Refactor: ModRM Handler replaced with compile-time lookup table.

Started refactoring to a library-based model to easily integrate
comprehensive unit testing. Added CPU exception handling system for
the GUI. Non-critical exceptions (though critical to execution of
emulated application) will not shutdown the application in the future
releases. Refactored Userspace::Run() logic so that the GUI-controls
reflect emulator behavior.

CMakeLists.txt

@@ -3,11 +3,12 @@ project(emulator C CXX)
 
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
-set(CMAKE_CXX_STANDDARD 23)
+set(CMAKE_CXX_STANDARD 26)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
 add_subdirectory(external/glfw)
+add_subdirectory(external/gtest)
 
 if(CMAKE_BUILD_TYPE STREQUAL "Debug")
     add_compile_definitions(Debug)
@@ -39,3 +40,5 @@ target_include_directories(emulator PRIVATE external/glfw/include
 					    external/imgui
 					    external/imgui_club/imgui_memory_editor
 					    )
+
+add_subdirectory(tests)

src/Arithmetic.cpp

@@ -10,7 +10,7 @@
 
 namespace executor_cases {
   void Add_rm32_r32(CPUContext& cc){
-    x86::ModRM modrm = cc.m_Instruction.optional.m_ModRM;
+    x86::ModRM modrm = cc.m_Instruction.m_ModRM;
 
     std::cout << "[Instruction] ";
     
@@ -55,7 +55,7 @@ namespace executor_cases {
   namespace helpers {
     void Add_rm_dst(CPUContext& cc, uint32_t address) {
       uint32_t dstPrevValue = cc.m_Bus->AccessX<uint32_t>(address);
-      uint32_t currRegValue = cc.m_Registers[cc.m_Instruction.optional.m_ModRM.m_Reg];
+      uint32_t currRegValue = cc.m_Registers[cc.m_Instruction.m_ModRM.m_Reg];
       uint32_t result = dstPrevValue + currRegValue;
       cc.m_Bus->WriteX<uint32_t>(address, result);
     }

src/Bus.h

@@ -9,6 +9,8 @@
 
 #include "RAM.h"
 
+#include "Exceptions.h"
+
 class Bus {
 public:
   Bus(std::shared_ptr<RAM> m_Bus);
@@ -31,7 +33,8 @@ public:
       break;
     }
     default:
-      throw std::runtime_error("Illegal Access!");
+      std::string exception = "Illegal access to: " + std::to_string(address);
+      throw CPUException(exception);
     }
   }
 
@@ -52,7 +55,8 @@ public:
       return value;
     }
     default:
-      throw std::runtime_error("Illegal Access!");
+      std::string exception = "Illegal access to: " +  std::to_string(address);
+      throw std::runtime_error(exception);
     }
   }
   

src/CPU.cpp

@@ -8,6 +8,11 @@
 #include <array>
 #include <cassert>
 
+#include "Exceptions.h"
+#include "InstructionModifierLookup.h"
+
+static const Instruction s_EmptyInstruction{};
+
 CPU::CPU(std::shared_ptr<Bus>& bus) : m_Bus(bus), m_IsHalted(false), m_Context({m_Instruction, m_InstructionPointer, m_Flags, m_Registers, m_Bus, m_IsHalted}) {
   m_InstructionPointer = 0x00008000;
 
@@ -54,12 +59,7 @@ void CPU::Reset() {
 
   std::cout << "[CPU] State Flushed!" << std::endl;
   m_IsHalted = false;
-  m_Instruction.m_Opcode = (Opcode)0;
+  m_Instruction = s_EmptyInstruction;
-  m_Instruction.m_Operand1 = 0;
-  m_Instruction.m_Operand2 = 0;
-  m_Instruction.m_Func = nullptr;
-  std::memset(m_Instruction.m_Displacement, 0, 4);
-  m_Instruction.optional.m_ModRM = (x86::ModRM){ .m_State = x86::ModRMState::INVALID, .m_Reg = 0, .m_Rm = 0 };
 }
 
 CPUStatus CPU::GetStatus() {
@@ -67,13 +67,16 @@ CPUStatus CPU::GetStatus() {
 }
 
 void CPU::FetchDecode() {
+  m_Instruction = s_EmptyInstruction;
+  
   uint8_t opcode_raw = m_Bus->AccessX<uint8_t>(m_InstructionPointer);
   Opcode opcode = static_cast<Opcode>(opcode_raw);
 
   auto& instruction_table = GetInstructionTable();
   if(instruction_table[opcode_raw].m_Executor == nullptr) {
     std::cout << "Encoding: " << opcode_raw << std::endl;
-    throw std::runtime_error("[CPU] [FetchDecode] Opcode is not in instruction table!");    
+    std::string exception = "[CPU] [FetchDecode] Opcode is not in instruction table! Opcode_raw: " + std::to_string(opcode_raw); 
+    throw CPUException(exception);
   }
 
   auto& instruction = instruction_table[opcode_raw];
@@ -82,42 +85,48 @@ void CPU::FetchDecode() {
   uint8_t encoding = instruction.m_Encoding & encoding_mask;
   uint8_t immediate = instruction.m_Encoding & ~encoding_mask;
 
+  m_Instruction.m_Opcode = opcode;
+  
   switch(encoding) {
   case OI:
     if (immediate == I32) {
       m_Instruction.m_Operand1 = m_Bus->AccessX<uint32_t>(m_InstructionPointer + 1);
       m_Instruction.m_Length = 5;
-    } else if (immediate = I16) {
+    } else if (immediate == I16) {
       m_Instruction.m_Operand1 = m_Bus->AccessX<uint16_t>(m_InstructionPointer + 1);
       m_Instruction.m_Length = 3;
-    } else if (immediate = I8) {
+    } else if (immediate == I8) {
       m_Instruction.m_Operand1 = m_Bus->AccessX<uint8_t>(m_InstructionPointer + 1);
       m_Instruction.m_Length = 2;
     } else {
-      throw std::runtime_error("[CPU] [FetchDecode] OI instruction encoding only supports imm8,imm16,imm32.");
+      throw CPUException("[CPU] [FetchDecode] OI instruction encoding only supports imm8,imm16,imm32.");
     }
     break;
   case ZO:
     m_Instruction.m_Length = 1;
     break;
   case RM:
-  case MR:
+  case MR: {
-    m_Instruction.optional.m_ModRM = x86::process_modrm(m_Bus->AccessX<uint8_t>(m_InstructionPointer + 1));
+    uint8_t modrm = m_Bus->AccessX<uint8_t>(m_InstructionPointer + 1);
+    m_Instruction.m_ModRM = x86::ProcessMODRM(modrm);
     m_Instruction.m_Length = 2;
-    FetchModRMFields();
+    FetchModRMFields(modrm);
+    if (m_Instruction.m_ModRM.m_SIB) {
+      uint8_t sib = m_Bus->AccessX<uint8_t>(m_InstructionPointer + m_Instruction.m_Length);
+      m_Instruction.m_Length += 1;
+      m_Instruction.m_SIB = x86::ProcessSIB(sib);        
+    }      
     break;
+  }
   default:
-    throw std::runtime_error("[CPU] [FetchDecode] Encoding was not found!");
+    throw CPUException("[CPU] [FetchDecode] Encoding was not found!");
   }
 
   m_Instruction.m_Func = instruction.m_Executor;
-  m_Instruction.m_Opcode = opcode;
   m_InstructionPointer += m_Instruction.m_Length;
 }
 
 void CPU::Execute() {
-  uint8_t opcode_value = static_cast<uint8_t>(m_Instruction.m_Opcode);
-  
   if(m_Instruction.m_Func != nullptr)
   {
     m_Instruction.m_Func(m_Context);
@@ -126,28 +135,8 @@ void CPU::Execute() {
   throw std::runtime_error("[CPU] [Execute] m_Func is nullptr!");
 }
 
-void CPU::FetchModRMFields() {
+void CPU::FetchModRMFields(uint8_t modrm) {
   assert(m_Instruction.m_Length != 0); // FetchDecode() must set m_Length before calling FetchModRMFields()
-  x86::ModRMState state = m_Instruction.optional.m_ModRM.m_State;
+  auto modrm_table = GetMod32Table();
-  switch(state) {
+  modrm_table[modrm](m_Context);
-  case x86::ModRMState::LR:
-  case x86::ModRMState::R:
-    break;
-  case x86::ModRMState::DISP32:
-  case x86::ModRMState::LR_DISP32:
-  {
-    uint32_t disp = m_Bus->AccessX<uint32_t>(m_InstructionPointer + m_Instruction.m_Length);
-    std::memcpy(&m_Instruction.m_Displacement, &disp, 4);
-    m_Instruction.m_DisplacementSize = 4;
-    m_Instruction.m_Length += 4;
-    break;
-  }
-  case x86::ModRMState::LR_DISP8:
-    m_Instruction.m_Displacement[0] = m_Bus->AccessX<uint8_t>(m_InstructionPointer + m_Instruction.m_Length);
-    m_Instruction.m_DisplacementSize = 1;
-    m_Instruction.m_Length += 1;
-    break;
-  default:
-    throw std::runtime_error("Instruction could not be modified according to the modrm field!");
-  }
 }

src/CPU.h

@@ -8,6 +8,10 @@
 #include "ExecutorCases.h"
 #include "CPUContext.h"
 
+enum class CPUExecutionMode {
+  BIT_32,
+};
+
 struct CPUStatus {
     uint32_t* m_Registers;
     uint32_t& m_IP;
@@ -34,6 +38,7 @@ private:
   void Execute();
 
 private:
+  CPUExecutionMode m_Mode;
   uint32_t m_Registers[8];
   uint16_t m_SegmentRegisters[8];
   uint32_t m_InstructionPointer;
@@ -47,5 +52,6 @@ private:
   CPUContext m_Context;
 private:
   // FetchDecode() must set m_Length before calling FetchModRMFields()
-  void FetchModRMFields();
+  void FetchModRMFields(uint8_t modrm);
+  void FetchSIB();
 };

src/DataTransfer.cpp

@@ -10,7 +10,7 @@
 namespace executor_cases {
   
   void Mov_rm32_r32(CPUContext& cc) {
-    x86::ModRM modrm = cc.m_Instruction.optional.m_ModRM;
+    x86::ModRM modrm = cc.m_Instruction.m_ModRM;
 
     std::cout << "[Instruction] ";
     

src/DataTransfer.h

@@ -15,7 +15,6 @@ namespace executor_cases {
     uint8_t reg = cc.m_Instruction.m_Opcode - O;
     std::cout << "[Instruction] mov " << x86::Register2Str((x86::Register)reg) << ", " << std::hex << cc.m_Instruction.m_Operand1 << std::endl;
     std::memcpy(&cc.m_Registers[reg], &cc.m_Instruction.m_Operand1, sizeof(T));
-    //    cc.m_Registers[reg] = cc.m_Instruction.m_Operand1;
   }
 
   void Mov_rm32_r32(CPUContext& cc);

src/Exceptions.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <stdexcept>
+
+class CPUException : std::runtime_error {
+public:
+  CPUException(std::string what) : std::runtime_error(what), m_What(what) { }
+  ~CPUException() = default;
+
+  std::string GetMessage() { return m_What; }
+  
+private:
+  std::string m_What;
+};

src/ExecutorCases.cpp

@@ -9,11 +9,13 @@
 #include "CPUContext.h"
 #include "Bus.h"
 
+#include <algorithm>
 #include <bitset>
 #include <iostream>
+#include <utility>
 
-constexpr std::array<InstructionEntry, 255> GenerateInstructionTable(){
+constexpr std::array<InstructionEntry, 256> GenerateInstructionTable(){
-  std::array<InstructionEntry, 255> table{};
+  std::array<InstructionEntry, 256> table{};
   
   for(uint8_t i = 0; i < 255; i++) {
     table[i] = (InstructionEntry){nullptr, (OperandEncoding)0};
@@ -21,7 +23,6 @@ constexpr std::array<InstructionEntry, 255> GenerateInstructionTable(){
   
   table[Opcode::NOP] = (InstructionEntry){executor_cases::Nop, ZO};
   table[Opcode::HLT] = (InstructionEntry){executor_cases::Hlt, ZO};
-  // table[Opcode::MOV_R32_IMM16] = (InstructionEntry){executor_cases::Mov_rX_immX<uint16_t, 0xB8>, (OperandEncoding)(I16 | OI)};
   table[Opcode::MOV_R32_IMM32] = (InstructionEntry){executor_cases::Mov_rX_immX<uint32_t, 0xB8>, (OperandEncoding)(I32 | OI)};
   table[Opcode::MOV_RM32_R32] = (InstructionEntry){executor_cases::Mov_rm32_r32, MR};
   table[Opcode::ADD_RM32_R32] = (InstructionEntry){executor_cases::Add_rm32_r32, MR};
@@ -31,9 +32,8 @@ constexpr std::array<InstructionEntry, 255> GenerateInstructionTable(){
   {
     if((table[i].m_Encoding & OI) != 0)
     {
-      for(int j = 1; j < 8 && (i + j) < 255; ++j) {
+      uint8_t end_offset = std::min(i + 8, 255);
-	table[i+j] = table[i]; 
+      std::fill(table.begin() + i, table.begin() + end_offset, table[i]);	
-      }
       i += 8;
     } else {
       i++;
@@ -42,16 +42,35 @@ constexpr std::array<InstructionEntry, 255> GenerateInstructionTable(){
   return table;
 }
 
-static constexpr std::array<InstructionEntry, 255> s_InstructionTable = GenerateInstructionTable();
+static constexpr std::array<InstructionEntry, 256> s_InstructionTable = GenerateInstructionTable();
 
-const std::array<InstructionEntry, 255>& GetInstructionTable() {
+const std::array<InstructionEntry, 256>& GetInstructionTable() {
   return s_InstructionTable;
 }
 
 namespace executor_cases::helpers {
 
+  // template<uint8_t... I>
+  // constexpr std::array<ExecutorCase, 256> GenerateEffectiveAddressCases(std::integer_sequence<uint8_t, I...>) {
+  //   return std::array<ExecutorCase, 256> {
+  //     []<uint8_t S>() {
+  // 	return [](CPUContext& c){};
+  //     }.template operator()<I>()...
+  //   };      
+  // }  
+
+  // constexpr static std::array<ExecutorCase, 256> s_EffectiveAddressCases = GenerateEffectiveAddressCases(std::make_integer_sequence<uint8_t, 255>());
+  
+  // uint32_t ResolveEffectiveAddress(CPUContext &cc) {
+  //   //    uint32_t req = cc.m_Instruction.m_ModRM.m_Rm << 24 | cc.m_Instruction.m_ModRM.m_Reg << 16 | cc.m_Instruction.m_ModRM.m_State << 8;
+
+  //   return 0;
+  // }  
+
   uint32_t ResolveModRMAddress(CPUContext& cc) {
-    x86::ModRM modrm = cc.m_Instruction.optional.m_ModRM;
+    x86::ModRM modrm = cc.m_Instruction.m_ModRM;
+    x86::SIB sib = cc.m_Instruction.m_SIB;
+    
     uint32_t value = 0;
     
     switch(modrm.m_State) {
@@ -75,5 +94,4 @@ namespace executor_cases::helpers {
 
     return value;
   }
-  
 }

src/ExecutorCases.h

@@ -24,9 +24,9 @@ struct InstructionEntry {
   OperandEncoding m_Encoding;
 };
 
-const std::array<InstructionEntry, 255>& GetInstructionTable();
+const std::array<InstructionEntry, 256>& GetInstructionTable();
 
-const std::array<ExecutorCase, 255>& GetExecutorTable();
+const std::array<ExecutorCase, 256>& GetExecutorTable();
 
 namespace executor_cases::helpers {
   uint32_t ResolveModRMAddress(CPUContext& cc);

src/GUI.cpp

@@ -31,7 +31,7 @@ GUI::GUI(Userspace& user) :m_Userspace(user) {
 }
 
 void GUI::CreateWindow() {
-  m_Window = glfwCreateWindow(1020, 640, "CPUSecX86", NULL, NULL);
+  m_Window = glfwCreateWindow(1200, 800, "CPUSecX86", NULL, NULL);
   if(!m_Window)
   {
     glfwTerminate();
@@ -70,6 +70,9 @@ void GUI::Run() {
     size_t data_size = 0x10000;
     process_mem.DrawWindow("Process Memory", m_Userspace.RetrieveMemory(), data_size);
 
+    if(m_Userspace.GetSystemStatus() == SystemStatus::EXCEPTION)
+      ErrorGUI();
+    
     glClear(GL_COLOR_BUFFER_BIT);
 
     ImGui::Render();
@@ -90,7 +93,7 @@ void GUI::Terminate() {
 
 void GUI::DockingSetup() {
   ImGuiID dockspace_id = ImGui::GetID("Dockspace");
-  ImGuiViewport* viewport = ImGui::GetMainViewport();
+  ImGuiViewport *viewport = ImGui::GetMainViewport();
 
   if (ImGui::DockBuilderGetNode(dockspace_id) == nullptr) {
     ImGui::DockBuilderAddNode(dockspace_id, ImGuiDockNodeFlags_DockSpace);
@@ -109,12 +112,23 @@ void GUI::DockingSetup() {
     ImGui::DockBuilderFinish(dockspace_id);
   }
 
-  ImGui::DockSpaceOverViewport(dockspace_id, viewport, ImGuiDockNodeFlags_PassthruCentralNode);
+  ImGuiDockNodeFlags dockspace_flags = ImGuiDockNodeFlags_NoTabBar;
+  dockspace_flags |= ImGuiDockNodeFlags_PassthruCentralNode;
+  
+  ImGui::DockSpaceOverViewport(dockspace_id, viewport, dockspace_flags);
 }
 
 void GUI::ControlGUI() {
   ImGui::Begin("Emulator Control");
-  ImGui::Text("Current State: %s", m_Userspace.IsRunning() ? "Running" : "Stopped");
+  static std::string state = "STOPPED";
+  if (m_Userspace.GetSystemStatus() == SystemStatus::STOPPED)
+    state = "STOPPED";
+  else if (m_Userspace.GetSystemStatus() == SystemStatus::RUNNING)
+    state = "RUNNING";
+  else if (m_Userspace.GetSystemStatus() == SystemStatus::EXCEPTION)
+    state = "EXCEPTION";
+  
+  ImGui::Text("Current State: %s", state.c_str());
   if(ImGui::Button("Start"))
     m_Userspace.Start();
   ImGui::SameLine();
@@ -128,9 +142,23 @@ void GUI::ControlGUI() {
   ImGui::End();
 }
 
+void GUI::ErrorGUI() {
+  ImGui::PushStyleColor(ImGuiCol_TitleBg, ImVec4(1.0f, 0.0f, 0.0f, 1.0f));
+  ImGui::PushStyleColor(ImGuiCol_Border, ImVec4(1.0f, 0.0f, 0.0f, 1.0f));
+  ImGui::PushStyleColor(ImGuiCol_TitleBgActive, ImVec4(1.0f, 0.0f, 0.0f, 1.0f));
+  ImGui::PushStyleColor(ImGuiCol_ResizeGrip, ImVec4(1.0f, 0.0f, 0.0f, 1.0f));
+  ImGui::Begin("Exception");
+  ImGui::Text("An exception was detected! Trace:");
+  auto& e = m_Userspace.GetException();
+  for (auto i : e)
+    ImGui::Text("Err: %s", i.c_str());
+  ImGui::End();
+  ImGui::PopStyleColor(4);
+}
+
 void GUI::DebugInfoGUI() {
   ImGuiInputTextFlags flags = ImGuiInputTextFlags_CharsHexadecimal;
-  if (m_Userspace.IsRunning()) flags |= ImGuiInputTextFlags_ReadOnly;
+  if (m_Userspace.GetSystemStatus() == SystemStatus::RUNNING) flags |= ImGuiInputTextFlags_ReadOnly;
   CPUStatus status = m_Userspace.GetCPUStatus();
 
   ImGui::Begin("Debug Information");
@@ -152,9 +180,12 @@ void GUI::DebugInfoGUI() {
   ImGui::Text("Opcode: %s", Opcode2Str(status.m_Instruction.m_Opcode).c_str());
   ImGui::Text("Operand1: %x", status.m_Instruction.m_Operand1);
   ImGui::Text("Operand2: %x", status.m_Instruction.m_Operand2);
-  ImGui::Text("ModRM R/M: %d", status.m_Instruction.optional.m_ModRM.m_Rm);
+  ImGui::Text("ModRM R/M: %d", status.m_Instruction.m_ModRM.m_Rm);
-  ImGui::Text("ModRM R: %d", status.m_Instruction.optional.m_ModRM.m_Reg);
+  ImGui::Text("ModRM R: %d", status.m_Instruction.m_ModRM.m_Reg);
-  ImGui::Text("ModRM Status: %d", status.m_Instruction.optional.m_ModRM.m_State);
+  ImGui::Text("ModRM Status: %d", status.m_Instruction.m_ModRM.m_State);
+  ImGui::Text("SIB Scale: %x", status.m_Instruction.m_SIB.m_Scale);
+  ImGui::Text("SIB Index: %x", status.m_Instruction.m_SIB.m_Index);
+  ImGui::Text("SIB Base: %x", status.m_Instruction.m_SIB.m_Base);
   ImGui::End();
 }
 

src/GUI.h

@@ -19,6 +19,8 @@ private:
   void ControlGUI();
   void DebugInfoGUI();
 
+  void ErrorGUI();
+
  private:
   GLFWwindow* m_Window;
   Userspace& m_Userspace;

src/Instruction.cpp

@@ -1,6 +1,6 @@
 #include "Instruction.h"
 
-#include <stdexcept>
+#include "Exceptions.h"
 
 namespace x86 { 
   std::string Register2Str(x86::Register reg) {
@@ -16,38 +16,20 @@ namespace x86 {
     }
     throw std::runtime_error("Register not found!");
   }
-  // Does not support SIB yet!
-  ModRM process_modrm(uint8_t modrm){
-    uint8_t mod_mask = 0b11000000;
-    uint8_t reg_mask = 0b00111000;
-    uint8_t rm_mask = 0b00000111;
 
-    uint8_t mod = modrm & mod_mask;
+  SIB ProcessSIB(uint8_t sib) {
-    uint8_t reg = (modrm & reg_mask) >> 3;
+    uint8_t scale_mask = 0b11000000;
-    uint8_t rm = modrm & rm_mask;
+    uint8_t index_mask = 0b00111000;
+    uint8_t base_mask  = 0b00000111;
 
-    ModRMState state = ModRMState::INVALID;
+    uint8_t scale = (sib & scale_mask) >> 6;
+    uint8_t index = (sib & index_mask) >> 3;
+    uint8_t base = sib & base_mask;
 
-    switch(mod) {
+    if(index == 4) index = x86::Register::SIB_NONE;
-    case 0b00000000:
+    if(base == 4)  base  = x86::Register::SIB_DISP;
-      state = ModRMState::LR;
-      if(rm == 0b00000101)
-	state = ModRMState::DISP32;
-      break;
-    case 0b01000000:
-      state = ModRMState::LR_DISP8;
-      break;
-    case 0b10000000:
-      state = ModRMState::LR_DISP32;
-      break;
-    case 0b11000000:
-      state = ModRMState::R;
-      break;
-    default:
-      throw std::runtime_error("Mod R/M does not support non-register operands right now!");
-    }
 
-    return {.m_State = state, .m_Reg = reg, .m_Rm = rm};  
+    return {.m_Scale = scale, .m_Index = (x86::Register)index, .m_Base = (x86::Register)base};	  
   }
 }
 
@@ -62,6 +44,6 @@ std::string Opcode2Str(Opcode op) {
   case Opcode::HLT:                        return "HLT";
   }
 
-  std::string what = "Opcode '" + std::to_string(op) + "' invalid!"; 
+  std::string what = "Opcode2Str could not find '" + std::to_string(op) + "'!"; 
-  throw std::runtime_error(what);
+  throw CPUException(what);
 }

src/Instruction.h

@@ -4,11 +4,15 @@
 #include <cstdint>
 
 #include "ExecutorCases.h"
+#include "Exceptions.h"
 
 namespace x86 {
   enum Register : uint8_t {
     EAX = 0,    ECX = 1,     EDX = 2,    EBX = 3,
-    ESP = 4,    EBP = 5,     ESI = 6,    EDI = 7
+    ESP = 4,    EBP = 5,     ESI = 6,    EDI = 7,
+
+    SIB_DISP = 254,
+    SIB_NONE = 255,
   };
 
   enum class ModRMState : uint8_t
@@ -23,11 +27,58 @@ namespace x86 {
 
   struct ModRM {
     ModRMState m_State;
-    uint8_t m_Reg;
+    x86::Register m_Reg;
     uint8_t m_Rm;
+    bool m_SIB;
   };
 
-  ModRM process_modrm(uint8_t modrm);
+  struct SIB {
+    uint8_t m_Scale;
+    x86::Register m_Index;
+    x86::Register m_Base;
+  };
+
+  constexpr ModRM ProcessMODRM(uint8_t modrm) {
+    uint8_t mod_mask = 0b11000000;
+    uint8_t reg_mask = 0b00111000;
+    uint8_t rm_mask = 0b00000111;
+  
+    uint8_t mod = modrm & mod_mask;
+    uint8_t reg = (modrm & reg_mask) >> 3;
+    uint8_t rm = modrm & rm_mask;
+
+    ModRMState state = ModRMState::INVALID;
+    bool SIB = false;
+  
+    switch(mod) {
+    case 0b00000000:
+      state = ModRMState::LR;
+      if(rm == 0b00000101)
+	state = ModRMState::DISP32;
+      else if(rm == 0b00000100)
+	SIB = true;
+      break;
+    case 0b01000000:
+      state = ModRMState::LR_DISP8;
+      if(rm == 0b00000100)
+	SIB = true;
+      break;
+    case 0b10000000:
+      state = ModRMState::LR_DISP32;
+      if(rm == 0b00000100)
+	SIB = true;
+      break;
+    case 0b11000000:
+      state = ModRMState::R;
+      break;
+    default:
+      throw CPUException("Mod R/M Unknown exception!");
+    }
+    
+    return {.m_State = state, .m_Reg = (x86::Register)reg, .m_Rm = rm, .m_SIB = SIB};  
+  }
+  
+  SIB ProcessSIB(uint8_t sib);
 
   // Helpers
   std::string Register2Str(x86::Register reg);
@@ -52,9 +103,8 @@ struct Instruction{
   size_t m_Length;
   uint32_t m_Operand1;
   uint32_t m_Operand2;
-  union {
+  x86::ModRM m_ModRM;
-    x86::ModRM m_ModRM;
+  x86::SIB m_SIB;
-  } optional;
   uint8_t m_Displacement[4];
   uint8_t m_DisplacementSize;
 };

src/InstructionModifierLookup.cpp

@@ -0,0 +1,76 @@
+#include "InstructionModifierLookup.h"
+
+#include "CPUContext.h"
+#include "Instruction.h"
+#include "Bus.h"
+
+#include <array>
+#include <cstdint>
+#include <utility>
+#include <cstring>
+#include <iostream>
+
+void Mod32_SIB(CPUContext &cc) {
+  std::cout << "[Mod32_SIB] SIB byte encountered." << std::endl; 
+}
+
+void Mod32_LR(CPUContext& cc) {
+  std::cout << "[Mod32_LR] Executed." << std::endl;
+  if (cc.m_Instruction.m_ModRM.m_SIB) {
+    uint32_t src = cc.m_Bus->AccessX<uint32_t>(cc.m_InstructionPointer + cc.m_Instruction.m_Length);
+    std::memcpy(&cc.m_Instruction.m_Displacement, &src, 4);
+    Mod32_SIB(cc);
+  }    
+}
+
+void Mod32_DISP32(CPUContext& cc) {
+  uint32_t src = cc.m_Bus->AccessX<uint32_t>(cc.m_InstructionPointer + cc.m_Instruction.m_Length);
+  std::memcpy(&cc.m_Instruction.m_Displacement, &src, 4);
+  cc.m_Instruction.m_Length += 4;
+  if (cc.m_Instruction.m_ModRM.m_SIB)
+    Mod32_SIB(cc);    
+}
+
+template<uint8_t RM>
+void Mod32_LRDISP32(CPUContext& cc) {
+  uint32_t disp = cc.m_Registers[RM] + cc.m_Bus->AccessX<uint32_t>(cc.m_Instruction.m_Length);
+  std::memcpy(&cc.m_Instruction.m_Displacement, &disp, 4);
+  cc.m_Instruction.m_Length += 4;
+  if (cc.m_Instruction.m_ModRM.m_SIB)
+    Mod32_SIB(cc);    
+ }
+
+template<uint8_t RM>
+void Mod32_LRDISP8(CPUContext& cc) {
+  uint32_t disp = cc.m_Registers[RM] + cc.m_Bus->AccessX<uint8_t>(cc.m_Instruction.m_Length);
+  cc.m_Instruction.m_Displacement[0] = disp;
+  cc.m_Instruction.m_Length += 1;
+  if (cc.m_Instruction.m_ModRM.m_SIB)
+    Mod32_SIB(cc);  
+}
+
+void Mod32_R(CPUContext& cc) {
+  std::cout << "[Mod32_R] Executed." << std::endl;
+}
+  
+template<uint8_t...Is>
+static constexpr std::array<ModifierEntry, 256> GenerateModRM32(std::integer_sequence<uint8_t, Is...>) {
+  return std::array<ModifierEntry, 256>{
+    ([]<uint8_t I>() {
+      constexpr auto modrm = x86::ProcessMODRM(I);
+
+      switch (modrm.m_State) {
+      case x86::ModRMState::LR:           return Mod32_LR;
+      case x86::ModRMState::LR_DISP32:    return Mod32_LRDISP32<modrm.m_Rm>;
+      case x86::ModRMState::LR_DISP8:     return Mod32_LRDISP8<modrm.m_Rm>;
+      case x86::ModRMState::DISP32:       return Mod32_DISP32;
+      case x86::ModRMState::R:            return Mod32_R;
+      }
+
+    }.template operator()<Is>())... // provide the lambda with a proper template argument
+  };
+}
+
+static constexpr std::array<ModifierEntry, 256> s_ModRM32 = GenerateModRM32(std::make_integer_sequence<uint8_t, 255>{});
+
+std::array<ModifierEntry, 256> GetMod32Table() { return s_ModRM32; }

src/InstructionModifierLookup.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <array>
+
+struct CPUContext;
+
+typedef void (*ModifierEntry)(CPUContext&);
+
+std::array<ModifierEntry, 256> GetMod32Table();
+
+std::array<ModifierEntry, 256> GetSIBTable();

src/Userspace.cpp

@@ -5,10 +5,10 @@
 
 #include <thread>
 
-Userspace::Userspace() : m_RAM(std::make_shared<RAM>()),m_ProgramMemory(std::make_shared<RAM>()), m_Bus(std::make_shared<Bus>(m_RAM)), m_CPU(m_Bus) {
+Userspace::Userspace() : m_RAM(std::make_shared<RAM>()), m_ProgramMemory(std::make_shared<RAM>()), m_Bus(std::make_shared<Bus>(m_RAM)), m_CPU(m_Bus) {
   std::cout << "[Userspace] Emulation started." << std::endl;
   m_ApplicationRunning = true;
-  m_Running = false;
+  m_Status = SystemStatus::STOPPED;
 }
 
 void Userspace::Upload2Memory(uint8_t bytes[], size_t len) {
@@ -27,36 +27,46 @@ void Userspace::Run() {
   }
 }
 
-void Userspace::EmulatorLoop() {
+void Userspace::StepCPU() {
-   while(m_Running) {
+  try {
-     if(!m_CPU.IsHalted()) {
+    m_CPU.Step();
-       m_CPU.Step();
+  } catch (CPUException& c) {
-     }
+    m_Status = SystemStatus::EXCEPTION;
+    m_ExceptionMessages.push_back(c.GetMessage());
+  }
+}
 
-     if (m_CPU.IsHalted()) {
+void Userspace::EmulatorLoop() {
-       if(m_Running)
+  while(m_Status == SystemStatus::RUNNING) {
-	 m_Running = false;
+    if(!m_CPU.IsHalted()) {
-       std::this_thread::sleep_for(std::chrono::milliseconds(10));
+      StepCPU();
-     }
+    }
-   }
+
-   if(!m_Running)
+    if (m_CPU.IsHalted()) {
-     std::this_thread::sleep_for(std::chrono::milliseconds(10));
+      if(m_Status == SystemStatus::RUNNING)
+	m_Status = SystemStatus::STOPPED;
+      std::this_thread::sleep_for(std::chrono::milliseconds(10));
+    }
+  }
+  if(m_Status == SystemStatus::STOPPED)
+    std::this_thread::sleep_for(std::chrono::milliseconds(10));
 }
 
 void Userspace::Reset() {
-  if(m_CPU.IsHalted()) m_Running = false;
+  if(m_CPU.IsHalted()) m_Status = SystemStatus::STOPPED;
   m_CPU.Reset();
   std::memcpy(m_RAM->Data(), m_ProgramMemory->Data(), m_ProgramMemoryLen);
+  m_ExceptionMessages.clear();
 }
 
 void Userspace::Start() {
-  m_Running = true;
+  m_Status = SystemStatus::RUNNING;
 }
 
 void Userspace::Step() {
   if(!m_CPU.IsHalted())
   {
-    m_CPU.Step();
+    StepCPU();
     std::cout << "[Userspace] CPU stepped!" << std::endl;
   }
   else
@@ -66,20 +76,24 @@ void Userspace::Step() {
 }
 
 void Userspace::Stop() {
-  m_Running = false;
+  m_Status = SystemStatus::STOPPED;
   std::cout << "[Userspace] Emulation stopped." << std::endl;
 }
 
 void Userspace::Exit() {
-  m_Running = false;
+  m_Status = SystemStatus::STOPPED;
   m_ApplicationRunning = false;
   std::cout << "[Userspace] Emulation exited." << std::endl;
 }
 
-bool Userspace::IsRunning() {
+SystemStatus Userspace::GetSystemStatus() {
-  return m_Running;
+  return m_Status;
 }
 
 CPUStatus Userspace::GetCPUStatus() {
   return m_CPU.GetStatus();
 }
+
+std::vector<std::string>& Userspace::GetException() {
+  return m_ExceptionMessages;
+}

src/Userspace.h

@@ -5,9 +5,16 @@ class Bus;
 #include <cstdint>
 #include <memory>
 #include <atomic>
+#include <vector>
 
 #include "CPU.h"
 
+enum class SystemStatus {
+  STOPPED,
+  RUNNING,
+  EXCEPTION
+};
+
 class Userspace {
  public:
   Userspace();
@@ -23,20 +30,23 @@ class Userspace {
   void Reset();
   void Exit();
 
-  bool IsRunning();
+  SystemStatus GetSystemStatus();
-
   CPUStatus GetCPUStatus();
+  std::vector<std::string>& GetException();
     
  private:
   std::shared_ptr<RAM> m_RAM;
   std::shared_ptr<Bus> m_Bus;
   CPU m_CPU;
-  std::atomic<bool> m_Running = false;
+  std::atomic<SystemStatus> m_Status = SystemStatus::STOPPED;
   std::atomic<bool> m_ApplicationRunning = false;
 
+  std::vector<std::string> m_ExceptionMessages;
+  
   std::shared_ptr<RAM> m_ProgramMemory;
   size_t m_ProgramMemoryLen;
 
  private:
+  void StepCPU();
   void EmulatorLoop();
 };

tests/CMakeLists.txt

@@ -0,0 +1,10 @@
+enable_testing()
+						
+add_executable(test test.cpp)
+
+target_link_libraries(test
+		      GTest::gtest_main
+		      )
+
+include(GoogleTest)
+gtest_discover_tests(test)

tests/test.cpp

@@ -0,0 +1,21 @@
+#include <gtest/gtest.h>
+
+//#include "../src/Userspace.cpp"
+
+// uint8_t test[] = {
+//   0xB8, 0x10,
+//   0xB9, 0x10,
+//   0x01, 0xC0,
+// };
+
+// Userspace user;
+
+TEST(ModRM_C0, ModRM) {
+  // auto a = user.GetCPUStatus();
+  
+  // user.Upload2Memory(test, 6);
+  // user.Run();
+  
+  //EXPECT_EQ(a.m_Registers[0], 0x20);
+  EXPECT_EQ(1, 1);
+}