diff --git a/.gitignore b/.gitignore
index 6f03a8e..ebebf77 100644
--- a/.gitignore
+++ b/.gitignore
@@ -24,3 +24,5 @@ __pycache__/
 autotester
 sictools.jar
 simulator_SIC_XE/CMakeLists.txt.user
+
+/build/
diff --git a/simulator_SIC_XE/CMakeLists.txt b/simulator_SIC_XE/CMakeLists.txt
index c6c37e1..cde4cd2 100644
--- a/simulator_SIC_XE/CMakeLists.txt
+++ b/simulator_SIC_XE/CMakeLists.txt
@@ -1,7 +1,7 @@
 cmake_minimum_required(VERSION 3.10)
 project(simulator_SIC_XE VERSION 1.0 LANGUAGES CXX)
 
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 # Put all build outputs under target/bin
@@ -13,6 +13,11 @@ set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${OUTPUT_DIR})
 # Collect all .cpp sources under src/
 file(GLOB_RECURSE SOURCES "${PROJECT_SOURCE_DIR}/src/*.cpp")
 
+
+set(MAIN_SRC "${PROJECT_SOURCE_DIR}/src/main.cpp")
+set(ASSEMBLER_SRC "${PROJECT_SOURCE_DIR}/src/assembler.cpp")
+list(REMOVE_ITEM SOURCES ${MAIN_SRC} ${ASSEMBLER_SRC})
+
 if(NOT SOURCES)
   message(WARNING "No source files found in ${PROJECT_SOURCE_DIR}/src — the build will create an empty library")
 endif()
@@ -28,15 +33,9 @@ if(EXISTS "${PROJECT_SOURCE_DIR}/src/main.cpp")
   target_link_libraries(simulator_exec PRIVATE simulator_lib)
 endif()
 
-
-if(TARGET simulator_exec)
-  add_custom_target(run
-    DEPENDS simulator_exec
-    COMMAND ${CMAKE_COMMAND} -E echo "Running simulator_exec..."
-    COMMAND $<TARGET_FILE:simulator_exec>
-    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
-    COMMENT "Builds and runs simulator_exec"
-  )
+if(EXISTS "${PROJECT_SOURCE_DIR}/src/assembler.cpp")
+  add_executable(assembler "${PROJECT_SOURCE_DIR}/src/assembler.cpp")
+  target_link_libraries(assembler PRIVATE simulator_lib)
 endif()
 
 message(STATUS "Project: ${PROJECT_NAME}")
diff --git a/simulator_SIC_XE/README.md b/simulator_SIC_XE/README.md
index aac6331..ae46366 100644
--- a/simulator_SIC_XE/README.md
+++ b/simulator_SIC_XE/README.md
@@ -1,30 +1,65 @@
 # SIC/XE Simulator
 
-A complete SIC/XE architecture simulator with instruction execution, device I/O, and memory management.
+A complete SIC/XE architecture simulator with instruction execution, device I/O, memory management, and assembler.
 
 ## Quick Start
 
-The easiest way to build and run the simulator:
+### Building the Project
+
+```bash
+make
+```
+
+This will build:
+- `target/bin/simulator_exec` - The main simulator
+- `target/bin/assembler` - The SIC/XE assembler
+- `target/bin/simulator_qt` - Qt GUI version (if Qt is available)
+
+### Using the Assembler
+
+Assemble a SIC/XE assembly file to object code:
+
+```bash
+./target/bin/assembler <file.asm>
+```
+
+**Example:**
+```bash
+./target/bin/assembler res/test_format4.asm
+```
+
+This will:
+- Parse and assemble the input file
+- Generate modification records (M records) for format 4 instructions
+- Create `<file>.obj` with the object code
+- Display the object code and symbol table
+
+**Sample Output:**
+```
+H TESTF4 0003E8 00001B
+T 0003E8 1B 031003F70F1003FA4B1003FD4F2C090000000000000100004F2BFD
+M 0003E9 05
+M 0003ED 05
+M 0003F1 05
+E 0003E8
+```
+
+### Running the Simulator
 
 ```bash
 make run
 ```
 
-This single command will:
-- Configure the build system (if needed)
-- Compile all source files
-- Link the executable 
-- Run the simulator
+This will build and run the simulator with the default program.
 
 ## Build Commands
 
 |    Command   |                   Description                      |
 |--------------|----------------------------------------------------|
-| `make`       | Build the project                                  |
+| `make`       | Build all executables                              |
 | `make build` | Build the project                                  |
-| `make run`   | Build run the simulator                            |
+| `make run`   | Build and run the simulator                        |
 | `make clean` | Clean build artifacts                              |
-| `make run`   | Clean build artifacts, build and run the simulator |
 
 
 ## Project Structure
diff --git a/simulator_SIC_XE/devices/FA.dev b/simulator_SIC_XE/devices/FA.dev
new file mode 100644
index 0000000..cb6e5ed
--- /dev/null
+++ b/simulator_SIC_XE/devices/FA.dev
@@ -0,0 +1,2 @@
+5
+0
diff --git a/simulator_SIC_XE/gui/qt/mainwindow.cpp b/simulator_SIC_XE/gui/qt/mainwindow.cpp
index b53aa72..da6fb69 100644
--- a/simulator_SIC_XE/gui/qt/mainwindow.cpp
+++ b/simulator_SIC_XE/gui/qt/mainwindow.cpp
@@ -5,7 +5,9 @@
 #include "../../include/instructions.h"
 #include "../../include/opcode.h"
 #include "../../include/constants.h"
-#include "../../../include/loader.h"
+#include "../../include/loader.h"
+#include "../../include/parser.h"
+#include "../../include/code.h"
 
 #include <QIntValidator>
 #include <QLineEdit>
@@ -20,6 +22,9 @@
 #include <QFileDialog>
 #include <QMessageBox>
 #include <QScrollBar>
+#include <QDir>
+#include <fstream>
+#include <sstream>
 
 class Loader;
 
@@ -102,6 +107,7 @@ MainWindow::MainWindow(QWidget *parent) :
     
     // Connect menu actions
     connect(ui->actionLoad_Object_File, &QAction::triggered, this, &MainWindow::loadObjectFile);
+    connect(ui->actionLoad_Asm_file, &QAction::triggered, this, &MainWindow::loadAsmFile);
     connect(ui->actionAbout, &QAction::triggered, this, &MainWindow::showAboutDialog);
     connect(ui->actionFrequency, &QAction::triggered, this, &MainWindow::showFrequencyDialog);
     
@@ -981,6 +987,100 @@ void MainWindow::loadObjectFile()
     }
 }
 
+void MainWindow::loadAsmFile()
+{
+    QString fileName = QFileDialog::getOpenFileName(this,
+        tr("Load Assembly File"), 
+        QString(),
+        tr("Assembly Files (*.asm);;All Files (*)"));
+    
+    if (fileName.isEmpty()) {
+        return;
+    }
+    
+    try {
+        // Stop execution if running
+        m_controller->stop();
+        
+        // Reset machine state
+        m_machine->reset();
+        
+        // Read assembly file
+        std::ifstream file(fileName.toStdString());
+        if (!file.is_open()) {
+            throw std::runtime_error("Could not open file: " + fileName.toStdString());
+        }
+        
+        std::string source((std::istreambuf_iterator<char>(file)), 
+                          std::istreambuf_iterator<char>());
+        file.close();
+        
+        // Parse and assemble
+        Parser parser;
+        Code code = parser.parse(source);
+        code.assemble();
+        
+        // Generate object code
+        std::string objCode = code.emitText();
+        
+        // Create resources directory if it doesn't exist
+        QDir dir;
+        if (!dir.exists("resources")) {
+            dir.mkpath("resources");
+        }
+        
+        // Save object file to resources directory
+        QFileInfo fileInfo(fileName);
+        QString objFileName = "resources/" + fileInfo.completeBaseName() + ".obj";
+        
+        std::ofstream objFile(objFileName.toStdString());
+        if (!objFile.is_open()) {
+            throw std::runtime_error("Could not create object file: " + objFileName.toStdString());
+        }
+        objFile << objCode;
+        objFile.close();
+        
+        // Generate and save log file
+        QString logFileName = "resources/" + fileInfo.completeBaseName() + ".log";
+        std::ofstream logFile(logFileName.toStdString());
+        if (!logFile.is_open()) {
+            throw std::runtime_error("Could not create log file: " + logFileName.toStdString());
+        }
+        
+        logFile << "=== SIC/XE Assembler Log ===\n\n";
+        logFile << "Source file: " << fileName.toStdString() << "\n";
+        logFile << "Object file: " << objFileName.toStdString() << "\n\n";
+        
+        logFile << "=== Symbols ===\n";
+        logFile << code.dumpSymbols() << "\n\n";
+        
+        logFile << "=== Code ===\n";
+        logFile << code.dumpCode() << "\n\n";
+        
+        logFile << "=== Object Code ===\n";
+        logFile << objCode << "\n";
+        
+        logFile.close();
+        
+        // Load the generated object file
+        Loader loader(m_machine, objFileName.toStdString());
+        loader.load();
+        
+        // Update displays
+        updateRegisterDisplays();
+        updateMemoryDisplay();
+        updateDisassemblyDisplay();
+        
+        QMessageBox::information(this, tr("Success"), 
+            tr("Assembly successful!\nObject file: %1\nLog file: %2")
+            .arg(objFileName).arg(logFileName));
+            
+    } catch (const std::exception &e) {
+        QMessageBox::critical(this, tr("Error"), 
+            tr("Failed to assemble file: %1").arg(e.what()));
+    }
+}
+
 void MainWindow::showAboutDialog()
 {
     QMessageBox::about(this, tr("About SIC/XE Simulator"),
diff --git a/simulator_SIC_XE/gui/qt/mainwindow.h b/simulator_SIC_XE/gui/qt/mainwindow.h
index 2dba671..cc8f042 100644
--- a/simulator_SIC_XE/gui/qt/mainwindow.h
+++ b/simulator_SIC_XE/gui/qt/mainwindow.h
@@ -51,6 +51,7 @@ private slots:
     void onDisassemblyGoToStart();
     void onDisassemblyGoToEnd();
     void loadObjectFile();
+    void loadAsmFile();
     void showAboutDialog();
     void showFrequencyDialog();
 
diff --git a/simulator_SIC_XE/gui/qt/mainwindow.ui b/simulator_SIC_XE/gui/qt/mainwindow.ui
index e9084bd..21085fa 100644
--- a/simulator_SIC_XE/gui/qt/mainwindow.ui
+++ b/simulator_SIC_XE/gui/qt/mainwindow.ui
@@ -892,6 +892,7 @@
      <string>File</string>
     </property>
     <addaction name="actionLoad_Object_File"/>
+    <addaction name="actionLoad_Asm_file"/>
    </widget>
    <widget class="QMenu" name="menuMachine">
     <property name="title">
@@ -924,6 +925,11 @@
     <string>About</string>
    </property>
   </action>
+  <action name="actionLoad_Asm_file">
+   <property name="text">
+    <string>Load Asm file</string>
+   </property>
+  </action>
  </widget>
  <resources/>
  <connections/>
diff --git a/simulator_SIC_XE/include/code.h b/simulator_SIC_XE/include/code.h
new file mode 100644
index 0000000..519d5f1
--- /dev/null
+++ b/simulator_SIC_XE/include/code.h
@@ -0,0 +1,68 @@
+#ifndef CODE_H
+#define CODE_H
+
+#include <vector>
+#include <memory>
+#include <unordered_map>
+#include <cstdint>
+
+#include "node.h"
+
+class Code {
+
+public:
+    Code() = default;
+
+    void addLine(const std::shared_ptr<Node>& line);
+
+    const std::vector<std::shared_ptr<Node>>& getLines() const;
+
+    const string toString() const;
+
+    // Two-pass assembler methods
+    void assemble();
+    std::vector<uint8_t> emitCode();
+    std::string emitText();
+    std::string dumpSymbols() const;
+    std::string dumpCode() const;
+
+private:
+    std::vector<std::shared_ptr<Node>> _lines;
+    
+    // Assembler state
+    std::unordered_map<std::string, int> _symbolTable;
+    std::vector<int> _locationCounters;  // Location counter per line
+    int _startAddress = 0;
+    int _programLength = 0;
+    std::string _programName;
+    int _baseRegister = -1;  // -1 means not set
+    
+    struct ModificationRecord {
+        int address;    
+        int halfBytes;  
+    };
+    mutable std::vector<ModificationRecord> _modificationRecords;
+
+    // Pass 1: build symbol table and assign addresses
+    void firstPass();
+    
+    // Pass 2: generate code
+    void secondPass();
+    
+    // Helper methods
+    int getInstructionLength(const std::shared_ptr<Node>& node, int locationCounter) const;
+    std::vector<uint8_t> generateInstruction(const InstructionNode* inst, int address);
+    std::vector<uint8_t> generateData(const DataNode* data);
+    
+    // Addressing mode selection
+    struct AddressingResult {
+        int nixbpe;  // ni, x, b, p, e bits
+        int displacement;  // 12-bit or 20-bit
+        bool success;
+    };
+    AddressingResult selectAddressingMode(int targetAddress, int pc, bool indexed, bool immediate, bool indirect, bool extended) const;
+};
+
+
+
+#endif // CODE_H
\ No newline at end of file
diff --git a/simulator_SIC_XE/include/lexer.h b/simulator_SIC_XE/include/lexer.h
new file mode 100644
index 0000000..f9bdd70
--- /dev/null
+++ b/simulator_SIC_XE/include/lexer.h
@@ -0,0 +1,55 @@
+#ifndef LEXER_H
+#define LEXER_H
+
+#include <string>
+#include <stdexcept>
+#include <cstddef>
+
+class SyntaxError : public std::runtime_error {
+public:
+    int row;
+    int col;
+
+    SyntaxError(const std::string& msg, int row_, int col_)
+        : std::runtime_error(msg), row(row_), col(col_) {}
+};
+
+
+class Lexer {
+public:
+    int row;
+    int col;
+
+    explicit Lexer(std::string input);
+
+    Lexer& mark();
+
+    std::string extract(int ofs);
+    std::string extract();
+
+    char peek(int ahead) const;
+    char peek() const;
+
+    char advance();
+
+    bool advanceIf(char ch);
+    void advance(char ch);  
+
+
+    bool skipWhitespace();
+
+    std::string readTo(char delimiter);
+
+    std::string readAlphanumeric();
+
+    std::string readDigits(int radix);
+
+private:
+    std::string input_;
+    std::size_t pos_;
+    std::size_t start_;
+
+    static int digitValue(char c, int radix);
+};
+
+#endif // LEXER_H
diff --git a/simulator_SIC_XE/include/loader.h b/simulator_SIC_XE/include/loader.h
index 41899c2..34c31a6 100644
--- a/simulator_SIC_XE/include/loader.h
+++ b/simulator_SIC_XE/include/loader.h
@@ -27,6 +27,7 @@ public:
     enum class RecordType {
         HEADER,
         TEXT,
+        MODIFICATION,
         END,
         UNKNOWN
     };
@@ -40,6 +41,11 @@ public:
         int start_address;
         std::vector<uint8_t> data;
     };
+    struct ModificationRecord {
+        int address;        // Address to be modified
+        int length;         // Length in nibbles
+        bool add;           // true for +, false for -
+    };
     struct EndRecord {
         int execution_start_address;
     };
@@ -54,10 +60,13 @@ private :
     shared_ptr<Machine> _machine;
     string _filename;
     shared_ptr<FileReader> _file_reader;
+    int _relocation_address;
     HeaderMetadata readHeader();
     TextRecord readTextRecord();
+    ModificationRecord readModificationRecord();
     EndRecord readEndRecord();
     bool load_into_memory(int start_address, const std::vector<uint8_t>& data);
+    void applyModification(const ModificationRecord& mod);
 
 };
 
diff --git a/simulator_SIC_XE/include/mnemonic.h b/simulator_SIC_XE/include/mnemonic.h
new file mode 100644
index 0000000..3cdddab
--- /dev/null
+++ b/simulator_SIC_XE/include/mnemonic.h
@@ -0,0 +1,45 @@
+// mnemonic.h
+#ifndef MNEMONIC_H
+#define MNEMONIC_H
+
+#include <cstdint>
+#include <string>
+#include <vector>
+#include <variant>
+
+#include "opcode.h"
+
+struct Empty {};
+struct Register { int num; };
+struct Immediate { int value; };
+struct SymbolRef { 
+    std::string name; 
+    bool indexed = false;
+    bool immediate = false;
+    bool indirect = false;
+};
+
+using Operand = std::variant<Empty, Register, Immediate, SymbolRef>;
+
+class Mnemonic {
+public:
+    Mnemonic(std::uint8_t opcode, InstructionType type, bool extended)
+        : _opcode(opcode), _extended(extended), _type(type) {}
+
+    std::uint8_t    opcode()   const { return _opcode; }
+    bool            extended() const { return _extended; }
+    InstructionType type()     const { return _type; }
+
+    std::vector<Operand>& operands() { return _operands; }
+    const std::vector<Operand>& operands() const { return _operands; }
+
+    std::string toString() const;
+
+private:
+    std::uint8_t    _opcode;
+    bool            _extended;
+    InstructionType _type;
+    std::vector<Operand> _operands;
+};
+
+#endif // MNEMONIC_H
diff --git a/simulator_SIC_XE/include/node.h b/simulator_SIC_XE/include/node.h
new file mode 100644
index 0000000..2ad0d86
--- /dev/null
+++ b/simulator_SIC_XE/include/node.h
@@ -0,0 +1,98 @@
+#ifndef NODE_H
+#define NODE_H
+
+#include <string>
+#include <memory>
+#include <vector>
+#include <variant>
+#include <cstdint>
+#include "mnemonic.h"
+
+using std::string;
+
+class Node {
+public:
+    virtual ~Node() = default;
+
+    string getLabel() const      { return _label; }
+    string getComment() const    { return _comment; }
+    std::shared_ptr<Mnemonic> getMnemonic() const { return _mnemonic; }
+
+    virtual string toString() const;
+
+protected:
+    string _label;
+    std::shared_ptr<Mnemonic> _mnemonic;
+    string _comment;
+};
+
+class InstructionNode : public Node {
+public:
+    InstructionNode(string label,
+                    std::shared_ptr<Mnemonic> mnemonic,
+                    string comment) {
+        _label    = std::move(label);
+        _mnemonic = std::move(mnemonic);
+        _comment  = std::move(comment);
+    }
+
+    string toString() const override;
+};
+
+class CommentNode : public Node {
+public:
+    explicit CommentNode(string text) {
+        _comment = std::move(text);
+    }
+
+    string toString() const override;
+};
+
+enum class DirectiveKind {
+    START, END, BASE, NOBASE, EQU, ORG, LTORG, 
+    EXTDEF, EXTREF, CSECT
+};
+
+using DirectiveArg = std::variant<std::monostate, int, std::string, std::vector<std::string>>;
+
+class DirectiveNode : public Node {
+public:
+    DirectiveNode(string label, DirectiveKind kind, DirectiveArg arg, string comment)
+        : _kind(kind), _arg(std::move(arg)) {
+        _label = std::move(label);
+        _comment = std::move(comment);
+    }
+
+    DirectiveKind kind() const { return _kind; }
+    const DirectiveArg& arg() const { return _arg; }
+
+    string toString() const override;
+
+private:
+    DirectiveKind _kind;
+    DirectiveArg _arg;
+};
+
+enum class DataKind { WORD, BYTE, RESW, RESB };
+
+using DataValue = std::variant<std::monostate, int, std::vector<uint8_t>>;
+
+class DataNode : public Node {
+public:
+    DataNode(string label, DataKind kind, DataValue value, string comment)
+        : _kind(kind), _value(std::move(value)) {
+        _label = std::move(label);
+        _comment = std::move(comment);
+    }
+
+    DataKind kind() const { return _kind; }
+    const DataValue& value() const { return _value; }
+
+    string toString() const override;
+
+private:
+    DataKind _kind;
+    DataValue _value;
+};
+
+#endif // NODE_H
diff --git a/simulator_SIC_XE/include/opcode.h b/simulator_SIC_XE/include/opcode.h
index a467488..05cb693 100644
--- a/simulator_SIC_XE/include/opcode.h
+++ b/simulator_SIC_XE/include/opcode.h
@@ -3,6 +3,11 @@
 
 #include "utils.h"
 
+#include <unordered_map>
+#include <string_view>
+#include <optional>
+#include <cstdint>
+
 // ==============================
 // Opcode definitions (SIC/XE)
 // ==============================
@@ -87,6 +92,8 @@
 #define LDVS    0x68
 #define LDVT    0x04
 
+static std::unordered_map<std::string_view, uint8_t> mnemonicToOpcode;
+static bool opcodeTablesInitialized = false;
 
 
 enum class InstructionType {
@@ -110,6 +117,10 @@ struct InstructionInfo {
 extern InstructionInfo instructions[];
 extern InstructionInfo instructionsEXEX[];
 
+extern std::optional<uint8_t> findOpcodeByMnemonic(std::string_view name);
+extern const InstructionInfo& getInstructionInfo(uint8_t opcode);
+
+
 // Initialize the instruction table
 void loadInstructionSet();
 
diff --git a/simulator_SIC_XE/include/parser.h b/simulator_SIC_XE/include/parser.h
new file mode 100644
index 0000000..8ae126a
--- /dev/null
+++ b/simulator_SIC_XE/include/parser.h
@@ -0,0 +1,52 @@
+// parser.h
+#ifndef PARSER_H
+#define PARSER_H
+
+#include <string>
+#include <vector>
+#include <memory>
+#include <unordered_map>
+#include <cstdint>
+
+#include "lexer.h"
+#include "code.h"
+#include "opcode.h"
+#include "mnemonic.h"
+
+class Parser {
+public:
+    Parser() = default;
+
+    Code parse(const std::string& input);
+
+private:
+    std::string parseLabel();
+    std::shared_ptr<Mnemonic> parseMnemonic();
+    std::string parseSymbol();
+    int  parseRegister();
+    void parseComma();
+    bool parseIndexed();
+    int  parseNumber(int lo, int hi);
+    std::vector<std::uint8_t> parseData();
+
+    void parseOperands(Mnemonic& m);
+    
+    bool isDirective(const std::string& name);
+    bool isDataDirective(const std::string& name);
+    std::shared_ptr<Node> parseDirective(const std::string& label, const std::string& directive);
+    std::shared_ptr<Node> parseDataDirective(const std::string& label, const std::string& directive);
+
+    std::shared_ptr<Node> parseInstruction();
+    Code parseCode();
+
+    std::shared_ptr<Mnemonic> makeMnemonic(const std::string& name, bool extended);
+    static void initMnemonicMap();
+
+private:
+    Lexer lexer_{""};
+
+    static inline std::unordered_map<std::string, std::uint8_t> s_nameToOpcode{};
+    static inline bool s_mnemonicMapInitialized = false;
+};
+
+#endif // PARSER_H
diff --git a/simulator_SIC_XE/res/rec.asm b/simulator_SIC_XE/res/rec.asm
new file mode 100644
index 0000000..212bf23
--- /dev/null
+++ b/simulator_SIC_XE/res/rec.asm
@@ -0,0 +1,223 @@
+prog        START   0
+
+.-------------------------------------------
+. MAIN LOOP
+.
+. Psevdo:
+.   sp = 0
+.   while true:
+.       n = readFA()
+.       if n == 0: halt
+.       acc = 1
+.       fact()       ; rekurzivno: acc = n!
+.       printStdout(acc)
+.-------------------------------------------
+            CLEAR   A
+            STA     sp              
+
+loop        JSUB    readFA          
+            COMP    #0
+            JEQ     halt            
+
+            STA     n               
+            LDA     #1
+            STA     acc             
+
+            JSUB    fact           
+            LDA     acc
+            JSUB    printStdout     
+
+            J       loop
+
+halt        J       halt
+
+.-------------------------------------------
+. readFA
+.
+. Psevdo:
+.   B = 0
+.   while true:
+.       ch = RD(FA)
+.       if ch == CR or ch == LF: break
+.       digit = ch - '0'
+.       B = B * 10 + digit
+.   return B
+.-------------------------------------------
+readFA      CLEAR   B
+            LDS     #10           
+
+rd_loopFA   RD      #0xFA
+            COMP    #0x0D          . CR?
+            JEQ     rd_doneCR_FA
+            COMP    #0x0A          . LF?
+            JEQ     rd_doneFA
+
+            SUB     #0x30          
+            MULR    S,B            . B = B * 10
+            ADDR    A,B            . B = B + digit
+            J       rd_loopFA
+
+rd_doneCR_FA RD     #0xFA          . pogoltni LF po CR
+rd_doneFA   CLEAR   A
+            RMO     B,A
+            RSUB
+
+.-------------------------------------------
+. fact
+.
+. Psevdo (globalni n, acc, sklad L):
+.   fact():
+.     push(L)
+.     if n <= 1:
+.         pop(L); return
+.     acc = acc * n
+.     n = n - 1
+.     fact()
+.     pop(L); return
+.-------------------------------------------
+fact        . push L
+            LDA     sp
+            ADD     #3
+            STA     sp
+            LDX     sp
+            STL     stackL,X
+
+            LDA     n
+            COMP    #1
+            JGT     fact_rec
+
+            . base case: n <= 1
+            LDX     sp
+            LDL     stackL,X
+            LDA     sp
+            SUB     #3
+            STA     sp
+            RSUB
+
+fact_rec    . recursive case: acc *= n; n--; fact()
+
+            LDB     acc
+            LDS     n
+            MULR    S,B
+            STB     acc
+
+            LDA     n
+            SUB     #1
+            STA     n
+
+            JSUB    fact
+
+            . pop L in return to caller
+            LDX     sp
+            LDL     stackL,X
+            LDA     sp
+            SUB     #3
+            STA     sp
+            RSUB
+
+.-------------------------------------------
+. printStdout
+.
+. Psevdo:
+.   if A == 0:
+.       print "0\n"
+.       return
+.   ps_val = A
+.   ps_len = 0
+.   while ps_val > 0:
+.       q = ps_val / 10
+.       r = ps_val % 10
+.       buf[ps_len] = '0' + r
+.       ps_len++
+.       ps_val = q
+.   for i = ps_len-1 .. 0:
+.       print buf[i]
+.   print "\r\n"
+.-------------------------------------------
+printStdout COMP    #0
+            JEQ     ps_zero
+
+            STA     ps_val
+            LDA     #0
+            STA     ps_len
+            LDS     #10
+            LDT     #0x30          . '0'
+
+ps_div      LDA     ps_val
+            COMP    #0
+            JEQ     ps_divdone
+
+            RMO     A,B
+            DIVR    S,B            . kvocient v B
+            RMO     B,X            . X = kvocient
+
+            MULR    S,B
+            SUBR    B,A            . A = ostanek
+            ADDR    T,A            . A = '0' + ostanek
+            STA     psdigit
+
+            LDA     ps_len
+            STA     ps_idx
+            LDA     #psbuf
+            ADD     ps_idx
+            STA     ps_ptr
+            LDA     psdigit
+            STCH    @ps_ptr
+
+            LDA     ps_len
+            ADD     #1
+            STA     ps_len
+
+            RMO     X,A
+            STA     ps_val
+            J       ps_div
+
+ps_divdone  LDA     ps_len
+            SUB     #1
+            STA     ps_idx
+
+ps_print    LDA     ps_idx
+            COMP    #0
+            JLT     ps_end
+
+            LDA     #psbuf
+            ADD     ps_idx
+            STA     ps_ptr
+            LDCH    @ps_ptr
+            WD      #1             
+
+            LDA     ps_idx
+            SUB     #1
+            STA     ps_idx
+            J       ps_print
+
+ps_end      LDA     #0x0D          . CR
+            WD      #1
+            LDA     #0x0A          . LF
+            WD      #1
+            RSUB
+
+ps_zero     LDA     #0x30          . "0"
+            WD      #1
+            LDA     #0x0D
+            WD      #1
+            LDA     #0x0A
+            WD      #1
+            RSUB
+
+.data
+. rekurzija faktoriala
+sp          WORD    0          . stack pointer
+n           WORD    0          
+acc         WORD    0          . akumulator za faktorial
+stackL      RESB    60         
+
+. printStdout 
+ps_val      WORD    0
+ps_len      WORD    0
+ps_idx      WORD    0
+psdigit     WORD    0
+ps_ptr      WORD    0
+psbuf       RESB    12         
+
+            END     prog
diff --git a/simulator_SIC_XE/res/simple.asm b/simulator_SIC_XE/res/simple.asm
new file mode 100644
index 0000000..20beca6
--- /dev/null
+++ b/simulator_SIC_XE/res/simple.asm
@@ -0,0 +1,32 @@
+SIMPLE  START   0
+
+        +LDA    NUM1        
+        +ADD    NUM2       
+        +STA    RESULT      
+        
+        LDX     NUM1        
+        LDL     NUM2      
+        
+        LDA     #0          
+        ADDR    X,A         
+        ADDR    L,A         
+        
+        +LDA    RESULT      
+        ADD     #48         
+        RMO     A,S         
+        SHIFTL  S,16       
+        SHIFTR  S,16        
+        RMO     S,A         
+        STCH    RESULT      
+        LDCH    RESULT      
+        WD      OUTPUT      
+        
+HALT    J       HALT            
+
+OUTPUT  BYTE    1    
+
+NUM1    WORD    1
+NUM2    WORD    2
+RESULT  RESW    1
+
+        END     SIMPLE
diff --git a/simulator_SIC_XE/res/test_format4.asm b/simulator_SIC_XE/res/test_format4.asm
new file mode 100644
index 0000000..6de136d
--- /dev/null
+++ b/simulator_SIC_XE/res/test_format4.asm
@@ -0,0 +1,11 @@
+TESTF4  START   1000
+        +LDA    BUFFER
+        +STA    OUTPUT
+        +JSUB   FUNC
+        RSUB
+
+BUFFER  RESW    1
+OUTPUT  RESW    1
+FUNC    LDA     #0
+        RSUB
+        END     TESTF4
diff --git a/simulator_SIC_XE/src/assembler.cpp b/simulator_SIC_XE/src/assembler.cpp
new file mode 100644
index 0000000..2ceece9
--- /dev/null
+++ b/simulator_SIC_XE/src/assembler.cpp
@@ -0,0 +1,77 @@
+#include <iostream>
+#include <fstream>
+#include <string>
+#include "code.h"
+#include "parser.h"
+#include "opcode.h"
+
+using std::cout;
+using std::endl;
+using std::cerr;
+
+int main(int argc, char* argv[]) {
+    if (argc != 2) {
+        cerr << "Usage: " << argv[0] << " <assembly_file.asm>" << endl;
+        return 1;
+    }
+    
+    std::string inputFile = argv[1];
+    
+    // Load instruction set
+    loadInstructionSet();
+    
+    try {
+        // Read assembly file
+        cout << "Assembling: " << inputFile << endl;
+        std::ifstream file(inputFile);
+        if (!file.is_open()) {
+            throw std::runtime_error("Failed to open file: " + inputFile);
+        }
+        
+        std::string input;
+        std::string line;
+        while (std::getline(file, line)) {
+            input += line + "\n";
+        }
+        file.close();
+        
+        // Parse
+        Parser parser;
+        Code code = parser.parse(input);
+        
+        // Assemble
+        code.assemble();
+        
+        // Generate object code
+        std::string objectCode = code.emitText();
+        
+        // Determine output filename
+        std::string outputFile = inputFile;
+        size_t lastDot = outputFile.find_last_of('.');
+        if (lastDot != std::string::npos) {
+            outputFile = outputFile.substr(0, lastDot);
+        }
+        outputFile += ".obj";
+        
+        // Write to file
+        std::ofstream out(outputFile);
+        if (!out.is_open()) {
+            throw std::runtime_error("Failed to create output file: " + outputFile);
+        }
+        out << objectCode;
+        out.close();
+        
+        // Display results
+        cout << "\n=== Object Code ===" << endl;
+        cout << objectCode;
+        cout << "\n=== Symbol Table ===" << endl;
+        cout << code.dumpSymbols();
+        cout << "\nOutput written to: " << outputFile << endl;
+        
+    } catch (const std::exception& e) {
+        cerr << "ERROR: " << e.what() << endl;
+        return 1;
+    }
+    
+    return 0;
+}
diff --git a/simulator_SIC_XE/src/code.cpp b/simulator_SIC_XE/src/code.cpp
new file mode 100644
index 0000000..2e87d12
--- /dev/null
+++ b/simulator_SIC_XE/src/code.cpp
@@ -0,0 +1,551 @@
+#include "code.h"
+#include "opcode.h"
+#include "constants.h"
+#include <sstream>
+#include <iomanip>
+#include <stdexcept>
+#include <cstring>
+
+void Code::addLine(const std::shared_ptr<Node> &line)
+{
+    _lines.emplace_back(line);
+}
+
+const std::vector<std::shared_ptr<Node>> &Code::getLines() const
+{
+    return _lines;
+}
+
+const string Code::toString() const
+{
+    string result;
+    for (const auto& line : _lines) {
+        result += line->toString() + "\n";
+    }
+    return result;
+}
+
+// ============================================================
+// TWO-PASS ASSEMBLER IMPLEMENTATION
+// ============================================================
+
+void Code::assemble() {
+    firstPass();
+    secondPass();
+}
+
+void Code::firstPass() {
+    _symbolTable.clear();
+    _locationCounters.clear();
+    _locationCounters.resize(_lines.size(), 0);
+    
+    int locationCounter = 0;
+    bool startFound = false;
+    
+    for (size_t i = 0; i < _lines.size(); ++i) {
+        auto& line = _lines[i];
+        _locationCounters[i] = locationCounter;
+        
+        // Handle label
+        std::string label = line->getLabel();
+        if (!label.empty()) {
+            if (_symbolTable.find(label) != _symbolTable.end()) {
+                throw std::runtime_error("Duplicate symbol: " + label);
+            }
+            _symbolTable[label] = locationCounter;
+        }
+        
+        // Check for directives
+        if (auto* directive = dynamic_cast<DirectiveNode*>(line.get())) {
+            switch (directive->kind()) {
+                case DirectiveKind::START: {
+                    if (std::holds_alternative<int>(directive->arg())) {
+                        _startAddress = std::get<int>(directive->arg());
+                        locationCounter = _startAddress;
+                        _locationCounters[i] = locationCounter;
+                        if (!label.empty()) {
+                            _symbolTable[label] = locationCounter;
+                            _programName = label;
+                        }
+                        startFound = true;
+                    }
+                    break;
+                }
+                case DirectiveKind::END:
+                    _programLength = locationCounter - _startAddress;
+                    break;
+                    
+                case DirectiveKind::BASE: {
+                    // BASE sets base register for addressing
+                    if (std::holds_alternative<std::string>(directive->arg())) {
+                        // Will resolve in second pass
+                    }
+                    break;
+                }
+                case DirectiveKind::NOBASE:
+                    _baseRegister = -1;
+                    break;
+                    
+                case DirectiveKind::EQU: {
+                    // EQU defines symbol value
+                    if (!label.empty() && std::holds_alternative<int>(directive->arg())) {
+                        _symbolTable[label] = std::get<int>(directive->arg());
+                    }
+                    break;
+                }
+                case DirectiveKind::ORG: {
+                    // ORG changes location counter
+                    if (std::holds_alternative<int>(directive->arg())) {
+                        locationCounter = std::get<int>(directive->arg());
+                    }
+                    break;
+                }
+                default:
+                    break;
+            }
+            continue;
+        }
+        
+        // Handle data directives
+        if (auto* data = dynamic_cast<DataNode*>(line.get())) {
+            int length = 0;
+            switch (data->kind()) {
+                case DataKind::WORD:
+                    length = 3;  // 24-bit word
+                    break;
+                case DataKind::BYTE: {
+                    if (std::holds_alternative<std::vector<uint8_t>>(data->value())) {
+                        length = std::get<std::vector<uint8_t>>(data->value()).size();
+                    }
+                    break;
+                }
+                case DataKind::RESW: {
+                    if (std::holds_alternative<int>(data->value())) {
+                        length = std::get<int>(data->value()) * 3;
+                    }
+                    break;
+                }
+                case DataKind::RESB: {
+                    if (std::holds_alternative<int>(data->value())) {
+                        length = std::get<int>(data->value());
+                    }
+                    break;
+                }
+            }
+            locationCounter += length;
+            continue;
+        }
+        
+        // Handle instructions
+        if (auto* inst = dynamic_cast<InstructionNode*>(line.get())) {
+            int length = getInstructionLength(line, locationCounter);
+            locationCounter += length;
+        }
+    }
+    
+    if (!startFound) {
+        _startAddress = 0;
+    }
+    _programLength = locationCounter - _startAddress;
+}
+
+int Code::getInstructionLength(const std::shared_ptr<Node>& node, int locationCounter) const {
+    auto* inst = dynamic_cast<InstructionNode*>(node.get());
+    if (!inst || !inst->getMnemonic()) {
+        return 0;
+    }
+    
+    auto mnemonic = inst->getMnemonic();
+    InstructionType type = mnemonic->type();
+    
+    switch (type) {
+        case InstructionType::TYPE1:
+            return 1;
+        case InstructionType::TYPE2:
+            return 2;
+        case InstructionType::TYPE3_4:
+            return mnemonic->extended() ? 4 : 3;
+        default:
+            return 0;
+    }
+}
+
+void Code::secondPass() {
+    // Generate code for all instructions and data
+    // This will be used by emitCode() and emitText()
+}
+
+std::vector<uint8_t> Code::emitCode() {
+    std::vector<uint8_t> code;
+    code.resize(_programLength, 0);
+    
+    for (size_t i = 0; i < _lines.size(); ++i) {
+        auto& line = _lines[i];
+        int address = _locationCounters[i];
+        int offset = address - _startAddress;
+        
+        if (offset < 0 || offset >= _programLength) {
+            continue;
+        }
+        
+        // Generate instruction
+        if (auto* inst = dynamic_cast<InstructionNode*>(line.get())) {
+            auto bytes = generateInstruction(inst, address);
+            for (size_t j = 0; j < bytes.size() && (offset + j) < code.size(); ++j) {
+                code[offset + j] = bytes[j];
+            }
+        }
+        
+        // Generate data
+        if (auto* data = dynamic_cast<DataNode*>(line.get())) {
+            auto bytes = generateData(data);
+            for (size_t j = 0; j < bytes.size() && (offset + j) < code.size(); ++j) {
+                code[offset + j] = bytes[j];
+            }
+        }
+    }
+    
+    return code;
+}
+
+std::vector<uint8_t> Code::generateInstruction(const InstructionNode* inst, int address) {
+    std::vector<uint8_t> bytes;
+    
+    if (!inst || !inst->getMnemonic()) {
+        return bytes;
+    }
+    
+    auto mnemonic = inst->getMnemonic();
+    uint8_t opcode = mnemonic->opcode();
+    InstructionType type = mnemonic->type();
+    bool extended = mnemonic->extended();
+    const auto& operands = mnemonic->operands();
+    
+    switch (type) {
+        case InstructionType::TYPE1: {
+            bytes.push_back(opcode);
+            break;
+        }
+        
+        case InstructionType::TYPE2: {
+            bytes.push_back(opcode);
+            uint8_t r1 = 0, r2 = 0;
+            if (operands.size() >= 1 && std::holds_alternative<Register>(operands[0])) {
+                r1 = std::get<Register>(operands[0]).num & 0xF;
+            }
+            if (operands.size() >= 2 && std::holds_alternative<Register>(operands[1])) {
+                r2 = std::get<Register>(operands[1]).num & 0xF;
+            }
+            bytes.push_back((r1 << 4) | r2);
+            break;
+        }
+        
+        case InstructionType::TYPE3_4: {
+            // Format 3 or 4 instruction
+            int ni = 0, x = 0, b = 0, p = 0, e = 0;
+            int targetAddress = 0;
+            bool immediate = false, indirect = false, indexed = false;
+            
+            // Parse operand
+            if (!operands.empty()) {
+                if (std::holds_alternative<Immediate>(operands[0])) {
+                    immediate = true;
+                    targetAddress = std::get<Immediate>(operands[0]).value;
+                    ni = 0x01;  // n=0, i=1
+                } else if (std::holds_alternative<SymbolRef>(operands[0])) {
+                    auto& sym = std::get<SymbolRef>(operands[0]);
+                    immediate = sym.immediate;
+                    indirect = sym.indirect;
+                    indexed = sym.indexed;
+                    
+                    // Look up symbol
+                    auto it = _symbolTable.find(sym.name);
+                    if (it != _symbolTable.end()) {
+                        targetAddress = it->second;
+                    }
+                    
+                    // Set ni bits
+                    if (immediate) {
+                        ni = 0x01;  // n=0, i=1
+                    } else if (indirect) {
+                        ni = 0x02;  // n=1, i=0
+                    } else {
+                        ni = 0x03;  // n=1, i=1 (simple/direct)
+                    }
+                }
+            } else {
+                // No operand (like RSUB)
+                ni = 0x03;
+            }
+            
+            if (indexed) {
+                x = 1;
+            }
+            
+            if (extended) {
+                e = 1;
+            }
+            
+            // Calculate PC for addressing
+            int pc = address + (extended ? 4 : 3);
+            
+            // Select addressing mode
+            auto result = selectAddressingMode(targetAddress, pc, indexed, immediate, indirect, extended);
+            
+            if (result.success) {
+                b = (result.nixbpe >> 2) & 1;
+                p = (result.nixbpe >> 1) & 1;
+                e = result.nixbpe & 1;
+            }
+            
+            int displacement = result.displacement;
+            
+            // Build instruction bytes
+            uint8_t byte1 = (opcode & 0xFC) | ni;
+            uint8_t byte2 = (x << 7) | (b << 6) | (p << 5) | (e << 4);
+            
+            bytes.push_back(byte1);
+            
+            if (extended) {
+                // Format 4: 20-bit address
+                byte2 |= (displacement >> 16) & 0x0F;
+                bytes.push_back(byte2);
+                bytes.push_back((displacement >> 8) & 0xFF);
+                bytes.push_back(displacement & 0xFF);
+                
+                // Format 4 instructions with symbol references (not immediate values) need M records
+                bool needsRelocation = false;
+                if (!operands.empty() && std::holds_alternative<SymbolRef>(operands[0])) {
+                    auto& sym = std::get<SymbolRef>(operands[0]);
+                    // If it's not an immediate mode with a constant, it needs relocation
+                    if (!sym.immediate || _symbolTable.find(sym.name) != _symbolTable.end()) {
+                        needsRelocation = true;
+                    }
+                }
+                
+                // Record modification if needed 
+                if (needsRelocation) {
+                    ModificationRecord mod;
+                    mod.address = address + 1;  // Skip the opcode+ni byte, start at xbpe+addr
+                    mod.halfBytes = 5;  // 5 half-bytes (20 bits) for format 4 address field
+                    _modificationRecords.push_back(mod);
+                }
+            } else {
+                // Format 3: 12-bit displacement
+                byte2 |= (displacement >> 8) & 0x0F;
+                bytes.push_back(byte2);
+                bytes.push_back(displacement & 0xFF);
+            }
+            break;
+        }
+        
+        default:
+            break;
+    }
+    
+    return bytes;
+}
+
+Code::AddressingResult Code::selectAddressingMode(int targetAddress, int pc, bool indexed, bool immediate, bool indirect, bool extended) const {
+    AddressingResult result;
+    result.success = false;
+    result.nixbpe = 0;
+    result.displacement = 0;
+    
+    // Immediate mode - use target address directly
+    if (immediate) {
+        if (extended) {
+            result.nixbpe = 0x01;  // e=1, b=0, p=0
+            result.displacement = targetAddress & 0xFFFFF;  // 20 bits
+        } else {
+            result.nixbpe = 0x00;  // e=0, b=0, p=0
+            result.displacement = targetAddress & 0xFFF;  // 12 bits
+        }
+        result.success = true;
+        return result;
+    }
+    
+    // Extended format - use absolute address
+    if (extended) {
+        result.nixbpe = 0x01;  // e=1, b=0, p=0
+        result.displacement = targetAddress & 0xFFFFF;
+        result.success = true;
+        return result;
+    }
+    
+    // Try PC-relative (-2048 to +2047)
+    int pcDisp = targetAddress - pc;
+    if (pcDisp >= -2048 && pcDisp <= 2047) {
+        result.nixbpe = 0x02;  // p=1, b=0, e=0
+        result.displacement = pcDisp & 0xFFF;
+        result.success = true;
+        return result;
+    }
+    
+    // Try base-relative (0 to 4095)
+    if (_baseRegister >= 0) {
+        int baseDisp = targetAddress - _baseRegister;
+        if (baseDisp >= 0 && baseDisp <= 4095) {
+            result.nixbpe = 0x04;  // b=1, p=0, e=0
+            result.displacement = baseDisp & 0xFFF;
+            result.success = true;
+            return result;
+        }
+    }
+    
+    // Try direct (0 to 4095)
+    if (targetAddress >= 0 && targetAddress <= 4095) {
+        result.nixbpe = 0x00;  // b=0, p=0, e=0
+        result.displacement = targetAddress & 0xFFF;
+        result.success = true;
+        return result;
+    }
+    
+    // Try SIC format (0 to 32767, 15 bits)
+    if (targetAddress >= 0 && targetAddress <= 32767) {
+        result.nixbpe = 0x00;
+        result.displacement = targetAddress & 0x7FFF;
+        result.success = true;
+        return result;
+    }
+    
+    // Could not find suitable addressing mode
+    result.success = false;
+    return result;
+}
+
+std::vector<uint8_t> Code::generateData(const DataNode* data) {
+    std::vector<uint8_t> bytes;
+    
+    if (!data) {
+        return bytes;
+    }
+    
+    switch (data->kind()) {
+        case DataKind::WORD: {
+            if (std::holds_alternative<int>(data->value())) {
+                int value = std::get<int>(data->value()) & 0xFFFFFF;
+                // SIC/XE stores words in big-endian (MSB first)
+                bytes.push_back((value >> 16) & 0xFF);
+                bytes.push_back((value >> 8) & 0xFF);
+                bytes.push_back(value & 0xFF);
+            }
+            break;
+        }
+        
+        case DataKind::BYTE: {
+            if (std::holds_alternative<std::vector<uint8_t>>(data->value())) {
+                bytes = std::get<std::vector<uint8_t>>(data->value());
+            }
+            break;
+        }
+        
+        case DataKind::RESW:
+        case DataKind::RESB:
+            // Reserved space - emit zeros (handled by initialized array)
+            break;
+    }
+    
+    return bytes;
+}
+
+std::string Code::emitText() {
+    std::ostringstream oss;
+    
+    // H record: program name, start address, length
+    oss << "H ";
+    std::string name = _programName.empty() ? "PROG" : _programName;
+    name.resize(6, ' ');
+    oss << name << " ";
+    oss << std::setfill('0') << std::setw(6) << std::hex << std::uppercase << _startAddress << " ";
+    oss << std::setfill('0') << std::setw(6) << std::hex << std::uppercase << _programLength;
+    oss << "\n";
+    
+    // Clear and rebuild modification records
+    _modificationRecords.clear();
+    
+    // T records: text (code/data)
+    std::vector<uint8_t> code = emitCode();
+    int textStart = 0;
+    
+    while (textStart < code.size()) {
+        int textLength = std::min(30, (int)code.size() - textStart);
+        
+        oss << "T ";
+        oss << std::setfill('0') << std::setw(6) << std::hex << std::uppercase << (_startAddress + textStart) << " ";
+        oss << std::setfill('0') << std::setw(2) << std::hex << std::uppercase << textLength << " ";
+        
+        for (int i = 0; i < textLength; ++i) {
+            oss << std::setfill('0') << std::setw(2) << std::hex << std::uppercase << (int)code[textStart + i];
+        }
+        oss << "\n";
+        
+        textStart += textLength;
+    }
+    
+    // M records: modifications for format 4 instructions
+    for (const auto& mod : _modificationRecords) {
+        oss << "M ";
+        oss << std::setfill('0') << std::setw(6) << std::hex << std::uppercase << mod.address << " ";
+        oss << std::setfill('0') << std::setw(2) << std::hex << std::uppercase << mod.halfBytes;
+        oss << "\n";
+    }
+    
+    // E record: execution start address
+    oss << "E ";
+    oss << std::setfill('0') << std::setw(6) << std::hex << std::uppercase << _startAddress;
+    oss << "\n";
+    
+    return oss.str();
+}
+
+std::string Code::dumpSymbols() const {
+    std::ostringstream oss;
+    oss << "=== Symbol Table ===\n";
+    oss << std::left << std::setw(20) << "Symbol" << "Address\n";
+    oss << std::string(30, '-') << "\n";
+    
+    for (const auto& [symbol, address] : _symbolTable) {
+        oss << std::left << std::setw(20) << symbol;
+        oss << std::hex << std::uppercase << std::setw(6) << std::setfill('0') << address << "\n";
+    }
+    
+    return oss.str();
+}
+
+std::string Code::dumpCode() const {
+    std::ostringstream oss;
+    oss << "=== Code Listing ===\n";
+    oss << std::hex << std::uppercase << std::setfill('0');
+    
+    std::vector<uint8_t> code = const_cast<Code*>(this)->emitCode();
+    
+    for (size_t i = 0; i < _lines.size(); ++i) {
+        auto& line = _lines[i];
+        int address = _locationCounters[i];
+        int offset = address - _startAddress;
+        
+        // Print address
+        oss << std::setw(6) << address << "  ";
+        
+        // Print generated bytes
+        int length = getInstructionLength(line, address);
+        if (auto* data = dynamic_cast<DataNode*>(line.get())) {
+            auto bytes = const_cast<Code*>(this)->generateData(data);
+            length = bytes.size();
+        }
+        
+        for (int j = 0; j < length && (offset + j) < code.size(); ++j) {
+            oss << std::setw(2) << (int)code[offset + j];
+        }
+        
+        // Pad for alignment
+        for (int j = length; j < 12; ++j) {
+            oss << "  ";
+        }
+        
+        oss << "  " << line->toString() << "\n";
+    }
+    
+    return oss.str();
+}
diff --git a/simulator_SIC_XE/src/lexer.cpp b/simulator_SIC_XE/src/lexer.cpp
new file mode 100644
index 0000000..7ac9344
--- /dev/null
+++ b/simulator_SIC_XE/src/lexer.cpp
@@ -0,0 +1,138 @@
+#include "lexer.h"
+#include <cctype>
+#include <algorithm>
+
+Lexer::Lexer(std::string input)
+    : input_(std::move(input)),
+      pos_(0),
+      start_(0),
+      row(1),
+      col(1)
+{
+}
+
+Lexer& Lexer::mark() {
+    start_ = pos_;
+    return *this;
+}
+
+std::string Lexer::extract(int ofs) {
+    std::size_t end = pos_ + static_cast<std::size_t>(ofs);
+    if (end > input_.size()) {
+        end = input_.size();
+    }
+    if (end < start_) {
+        end = start_;
+    }
+    return input_.substr(start_, end - start_);
+}
+
+std::string Lexer::extract() {
+    return extract(0);
+}
+
+char Lexer::peek(int ahead) const {
+    std::size_t idx = pos_ + static_cast<std::size_t>(ahead);
+    if (idx < input_.size()) {
+        return input_[idx];
+    }
+    return '\0'; // sentinel for "no more chars"
+}
+
+char Lexer::peek() const {
+    return peek(0);
+}
+
+char Lexer::advance() {
+    char ch = peek();
+    if (ch == '\0') {
+        return '\0';  // don't move past end
+    }
+
+    ++pos_;
+
+    // update logical location
+    if (ch == '\n') {
+        ++row;
+        col = 1;
+    } else if (ch == '\t') {
+        col = ((col - 1) / 4) * 4 + 5;
+    } else {
+        ++col;
+    }
+    return ch;
+}
+
+bool Lexer::advanceIf(char ch) {
+    if (peek() != ch) {
+        return false;
+    }
+    advance();
+    return true;
+}
+
+void Lexer::advance(char ch) {
+    if (!advanceIf(ch)) {
+        throw SyntaxError(std::string("'") + ch + "' expected", row, col);
+    }
+}
+
+bool Lexer::skipWhitespace() {
+    while (true) {
+        char p = peek();
+        if (p == ' ' || p == '\t') {
+            advance();
+        } else {
+            break;
+        }
+    }
+    char p = peek();
+    return (p == '\n' || p == '\0');
+}
+
+std::string Lexer::readTo(char delimiter) {
+    mark();
+    while (peek() > 0 && peek() != delimiter) {
+        advance();
+    }
+    if (peek() == delimiter) {
+        advance(); // consume delimiter
+    }
+    // exclude delimiter itself (like Java's extract(-1))
+    return extract(-1);
+}
+
+std::string Lexer::readAlphanumeric() {
+    mark();
+    while (true) {
+        char c = peek();
+        if (std::isalnum(static_cast<unsigned char>(c)) || c == '_') {
+            advance();
+        } else {
+            break;
+        }
+    }
+    return extract();
+}
+
+int Lexer::digitValue(char c, int radix) {
+    if (radix < 2 || radix > 36) return -1;
+    int v = -1;
+    if (c >= '0' && c <= '9') {
+        v = c - '0';
+    } else if (c >= 'A' && c <= 'Z') {
+        v = c - 'A' + 10;
+    } else if (c >= 'a' && c <= 'z') {
+        v = c - 'a' + 10;
+    }
+    if (v >= 0 && v < radix) return v;
+    return -1;
+}
+
+std::string Lexer::readDigits(int radix) {
+    mark();
+    while (digitValue(peek(), radix) != -1) {
+        advance();
+    }
+    return extract();
+}
diff --git a/simulator_SIC_XE/src/loader.cpp b/simulator_SIC_XE/src/loader.cpp
index 8d3aa17..1d00ac5 100644
--- a/simulator_SIC_XE/src/loader.cpp
+++ b/simulator_SIC_XE/src/loader.cpp
@@ -17,6 +17,7 @@ Loader::~Loader()
 void Loader::load()
 {
     HeaderMetadata header = readHeader();
+    _relocation_address = header.start_address;
       
     while(true) {
         RecordType type = parseRecordType(static_cast<char>(_file_reader->readByte()));
@@ -28,6 +29,11 @@ void Loader::load()
                 }
                 break;
             }
+            case RecordType::MODIFICATION: {
+                ModificationRecord modRecord = readModificationRecord();
+                applyModification(modRecord);
+                break;
+            }
             case RecordType::END: {
                 EndRecord endRecord = readEndRecord();
                 _machine->setPC(endRecord.execution_start_address);
@@ -45,6 +51,7 @@ Loader::RecordType Loader::parseRecordType(char c)
     switch (c) {
         case 'H': return RecordType::HEADER;
         case 'T': return RecordType::TEXT;
+        case 'M': return RecordType::MODIFICATION;
         case 'E': return RecordType::END;
         default: return RecordType::UNKNOWN; // fallback; adjust as needed
     }
@@ -105,6 +112,29 @@ Loader::TextRecord Loader::readTextRecord()
     return record;
 }
 
+Loader::ModificationRecord Loader::readModificationRecord()
+{
+    ModificationRecord record;
+    if(FILE_CONTAINS_WHITE_SPACES) _file_reader->readByte();
+    
+    record.address = std::stoi(_file_reader->readString(6), nullptr, 16);
+    if(FILE_CONTAINS_WHITE_SPACES) _file_reader->readByte();
+    
+    record.length = std::stoi(_file_reader->readString(2), nullptr, 16);
+    
+    record.add = true;
+    std::string rest = _file_reader->readLine();
+    // Remove whitespace
+    rest.erase(std::remove_if(rest.begin(), rest.end(), ::isspace), rest.end());
+    if (!rest.empty()) {
+        if (rest[0] == '-') {
+            record.add = false;
+        }
+    }
+    
+    return record;
+}
+
 Loader::EndRecord Loader::readEndRecord()
 {
     EndRecord record;
@@ -132,3 +162,54 @@ bool Loader::load_into_memory(int start_address, const std::vector<uint8_t> &dat
     }
     return true;
 }
+
+void Loader::applyModification(const ModificationRecord& mod)
+{
+    // M record specifies address and length in half-bytes (nibbles)
+    // We need to modify the value at that address by adding or subtracting
+    // the relocation address
+    
+    int address = mod.address;
+    int halfBytes = mod.length;
+    
+    // Calculate how many full bytes we need to read
+    // halfBytes can be odd or even
+    int numBytes = (halfBytes + 1) / 2;
+    
+    if (address < 0 || address + numBytes > MEMORY_SIZE) {
+        throw std::runtime_error("Modification address out of bounds");
+    }
+    
+    // Read the current value from memory
+    int currentValue = 0;
+    for (int i = 0; i < numBytes; ++i) {
+        currentValue = (currentValue << 8) | _machine->getByte(address + i);
+    }
+    
+    // If odd number of half-bytes, we only modify the relevant nibbles
+    // For simplicity, we'll work with the full bytes and mask appropriately
+    int mask = 0;
+    for (int i = 0; i < halfBytes; ++i) {
+        mask = (mask << 4) | 0xF;
+    }
+    
+    // Extract the value to modify
+    int shift = (numBytes * 2 - halfBytes) * 4;
+    int valueToModify = (currentValue >> shift) & mask;
+    
+    // Apply modification
+    int newValue = mod.add ? (valueToModify + _relocation_address) 
+                           : (valueToModify - _relocation_address);
+    
+    // Mask to keep only the relevant bits
+    newValue &= mask;
+    
+    // Reconstruct the full value
+    int preservedBits = currentValue & ~(mask << shift);
+    int finalValue = preservedBits | (newValue << shift);
+    
+    // Write back to memory (big-endian)
+    for (int i = 0; i < numBytes; ++i) {
+        _machine->setByte(address + i, (finalValue >> ((numBytes - 1 - i) * 8)) & 0xFF);
+    }
+}
diff --git a/simulator_SIC_XE/src/node.cpp b/simulator_SIC_XE/src/node.cpp
new file mode 100644
index 0000000..28c31fe
--- /dev/null
+++ b/simulator_SIC_XE/src/node.cpp
@@ -0,0 +1,121 @@
+#include "node.h"
+#include <sstream>
+#include <iomanip>
+#include <algorithm>
+
+string Node::toString() const {
+    std::ostringstream oss;
+    if (!_label.empty()) oss << _label << " ";
+    if (_mnemonic) oss << _mnemonic->toString() << " ";
+    if (!_comment.empty()) oss << "." << _comment;
+    return oss.str();
+}
+
+std::string Mnemonic::toString() const {
+    std::ostringstream oss;
+    oss << "[OP:" << std::hex << (int)_opcode << "]";
+    if (_extended) oss << "+";
+    // Print operands
+    for (size_t i = 0; i < _operands.size(); ++i) {
+        if (i > 0) oss << ",";
+        std::visit([&](auto&& arg) {
+            using T = std::decay_t<decltype(arg)>;
+            if constexpr (std::is_same_v<T, Empty>) {
+                // nothing
+            } else if constexpr (std::is_same_v<T, Register>) {
+                oss << "R" << arg.num;
+            } else if constexpr (std::is_same_v<T, Immediate>) {
+                oss << "#" << arg.value;
+            } else if constexpr (std::is_same_v<T, SymbolRef>) {
+                oss << arg.name;
+                if (arg.indexed) oss << ",X";
+            }
+        }, _operands[i]);
+    }
+    return oss.str();
+}
+
+string InstructionNode::toString() const {
+    std::ostringstream oss;
+    if (!_label.empty()) oss << _label << " ";
+    if (_mnemonic) oss << _mnemonic->toString();
+    if (!_comment.empty()) oss << " ." << _comment;
+    return oss.str();
+}
+
+string CommentNode::toString() const {
+    return "." + _comment;
+}
+
+string DirectiveNode::toString() const {
+    std::ostringstream oss;
+    if (!_label.empty()) oss << _label << " ";
+    switch (_kind) {
+        case DirectiveKind::START:  oss << "START"; break;
+        case DirectiveKind::END:    oss << "END"; break;
+        case DirectiveKind::BASE:   oss << "BASE"; break;
+        case DirectiveKind::NOBASE: oss << "NOBASE"; break;
+        case DirectiveKind::EQU:    oss << "EQU"; break;
+        case DirectiveKind::ORG:    oss << "ORG"; break;
+        case DirectiveKind::LTORG:  oss << "LTORG"; break;
+        case DirectiveKind::EXTDEF: oss << "EXTDEF"; break;
+        case DirectiveKind::EXTREF: oss << "EXTREF"; break;
+        case DirectiveKind::CSECT:  oss << "CSECT"; break;
+    }
+    std::visit([&](auto&& arg) {
+        using T = std::decay_t<decltype(arg)>;
+        if constexpr (std::is_same_v<T, std::monostate>) {
+            // no arg
+        } else if constexpr (std::is_same_v<T, int>) {
+            oss << " " << std::hex << arg;
+        } else if constexpr (std::is_same_v<T, std::string>) {
+            oss << " " << arg;
+        } else if constexpr (std::is_same_v<T, std::vector<std::string>>) {
+            for (size_t i = 0; i < arg.size(); ++i) {
+                if (i > 0) oss << ",";
+                oss << arg[i];
+            }
+        }
+    }, _arg);
+    if (!_comment.empty()) oss << " ." << _comment;
+    return oss.str();
+}
+
+string DataNode::toString() const {
+    std::ostringstream oss;
+    if (!_label.empty()) oss << _label << " ";
+    switch (_kind) {
+        case DataKind::WORD: oss << "WORD"; break;
+        case DataKind::BYTE: oss << "BYTE"; break;
+        case DataKind::RESW: oss << "RESW"; break;
+        case DataKind::RESB: oss << "RESB"; break;
+    }
+    std::visit([&](auto&& arg) {
+        using T = std::decay_t<decltype(arg)>;
+        if constexpr (std::is_same_v<T, std::monostate>) {
+            // no value
+        } else if constexpr (std::is_same_v<T, int>) {
+            oss << " " << arg;
+        } else if constexpr (std::is_same_v<T, std::vector<uint8_t>>) {
+            // Try to display as string if all printable ASCII
+            bool isPrintable = !arg.empty() && std::all_of(arg.begin(), arg.end(), 
+                [](uint8_t b) { return b >= 32 && b <= 126; });
+            
+            if (isPrintable) {
+                oss << " C'";
+                for (uint8_t b : arg) oss << static_cast<char>(b);
+                oss << "'";
+            } else {
+                // Display as hex
+                oss << " X'";
+                for (uint8_t b : arg) {
+                    oss << std::hex << std::setw(2) << std::setfill('0') << (int)b;
+                }
+                oss << "'";
+            }
+        }
+    }, _value);
+    if (!_comment.empty()) oss << " ." << _comment;
+    return oss.str();
+}
+    
\ No newline at end of file
diff --git a/simulator_SIC_XE/src/opcode.cpp b/simulator_SIC_XE/src/opcode.cpp
index 0b63ce8..fcda0c4 100644
--- a/simulator_SIC_XE/src/opcode.cpp
+++ b/simulator_SIC_XE/src/opcode.cpp
@@ -95,8 +95,36 @@ void loadInstructionSet()
         if (instructions[i].name == nullptr) instructions[i] = {"INVALID", InstructionType::INVALID, nullptr};
         if (instructionsEXEX[i].name == nullptr) instructionsEXEX[i] = {"INVALID", InstructionType::INVALID, nullptr};
     }
+
+    // Initialize mnemonicToOpcode map
+    for (int i = 0; i < 0xff; ++i) {
+        if (instructions[i].type != InstructionType::INVALID) {
+            mnemonicToOpcode.emplace(instructions[i].name, static_cast<uint8_t>(i));
+        }
+        if (instructionsEXEX[i].type != InstructionType::INVALID) {
+            mnemonicToOpcode.emplace(instructionsEXEX[i].name, static_cast<uint8_t>(i));
+        }
+    }
+    opcodeTablesInitialized = true;
 }
 
+std::optional<uint8_t> findOpcodeByMnemonic(std::string_view name)
+{
+    auto it = mnemonicToOpcode.find(name);
+    if (it == mnemonicToOpcode.end())
+        return std::nullopt;
+    return it->second;
+}
+
+const InstructionInfo& getInstructionInfo(uint8_t opcode)
+{
+    if (instructions[opcode].type != InstructionType::INVALID)
+        return instructions[opcode];
+    return instructionsEXEX[opcode];
+}
+
+
+
 AddressingMode getAddressingMode(int ni)
 {
     switch (ni) {
diff --git a/simulator_SIC_XE/src/parser.cpp b/simulator_SIC_XE/src/parser.cpp
new file mode 100644
index 0000000..3decfe9
--- /dev/null
+++ b/simulator_SIC_XE/src/parser.cpp
@@ -0,0 +1,465 @@
+// parser.cpp
+#include "parser.h"
+#include <cctype>
+#include <limits>
+#include <string_view>
+
+void Parser::initMnemonicMap() {
+    if (s_mnemonicMapInitialized) return;
+
+    loadInstructionSet();
+
+    for (int op = 0; op < 0xFF; ++op) {
+        const auto& info = instructions[op];
+        if (info.name && info.type != InstructionType::INVALID) {
+            s_nameToOpcode.emplace(info.name, static_cast<std::uint8_t>(op));
+        }
+        const auto& ex = instructionsEXEX[op];
+        if (ex.name && ex.type != InstructionType::INVALID) {
+            s_nameToOpcode.emplace(ex.name, static_cast<std::uint8_t>(op));
+        }
+    }
+
+    s_mnemonicMapInitialized = true;
+}
+
+std::shared_ptr<Mnemonic> Parser::makeMnemonic(const std::string& name, bool extended) {
+    initMnemonicMap();
+
+    auto it = s_nameToOpcode.find(name);
+    if (it == s_nameToOpcode.end()) {
+        throw SyntaxError("Invalid mnemonic '" + name + "'", lexer_.row, lexer_.col);
+    }
+
+    std::uint8_t opcode = it->second;
+    const InstructionInfo* info = nullptr;
+
+    if (instructions[opcode].type != InstructionType::INVALID) {
+        info = &instructions[opcode];
+    } else if (instructionsEXEX[opcode].type != InstructionType::INVALID) {
+        info = &instructionsEXEX[opcode];
+    }
+
+    if (!info) {
+        throw SyntaxError("Invalid mnemonic '" + name + "'", lexer_.row, lexer_.col);
+    }
+
+    if (extended && info->type != InstructionType::TYPE3_4) {
+        throw SyntaxError(
+            "Extended format not allowed for mnemonic '" + name + "'",
+            lexer_.row,
+            lexer_.col
+        );
+    }
+
+    return std::make_shared<Mnemonic>(opcode, info->type, extended);
+}
+
+std::string Parser::parseLabel() {
+    if (lexer_.col == 1 && std::isalpha(static_cast<unsigned char>(lexer_.peek()))) {
+        return std::string(lexer_.readAlphanumeric());
+    }
+    return {};
+}
+
+std::shared_ptr<Mnemonic> Parser::parseMnemonic() {
+    bool isExtended = lexer_.advanceIf('+');
+    std::string name(lexer_.readAlphanumeric());
+    if (name.empty()) {
+        throw SyntaxError("Mnemonic expected", lexer_.row, lexer_.col);
+    }
+    return makeMnemonic(name, isExtended);
+}
+
+std::string Parser::parseSymbol() {
+    return std::string(lexer_.readAlphanumeric());
+}
+
+int Parser::parseRegister() {
+    char ch = lexer_.advance();
+    constexpr std::string_view regs = "AXLBSTF";
+    auto pos = regs.find(ch);
+    if (pos == std::string_view::npos) {
+        throw SyntaxError(std::string("Invalid register '") + ch + "'", lexer_.row, lexer_.col);
+    }
+    return static_cast<int>(pos);
+}
+
+void Parser::parseComma() {
+    lexer_.skipWhitespace();
+    lexer_.advance(',');
+    lexer_.skipWhitespace();
+}
+
+bool Parser::parseIndexed() {
+    lexer_.skipWhitespace();
+    if (lexer_.advanceIf(',')) {
+        lexer_.skipWhitespace();
+        lexer_.advance('X');
+        return true;
+    }
+    return false;
+}
+
+static int digitValue(char c, int radix) {
+    if (radix < 2 || radix > 36) return -1;
+    int v = -1;
+    if (c >= '0' && c <= '9')      v = c - '0';
+    else if (c >= 'A' && c <= 'Z') v = c - 'A' + 10;
+    else if (c >= 'a' && c <= 'z') v = c - 'a' + 10;
+    if (v >= 0 && v < radix) return v;
+    return -1;
+}
+
+int Parser::parseNumber(int lo, int hi) {
+    auto parseDigits = [&](int radix) -> int {
+        std::string digits(lexer_.readDigits(radix));
+        if (digits.empty()) {
+            throw SyntaxError("Invalid number", lexer_.row, lexer_.col);
+        }
+
+        long long value = 0;
+        for (char c : digits) {
+            int d = digitValue(c, radix);
+            if (d < 0) throw SyntaxError("Invalid number", lexer_.row, lexer_.col);
+            value = value * radix + d;
+            if (value > std::numeric_limits<int>::max()) {
+                throw SyntaxError("Invalid number", lexer_.row, lexer_.col);
+            }
+        }
+        return static_cast<int>(value);
+    };
+
+    int num = 0;
+
+    if (lexer_.peek() == '0') {
+        int radix = -1;
+        switch (lexer_.peek(1)) {
+        case 'b': radix = 2;  break;
+        case 'o': radix = 8;  break;
+        case 'x': radix = 16; break;
+        default:  break;
+        }
+        if (radix != -1) {
+            lexer_.advance();
+            lexer_.advance();
+            num = parseDigits(radix);
+        } else {
+            num = parseDigits(10);
+        }
+    } else if (std::isdigit(static_cast<unsigned char>(lexer_.peek()))) {
+        num = parseDigits(10);
+    } else {
+        throw SyntaxError("Number expected", lexer_.row, lexer_.col);
+    }
+
+    if (std::isalnum(static_cast<unsigned char>(lexer_.peek()))) {
+        throw SyntaxError(
+            std::string("invalid digit '") + lexer_.peek() + "'",
+            lexer_.row,
+            lexer_.col
+        );
+    }
+
+    if (num < lo || num > hi) {
+        throw SyntaxError(
+            "Number '" + std::to_string(num) + "' out of range [" +
+            std::to_string(lo) + ".." + std::to_string(hi) + "]",
+            lexer_.row,
+            lexer_.col
+        );
+    }
+
+    return num;
+}
+
+std::vector<std::uint8_t> Parser::parseData() {
+    if (lexer_.advanceIf('C')) {
+        lexer_.advance('\'');
+        std::string s(lexer_.readTo('\''));
+        std::vector<std::uint8_t> data;
+        data.reserve(s.size());
+        for (unsigned char c : s) {
+            data.push_back(static_cast<std::uint8_t>(c));
+        }
+        return data;
+    }
+
+    if (lexer_.advanceIf('X')) {
+        lexer_.advance('\'');
+        std::string s(lexer_.readTo('\''));
+        if (s.size() % 2 != 0) {
+            throw SyntaxError("Invalid hex literal length", lexer_.row, lexer_.col);
+        }
+
+        std::vector<std::uint8_t> data;
+        data.reserve(s.size() / 2);
+
+        auto hexVal = [](char c) -> int {
+            if (c >= '0' && c <= '9') return c - '0';
+            if (c >= 'A' && c <= 'F') return c - 'A' + 10;
+            if (c >= 'a' && c <= 'f') return c - 'a' + 10;
+            return -1;
+        };
+
+        for (std::size_t i = 0; i < s.size(); i += 2) {
+            int hi = hexVal(s[i]);
+            int lo = hexVal(s[i + 1]);
+            if (hi < 0 || lo < 0) {
+                throw SyntaxError("Invalid hex digit in literal", lexer_.row, lexer_.col);
+            }
+            data.push_back(static_cast<std::uint8_t>((hi << 4) | lo));
+        }
+        return data;
+    }
+
+    if (std::isdigit(static_cast<unsigned char>(lexer_.peek()))) {
+        constexpr int MAX_WORD = 0xFFFFFF;
+        int num = parseNumber(0, MAX_WORD);
+        return {
+            static_cast<std::uint8_t>((num >> 16) & 0xFF),
+            static_cast<std::uint8_t>((num >> 8) & 0xFF),
+            static_cast<std::uint8_t>(num & 0xFF)
+        };
+    }
+
+    throw SyntaxError(
+        std::string("Invalid storage specifier '") + lexer_.peek() + "'",
+        lexer_.row,
+        lexer_.col
+    );
+}
+
+void Parser::parseOperands(Mnemonic& m) {
+    InstructionType t = m.type();
+    char c = lexer_.peek();
+
+    if (t == InstructionType::TYPE1) {
+        // TYPE1 has no operands
+        return;
+    }
+
+    if (t == InstructionType::TYPE2) {
+        // TYPE2: r1 or r1,r2 or r1,n
+        if (c == '\n' || c == '\0') return;
+
+        int r1 = parseRegister();
+        m.operands().emplace_back(Register{r1});
+        lexer_.skipWhitespace();
+
+        if (lexer_.peek() == ',') {
+            parseComma();
+            char c2 = lexer_.peek();
+            if (std::isalpha(static_cast<unsigned char>(c2))) {
+                int r2 = parseRegister();
+                m.operands().emplace_back(Register{r2});
+            } else if (std::isdigit(static_cast<unsigned char>(c2))) {
+                int n = parseNumber(0, 0xFFFF);
+                m.operands().emplace_back(Immediate{n});
+            } else {
+                throw SyntaxError("Invalid second operand", lexer_.row, lexer_.col);
+            }
+        }
+
+        return;
+    }
+
+    if (t == InstructionType::TYPE3_4) {
+        lexer_.skipWhitespace();
+        char c0 = lexer_.peek();
+        if (c0 == '\n' || c0 == '\0') {
+            // No operand (e.g., RSUB)
+            return;
+        }
+
+        bool immediate = false;
+        bool indirect  = false;
+
+        if (lexer_.advanceIf('#')) {
+            immediate = true;
+        } else if (lexer_.advanceIf('@')) {
+            indirect = true;
+        }
+
+        char c1 = lexer_.peek();
+        if (std::isdigit(static_cast<unsigned char>(c1))) {
+            int num = parseNumber(0, 0x7FFFFF);
+            if (immediate) {
+                m.operands().emplace_back(Immediate{num});
+            } else {
+                // Direct numeric addressing (rare, treat as immediate)
+                m.operands().emplace_back(Immediate{num});
+            }
+        } else if (std::isalpha(static_cast<unsigned char>(c1))) {
+            std::string symbol = parseSymbol();
+            bool indexed = parseIndexed();
+            m.operands().emplace_back(SymbolRef{symbol, indexed, immediate, indirect});
+        } else {
+            throw SyntaxError("Invalid operand", lexer_.row, lexer_.col);
+        }
+
+        return;
+    }
+}
+
+bool Parser::isDirective(const std::string& name) {
+    return name == "START" || name == "END" || name == "BASE" || name == "NOBASE" || 
+           name == "EQU" || name == "ORG" || name == "LTORG" || 
+           name == "EXTDEF" || name == "EXTREF" || name == "CSECT";
+}
+
+bool Parser::isDataDirective(const std::string& name) {
+    return name == "WORD" || name == "BYTE" || name == "RESW" || name == "RESB";
+}
+
+std::shared_ptr<Node> Parser::parseDirective(const std::string& label, const std::string& directive) {
+    lexer_.skipWhitespace();
+    
+    DirectiveArg argValue;
+    char c = lexer_.peek();
+    
+    // Parse argument based on first character
+    if (std::isalpha(c)) {
+        std::string arg = std::string(lexer_.readAlphanumeric());
+        argValue = arg;
+    } else if (std::isdigit(c) || c == '0') {
+        int num = parseNumber(0, 0xFFFFFF);
+        argValue = num;
+    } else {
+        // No argument
+        argValue = std::monostate{};
+    }
+    
+    lexer_.skipWhitespace();
+    std::string comment = std::string(lexer_.readTo('\n'));
+    
+    DirectiveKind kind;
+    if (directive == "START") kind = DirectiveKind::START;
+    else if (directive == "END") kind = DirectiveKind::END;
+    else if (directive == "BASE") kind = DirectiveKind::BASE;
+    else if (directive == "NOBASE") kind = DirectiveKind::NOBASE;
+    else if (directive == "EQU") kind = DirectiveKind::EQU;
+    else if (directive == "ORG") kind = DirectiveKind::ORG;
+    else if (directive == "LTORG") kind = DirectiveKind::LTORG;
+    else if (directive == "EXTDEF") kind = DirectiveKind::EXTDEF;
+    else if (directive == "EXTREF") kind = DirectiveKind::EXTREF;
+    else if (directive == "CSECT") kind = DirectiveKind::CSECT;
+    else throw SyntaxError("Unknown directive", lexer_.row, lexer_.col);
+    
+    return std::make_shared<DirectiveNode>(label, kind, argValue, comment);
+}
+
+std::shared_ptr<Node> Parser::parseDataDirective(const std::string& label, const std::string& directive) {
+    lexer_.skipWhitespace();
+    
+    DataKind kind;
+    if (directive == "WORD") kind = DataKind::WORD;
+    else if (directive == "BYTE") kind = DataKind::BYTE;
+    else if (directive == "RESW") kind = DataKind::RESW;
+    else if (directive == "RESB") kind = DataKind::RESB;
+    else throw SyntaxError("Unknown data directive", lexer_.row, lexer_.col);
+    
+    DataValue value;
+    if (kind == DataKind::WORD || kind == DataKind::RESW || kind == DataKind::RESB) {
+        int num = parseNumber(0, 0xFFFFFF);
+        value = num;
+    } else { // BYTE
+        auto bytes = parseData();
+        value = bytes;
+    }
+    
+    lexer_.skipWhitespace();
+    std::string comment = std::string(lexer_.readTo('\n'));
+    
+    return std::make_shared<DataNode>(label, kind, value, comment);
+}
+
+std::shared_ptr<Node> Parser::parseInstruction() {
+    if (lexer_.col == 1 && lexer_.peek() == '.') {
+        return std::make_shared<CommentNode>(
+            std::string(lexer_.readTo('\n'))
+        );
+    }
+
+    std::string label = parseLabel();
+
+    if (lexer_.skipWhitespace() && label.empty()) {
+        lexer_.advance();
+        return nullptr;
+    }
+
+    lexer_.skipWhitespace();
+    
+    // Check for comment after label - create a label-only instruction node
+    if (lexer_.peek() == '.') {
+        std::string comment = std::string(lexer_.readTo('\n'));
+        // Return an instruction node with just the label (null mnemonic)
+        auto node = std::make_shared<InstructionNode>(
+            std::move(label),
+            nullptr,
+            std::move(comment)
+        );
+        return node;
+    }
+    
+    // Check for extended format prefix
+    bool isExtended = lexer_.peek() == '+';
+    if (isExtended) {
+        lexer_.advance();
+    }
+    
+    std::string name = std::string(lexer_.readAlphanumeric());
+    
+    if (name.empty()) {
+        throw SyntaxError(
+            "Mnemonic or directive expected (label='" + label + "')",
+            lexer_.row,
+            lexer_.col
+        );
+    }
+    
+    // Check if it's a directive or data directive
+    if (isDirective(name)) {
+        return parseDirective(label, name);
+    }
+    
+    if (isDataDirective(name)) {
+        return parseDataDirective(label, name);
+    }
+    
+    // It's an instruction - create mnemonic
+    auto mnemonic = makeMnemonic(name, isExtended);
+    lexer_.skipWhitespace();
+
+    parseOperands(*mnemonic);
+    lexer_.skipWhitespace();
+
+    std::string comment(lexer_.readTo('\n'));
+
+    return std::make_shared<InstructionNode>(
+        std::move(label),
+        std::move(mnemonic),
+        std::move(comment)
+    );
+}
+
+Code Parser::parseCode() {
+    Code code;
+
+    while (lexer_.peek() > 0) {
+        while (lexer_.peek() > 0 && lexer_.col > 1) {
+            lexer_.readTo('\n');
+        }
+
+        if (auto node = parseInstruction()) {
+            code.addLine(node);
+        }
+    }
+
+    return code;
+}
+
+Code Parser::parse(const std::string& input) {
+    lexer_ = Lexer(input);
+    return parseCode();
+}