public NTPUCPU(uint ram_size, ushort[] rom)
{
_memory = new ushort[ram_size];
_rom = rom;
_registers = new ushort[8]; // PC stored seperately.
_pc = 0x0000;
_interrupt_enable = false;
_pt_enable = false;
_sys_curr_mode = NTPUMode.System;
Halted = false;
_active_interrupt = 0;
_flag_neg = false;
_flag_zero = false;
_flag_carry = false;
_flag_overflow = false;
_last_reset_addr = 0;
_interrupt_stack = 0;
}
public void ExecuteInstruction()
{
var instr = ReadMemoryPaged(_pc);
if (instr == 0x0000)
{
Halted = true;
return;
}
byte opcode = (byte)(instr & 0xFF);
byte upbyte = (byte)((instr & 0xFF00) >> 8);
Console.WriteLine("Executing instr {0:X4}", instr);
byte src = (byte)((instr & 0x700) >> 8);
byte ctrl = (byte)((instr & 0x1800) >> 11);
byte r7offs = (byte)((instr & 0xF000) >> 12);
byte dest = (byte)((instr & 0xE000) >> 13);
bool standard_encode_1 = false;
bool standard_encode_2 = false;
bool calc_flags = true;
bool do_pc_increment = true;
Console.WriteLine("Dest: {0:x1}, ctrl: {1:x1}, r7offs: {2:x1}, src: {3:x1}", dest, ctrl, r7offs, src);
// Ew. handles if an instr should use the normal load/store setup.
switch (opcode)
{
case byte n when(n <= 0x20 && ((n & 0x01) == 0)):
standard_encode_1 = true;
break;
case byte n when(n <= 0x20 && ((n & 0x01) == 1)):
standard_encode_2 = true;
break;
case 0xFC: // Jcc Rn
case 0xFD: // Jcc imm
case 0xFE: // rst vector
case 0xFF: // hlt code
// neither of the normal encodes.
break;
}
ushort src_val;
ushort dest_val;
ushort dest_write = 0;
ushort imm = 0; // Yes, globalizing this is nasty. But it removes
// prevents a spurious memory read that shouldn't
// occur
if (standard_encode_1)
{
dest_val = _registers[dest];
if (ctrl == 0)
{
src_val = _registers[src];
}
else if (ctrl == 2)
{
var addr = _registers[src];
src_val = ReadMemoryPaged(addr);
}
else
{
var addr = (ushort)(_registers[7] + (ushort)SextR7Offs(r7offs));
src_val = ReadMemoryPaged(addr);
}
}
else if (standard_encode_2)
{
if (ctrl == 0)
{
// Immediate modes
imm = ReadMemoryPaged((ushort)(_pc + 1));
_pc += 1;
switch (src)
{
case 0:
dest_val = _registers[dest];
src_val = imm;
break;
case 1:
src_val = ReadMemoryPaged(imm);
dest_val = _registers[dest];
break;
case 2:
src_val = _registers[dest]; // swap mode.
dest_val = ReadMemoryPaged(imm);
break;
default:
_active_interrupt = 1; // Invalid instr
return;
}
}
else
{
src_val = _registers[src];
if (ctrl == 1)
{
dest_val = ReadMemoryPaged(_registers[dest]);
}
else
{
var addr = (ushort)(_registers[7] + (ushort)SextR7Offs(r7offs));
dest_val = ReadMemoryPaged(addr);
}
}
}
else
{
src_val = 0;
dest_val = 0;
}
switch (opcode)
{
case 0x00:
case 0x01:
calc_flags = false;
dest_write = src_val;
break;
case 0x02:
case 0x03:
InstrAdd(src_val, dest_val, out dest_write);
break;
case 0x04:
case 0x05:
InstrSub(src_val, dest_val, out dest_write);
break;
case 0x06:
case 0x07:
InstrAnd(src_val, dest_val, out dest_write);
break;
case 0x08:
case 0x09:
InstrOr(src_val, dest_val, out dest_write);
break;
case 0x0A:
case 0x0B:
InstrXor(src_val, dest_val, out dest_write);
break;
case 0x0C:
case 0x0D:
InstrShl(src_val, dest_val, out dest_write);
break;
case 0x0E:
case 0x0F:
InstrShr(src_val, dest_val, out dest_write);
break;
case 0x14:
case 0x15:
InstrCmp(src_val, dest_val, out dest_write);
break;
case 0x16:
case 0x17:
InstrTst(src_val, dest_val, out dest_write);
break;
case 0xF8:
do_pc_increment = false;
calc_flags = false;
InstrCall(dest);
break;
case 0xF9:
do_pc_increment = false;
calc_flags = false;
InstrRet(dest);
break;
case 0xFA:
case 0xFB:
InstrLdpr(instr);
calc_flags = false;
break;
case 0xFC:
case 0xFD:
do_pc_increment = false;
InstrJCC(opcode, upbyte, src);
calc_flags = false;
break;
case 0xFF:
if (_sys_curr_mode == NTPUMode.User)
{
goto case 0xFE; //would be fallthrough, but C#.
}
calc_flags = false;
Halted = true;
do_pc_increment = false;
_last_reset_addr = CalculateFullAddress(_pc, (_sys_curr_mode == NTPUMode.System) ? _sys_page_table_r : _user_page_table_r);
_last_reset_addr_virt = _pc;
break;
case 0xFE:
calc_flags = false;
do_pc_increment = false;
_last_reset_addr = CalculateFullAddress(_pc, (_sys_curr_mode == NTPUMode.System) ? _sys_page_table_r : _user_page_table_r);
_last_reset_addr_virt = _pc;
if (_sys_curr_mode == NTPUMode.User)
{
_sys_curr_mode = NTPUMode.System;
_pc = _syscall_entry; // Go to wherever the system says to.
}
else
{
_pc = 0;
_pt_enable = false;
_interrupt_enable = false;
}
break;
default:
break;
}
if (standard_encode_1)
{
_registers[dest] = dest_write;
}
else if (standard_encode_2)
{
if (ctrl == 0)
{
switch (src)
{
case 0: // LD_1
case 1:
_registers[dest] = dest_write;
break;
case 2:
WriteMemoryPaged(imm, dest_write);
break;
}
}
else if (ctrl == 1)
{
WriteMemoryPaged(_registers[dest], dest_write);
}
else
{
var addr = (ushort)(_registers[7] + (ushort)SextR7Offs(r7offs));
WriteMemoryPaged(addr, dest_write);
}
}
if (calc_flags)
{
_flag_zero |= dest_write == 0;
_flag_neg |= dest_write >= 0x7FFF;
}
if (do_pc_increment)
{
_pc += 1;
}
if (_pc > 513)
{
Halted = true;
}
}