode, 0);
currentInstruction = new Instruction(instructionCode.InstructionTypeIndex, instructionCode.InstructionTypeParamVariant);
DecodeInstructionExecutionMethod();
// Correct the actual duration of the current opcode fetch cycle (depends on the instruction)
currentMachineCycle = currentInstruction.ExecutionTimings.MachineCycles[machineCycleIndex];
// Decrease the half T state index to execute the decoded instruction as if it was fetched from memory
halfTStateIndex = 4;
}
// We need to read more bytes to recognize a multi-byte opcode
else
{
throw new NotSupportedException("Multi bytes opcodes are not supported in interrupt mode 0 by this simulator");
}
if (TraceMicroInstructions)
{
TraceMicroInstruction(new MicroInstruction(Z80MicroInstructionTypes.CPUControlDecodeInterruptingDeviceInstruction));
}
}
