private Tms7000Instruction Decode(Opcode opcode, string format)
{
int iOp = 0;
var instr = new Tms7000Instruction
{
Opcode = opcode,
};
for (int i = 0; i < format.Length; ++i)
{
MachineOperand op;
byte b;
ushort w;
switch (format[i])
{
case ',': continue;
case 'A':
op = new RegisterOperand(arch.a);
break;
case 'B':
op = new RegisterOperand(arch.b);
break;
case 'R':
if (!rdr.TryReadByte(out b))
{
return(Invalid());
}
op = new RegisterOperand(arch.GpRegs[b]);
break;
case 'P':
if (!rdr.TryReadByte(out b))
{
return(Invalid());
}
op = new RegisterOperand(arch.Ports[b]);
break;
case 'S':
; op = new RegisterOperand(arch.st);
break;
case 'i':
if (!rdr.TryReadByte(out b))
{
return(Invalid());
}
op = ImmediateOperand.Byte(b);
break;
case 'I':
if (!rdr.TryReadBeUInt16(out w))
{
return(Invalid());
}
op = ImmediateOperand.Word16(w);
break;
case 'j': // short PC-relative jump
if (!rdr.TryReadByte(out b))
{
return(Invalid());
}
op = AddressOperand.Create(rdr.Address + (sbyte)b);
break;
case 'D': // direct, absolute address
if (!rdr.TryReadBeUInt16(out w))
{
return(Invalid());
}
if (i < format.Length - 1 && format[i + 1] == 'B')
{
++i;
op = MemoryOperand.Indexed(Address.Ptr16(w), arch.b);
}
else
{
op = MemoryOperand.Direct(Address.Ptr16(w));
}
break;
case '*': // indirect
if (!rdr.TryReadByte(out b))
{
return(Invalid());
}
op = MemoryOperand.Indirect(arch.GpRegs[b]);
break;
default: throw new NotImplementedException($"Addressing mode {format[i]}");
}
switch (iOp++)
{
case 0: instr.op1 = op; break;
case 1: instr.op2 = op; break;
case 2: instr.op3 = op; break;
}
}
return(instr);
}