diff --git a/src/main/java/net/simon987/mar/cubot/event/UserCreationListener.java b/src/main/java/net/simon987/mar/cubot/event/UserCreationListener.java
index 414d7f1..3322a04 100644
--- a/src/main/java/net/simon987/mar/cubot/event/UserCreationListener.java
+++ b/src/main/java/net/simon987/mar/cubot/event/UserCreationListener.java
@@ -8,6 +8,7 @@ import net.simon987.mar.server.assembly.Assembler;
 import net.simon987.mar.server.assembly.AssemblyResult;
 import net.simon987.mar.server.assembly.CPU;
 import net.simon987.mar.server.assembly.exception.CancelledException;
+import net.simon987.mar.server.event.CpuInitialisationEvent;
 import net.simon987.mar.server.event.GameEvent;
 import net.simon987.mar.server.event.GameEventListener;
 import net.simon987.mar.server.event.UserCreationEvent;
@@ -55,13 +56,20 @@ public class UserCreationListener implements GameEventListener {
 
         //Create CPU
         try {
-            cubot.setCpu(new CPU(GameServer.INSTANCE.getConfig(), cubot));
+            CPU cpu = new CPU(config);
+            cubot.setCpu(cpu);
             cubot.getCpu().setHardwareHost(cubot);
             user.setUserCode(config.getString("new_user_code"));
 
+            GameEvent initEvent = new CpuInitialisationEvent(cpu, cubot);
+            GameServer.INSTANCE.getEventDispatcher().dispatch(event);
+            if (initEvent.isCancelled()) {
+                throw new CancelledException();
+            }
+
             //Compile user code
-            AssemblyResult ar = new Assembler(cubot.getCpu().getInstructionSet(), cubot.getCpu().getRegisterSet(),
-                    GameServer.INSTANCE.getConfig()).parse(user.getUserCode());
+            AssemblyResult ar = new Assembler(cpu.getInstructionSet(), cpu.getRegisterSet(),
+                    config).parse(user.getUserCode());
 
             cubot.getCpu().getMemory().clear();
 
diff --git a/src/main/java/net/simon987/mar/server/assembly/AssemblyResult.java b/src/main/java/net/simon987/mar/server/assembly/AssemblyResult.java
index 7f3a16c..5e6f5a2 100755
--- a/src/main/java/net/simon987/mar/server/assembly/AssemblyResult.java
+++ b/src/main/java/net/simon987/mar/server/assembly/AssemblyResult.java
@@ -22,7 +22,7 @@ public class AssemblyResult {
     /**
      * A list of labels
      */
-    HashMap<String, Character> labels = new HashMap<>(20);
+    public HashMap<String, Character> labels = new HashMap<>(20);
     /**
      * List of exceptions encountered during the assembly attempt,
      * they will be displayed in the editor
diff --git a/src/main/java/net/simon987/mar/server/assembly/CPU.java b/src/main/java/net/simon987/mar/server/assembly/CPU.java
index 5f6a16f..b8e3597 100755
--- a/src/main/java/net/simon987/mar/server/assembly/CPU.java
+++ b/src/main/java/net/simon987/mar/server/assembly/CPU.java
@@ -98,7 +98,7 @@ public class CPU implements MongoSerializable {
     /**
      * Creates a new CPU
      */
-    public CPU(IServerConfiguration config, ControllableUnit unit) throws CancelledException {
+    public CPU(IServerConfiguration config) throws CancelledException {
         instructionSet = new DefaultInstructionSet();
         registerSet = new DefaultRegisterSet();
         codeSectionOffset = config.getInt("org_offset");
@@ -132,12 +132,6 @@ public class CPU implements MongoSerializable {
 
         status = new Status();
         memory = new Memory(config.getInt("memory_size"));
-
-        GameEvent event = new CpuInitialisationEvent(this, unit);
-        GameServer.INSTANCE.getEventDispatcher().dispatch(event);
-        if (event.isCancelled()) {
-            throw new CancelledException();
-        }
     }
 
     public void reset() {
@@ -398,15 +392,20 @@ public class CPU implements MongoSerializable {
 
     public static CPU deserialize(Document obj, ControllableUnit unit) throws CancelledException {
 
-        CPU cpu = new CPU(GameServer.INSTANCE.getConfig(), unit);
+        CPU cpu = new CPU(GameServer.INSTANCE.getConfig());
 
         cpu.codeSectionOffset = obj.getInteger("codeSegmentOffset");
 
         cpu.memory = new Memory((Document) obj.get("memory"));
         cpu.registerSet = RegisterSet.deserialize((Document) obj.get("registerSet"));
 
-        return cpu;
+        GameEvent event = new CpuInitialisationEvent(cpu, unit);
+        GameServer.INSTANCE.getEventDispatcher().dispatch(event);
+        if (event.isCancelled()) {
+            throw new CancelledException();
+        }
 
+        return cpu;
     }
 
     public InstructionSet getInstructionSet() {
@@ -466,4 +465,15 @@ public class CPU implements MongoSerializable {
     public void setHardwareHost(HardwareHost hardwareHost) {
         this.hardwareHost = hardwareHost;
     }
+
+    /**
+     * For testing/debugging, this creates an copy (please be mindful of the memory usage)
+     */
+    public CpuState getState() {
+        return new CpuState(
+                registerSet.clone(),
+                memory.clone(),
+                status.clone()
+        );
+    }
 }
diff --git a/src/main/java/net/simon987/mar/server/assembly/CpuState.java b/src/main/java/net/simon987/mar/server/assembly/CpuState.java
new file mode 100644
index 0000000..60115ba
--- /dev/null
+++ b/src/main/java/net/simon987/mar/server/assembly/CpuState.java
@@ -0,0 +1,16 @@
+package net.simon987.mar.server.assembly;
+
+public class CpuState {
+
+    public RegisterSet registers;
+
+    public Memory memory;
+
+    public Status status;
+
+    public CpuState(RegisterSet registers, Memory memory, Status status) {
+        this.registers = registers;
+        this.memory = memory;
+        this.status = status;
+    }
+}
diff --git a/src/main/java/net/simon987/mar/server/assembly/DefaultRegisterSet.java b/src/main/java/net/simon987/mar/server/assembly/DefaultRegisterSet.java
index 8ae4cec..3fd3189 100755
--- a/src/main/java/net/simon987/mar/server/assembly/DefaultRegisterSet.java
+++ b/src/main/java/net/simon987/mar/server/assembly/DefaultRegisterSet.java
@@ -9,14 +9,13 @@ class DefaultRegisterSet extends RegisterSet {
     DefaultRegisterSet() {
         super();
 
-        addRegister(1, new Register("A"));
-        addRegister(2, new Register("B"));
-        addRegister(3, new Register("C"));
-        addRegister(4, new Register("D"));
-        addRegister(5, new Register("X"));
-        addRegister(6, new Register("Y"));
-        addRegister(7, new Register("SP"));
-        addRegister(8, new Register("BP"));
-
+        put(1, new Register("A"));
+        put(2, new Register("B"));
+        put(3, new Register("C"));
+        put(4, new Register("D"));
+        put(5, new Register("X"));
+        put(6, new Register("Y"));
+        put(7, new Register("SP"));
+        put(8, new Register("BP"));
     }
 }
diff --git a/src/main/java/net/simon987/mar/server/assembly/Memory.java b/src/main/java/net/simon987/mar/server/assembly/Memory.java
index 1d2a9d5..bc30408 100755
--- a/src/main/java/net/simon987/mar/server/assembly/Memory.java
+++ b/src/main/java/net/simon987/mar/server/assembly/Memory.java
@@ -21,7 +21,7 @@ import java.util.zip.InflaterOutputStream;
 /**
  * Represents the available memory for a CPU in the game universe
  */
-public class Memory implements Target, MongoSerializable {
+public class Memory implements Target, MongoSerializable, Cloneable {
 
 
     /**
@@ -105,6 +105,7 @@ public class Memory implements Target, MongoSerializable {
         address = (char) address;
 
         if (address >= words.length) {
+            Thread.dumpStack();
             LogManager.LOGGER.info("DEBUG: Trying to set memory out of bounds: " + address);
             return;
         }
@@ -117,25 +118,25 @@ public class Memory implements Target, MongoSerializable {
      *
      * @param blockSize Block size (in words) in which to randomly flip one bit
      */
-     public void corrupt(int blockSize) {
-         Random rand = new Random();
+    public void corrupt(int blockSize) {
+        Random rand = new Random();
 
-         // Increment offset by blockSize
-         for (int offset = 0; offset < words.length; offset += blockSize) {
+        // Increment offset by blockSize
+        for (int offset = 0; offset < words.length; offset += blockSize) {
 
-             // Calculate address to corrupt by adding a random value between 0 to (blocksize-1) to offset
-             int address = rand.nextInt(blockSize) + offset;
+            // Calculate address to corrupt by adding a random value between 0 to (blocksize-1) to offset
+            int address = rand.nextInt(blockSize) + offset;
 
-             // Checking here avoids having a protected area at the end of the address space
-             if(address < words.length) {
+            // Checking here avoids having a protected area at the end of the address space
+            if (address < words.length) {
 
-                 // Calculate bitmask by left-shifting 1 by a random value between 0 and 15
-                 int bitmask = 1 << rand.nextInt(16);
+                // Calculate bitmask by left-shifting 1 by a random value between 0 and 15
+                int bitmask = 1 << rand.nextInt(16);
 
-                 // Flip the bit with XOR
-                 words[address] ^= bitmask;
-             }
-         }
+                // Flip the bit with XOR
+                words[address] ^= bitmask;
+            }
+        }
     }
 
     /**
@@ -186,4 +187,12 @@ public class Memory implements Target, MongoSerializable {
     public char[] getWords() {
         return words;
     }
+
+    @Override
+    public Memory clone() {
+        Memory memory = new Memory(words.length);
+        memory.words = new char[words.length];
+        System.arraycopy(memory.words, 0, words, 0, words.length);
+        return memory;
+    }
 }
diff --git a/src/main/java/net/simon987/mar/server/assembly/Register.java b/src/main/java/net/simon987/mar/server/assembly/Register.java
index 2171c5d..80cea2b 100755
--- a/src/main/java/net/simon987/mar/server/assembly/Register.java
+++ b/src/main/java/net/simon987/mar/server/assembly/Register.java
@@ -3,7 +3,7 @@ package net.simon987.mar.server.assembly;
 /**
  * Represents a register in a cpu
  */
-public class Register {
+public class Register implements Cloneable {
 
     /**
      * Name of the register
@@ -47,4 +47,14 @@ public class Register {
         return name + "=" + value;
 
     }
+
+    @Override
+    public Register clone() {
+        try {
+            return (Register) super.clone();
+        } catch (CloneNotSupportedException e) {
+            e.printStackTrace();
+        }
+        return null;
+    }
 }
diff --git a/src/main/java/net/simon987/mar/server/assembly/RegisterSet.java b/src/main/java/net/simon987/mar/server/assembly/RegisterSet.java
index f0ca554..f0fcbe7 100755
--- a/src/main/java/net/simon987/mar/server/assembly/RegisterSet.java
+++ b/src/main/java/net/simon987/mar/server/assembly/RegisterSet.java
@@ -2,29 +2,23 @@ package net.simon987.mar.server.assembly;
 
 
 import net.simon987.mar.server.io.MongoSerializable;
-import net.simon987.mar.server.logging.LogManager;
 import org.bson.Document;
 import org.json.simple.JSONArray;
 import org.json.simple.JSONObject;
 
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.List;
 
 /**
  * A set of registers for a CPU
  */
-public class RegisterSet implements Target, MongoSerializable {
+public class RegisterSet implements Target, MongoSerializable, Cloneable {
 
-    /**
-     * List of registers
-     */
-    private final HashMap<Integer, Register> registers = new HashMap<>(8);
+    // TODO configurable number of registers
+    private static final int REG_COUNT = 8 + 1;
 
+    private final Register[] registers = new Register[REG_COUNT];
 
-    /**
-     * Create an empty Register set
-     */
     public RegisterSet() {
 
     }
@@ -40,9 +34,8 @@ public class RegisterSet implements Target, MongoSerializable {
 
         name = name.toUpperCase();
 
-        for (Integer i : registers.keySet()) {
-            if (registers.get(i).getName().equals(name)) {
-
+        for (int i = 1; i < REG_COUNT; i++) {
+            if (registers[i].getName().equals(name)) {
                 return i;
             }
         }
@@ -56,7 +49,7 @@ public class RegisterSet implements Target, MongoSerializable {
      * @param index index of the register
      */
     public Register getRegister(int index) {
-        return registers.get(index);
+        return registers[index];
     }
 
     /**
@@ -68,7 +61,8 @@ public class RegisterSet implements Target, MongoSerializable {
 
         name = name.toUpperCase();
 
-        for (Register r : registers.values()) {
+        for (Register r : registers) {
+            if (r == null) continue;
             if (r.getName().equals(name)) {
                 return r;
             }
@@ -86,15 +80,11 @@ public class RegisterSet implements Target, MongoSerializable {
      */
     @Override
     public int get(int address) {
-
-        Register register = registers.get(address);
-
-        if (register != null) {
-            return register.getValue();
-        } else {
+        if (address <= 0 || address >= REG_COUNT) {
             return 0;
         }
 
+        return registers[address].getValue();
     }
 
     /**
@@ -105,23 +95,17 @@ public class RegisterSet implements Target, MongoSerializable {
      */
     @Override
     public void set(int address, int value) {
-
-        Register register = registers.get(address);
-
-        if (register != null) {
-            register.setValue(value);
-        } else {
-            LogManager.LOGGER.info("DEBUG: trying to set unknown reg index : " + address);
+        if (address <= 0 || address >= REG_COUNT) {
+            return;
         }
-    }
 
-    public void put(int index, Register register) {
-        registers.put(index, register);
+        registers[address].setValue(value);
     }
 
     public void clear() {
-        for (Register register : registers.values()) {
-            register.setValue(0);
+        for (Register r : registers) {
+            if (r == null) continue;
+            r.setValue(0);
         }
     }
 
@@ -134,24 +118,24 @@ public class RegisterSet implements Target, MongoSerializable {
      * @param index Index of the register
      * @param reg   Register to add
      */
-    void addRegister(int index, Register reg) {
-        registers.put(index, reg);
+    public void put(int index, Register reg) {
+        registers[index] = reg;
     }
 
     int size() {
-        return registers.size();
+        return REG_COUNT - 1;
     }
 
 
     @Override
     public Document mongoSerialise() {
         List<Document> registers = new ArrayList<>();
-        for (Integer index : this.registers.keySet()) {
+        for (int i = 1; i < REG_COUNT; i++) {
             Document register = new Document();
 
-            register.put("index", index);
-            register.put("name", getRegister(index).getName());
-            register.put("value", (int) getRegister(index).getValue());
+            register.put("index", i);
+            register.put("name", getRegister(i).getName());
+            register.put("value", (int) getRegister(i).getValue());
 
             registers.add(register);
         }
@@ -169,11 +153,12 @@ public class RegisterSet implements Target, MongoSerializable {
         List registers = (ArrayList) obj.get("registers");
 
         for (Object sRegister : registers) {
+            if (sRegister == null) continue;
 
             Register register = new Register((String) ((Document) sRegister).get("name"));
             register.setValue(((Document) sRegister).getInteger("value"));
 
-            registerSet.registers.put(((Document) sRegister).getInteger("index"), register);
+            registerSet.put(((Document) sRegister).getInteger("index"), register);
 
         }
 
@@ -191,7 +176,7 @@ public class RegisterSet implements Target, MongoSerializable {
             Register register = new Register((String) jsonRegister.get("name"));
             register.setValue((int) (long) jsonRegister.get("value"));
 
-            registerSet.registers.put((int) (long) jsonRegister.get("index"), register);
+            registerSet.put((int) (long) jsonRegister.get("index"), register);
 
         }
 
@@ -202,10 +187,20 @@ public class RegisterSet implements Target, MongoSerializable {
     public String toString() {
         String str = "";
 
-        for (Integer index : registers.keySet()) {
-            str += index + " " + registers.get(index).getName() + "=" + Util.toHex(registers.get(index).getValue()) + "\n";
+        for (int i = 1; i < REG_COUNT; i++) {
+            str += i + " " + getRegister(i).getName() + "=" + Util.toHex(getRegister(i).getValue()) + "\n";
         }
 
         return str;
     }
+
+    @Override
+    public RegisterSet clone() {
+        RegisterSet rs = new RegisterSet();
+
+        for (int i = 1; i < REG_COUNT; i++) {
+            rs.put(i, getRegister(i).clone());
+        }
+        return rs;
+    }
 }
diff --git a/src/main/java/net/simon987/mar/server/assembly/Status.java b/src/main/java/net/simon987/mar/server/assembly/Status.java
index 6393282..21c0e82 100755
--- a/src/main/java/net/simon987/mar/server/assembly/Status.java
+++ b/src/main/java/net/simon987/mar/server/assembly/Status.java
@@ -3,7 +3,7 @@ package net.simon987.mar.server.assembly;
 /**
  * Represents the state of the processor
  */
-public class Status {
+public class Status implements Cloneable {
 
     /**
      * Set to true when the result of
@@ -148,4 +148,14 @@ public class Status {
         setCarryFlag((stat & (1 << 1)) != 0);
         setOverflowFlag((stat & 1) != 0);
     }
+
+    @Override
+    public Status clone() {
+        try {
+            return (Status) super.clone();
+        } catch (CloneNotSupportedException e) {
+            e.printStackTrace();
+        }
+        return null;
+    }
 }
diff --git a/src/main/java/net/simon987/mar/server/assembly/instruction/HwiInstruction.java b/src/main/java/net/simon987/mar/server/assembly/instruction/HwiInstruction.java
index 8a057ac..fb059aa 100755
--- a/src/main/java/net/simon987/mar/server/assembly/instruction/HwiInstruction.java
+++ b/src/main/java/net/simon987/mar/server/assembly/instruction/HwiInstruction.java
@@ -23,17 +23,12 @@ public class HwiInstruction extends Instruction {
     @Override
     public Status execute(Target src, int srcIndex, Status status) {
         status.setErrorFlag(cpu.getHardwareHost().hardwareInterrupt(src.get(srcIndex), cpu.getStatus()));
-
         return status;
     }
 
     @Override
     public Status execute(int src, Status status) {
-
         status.setErrorFlag(cpu.getHardwareHost().hardwareInterrupt(src, cpu.getStatus()));
-
         return status;
     }
-
-
 }
diff --git a/src/main/java/net/simon987/mar/server/game/objects/HardwareHost.java b/src/main/java/net/simon987/mar/server/game/objects/HardwareHost.java
index 01946d8..79df104 100644
--- a/src/main/java/net/simon987/mar/server/game/objects/HardwareHost.java
+++ b/src/main/java/net/simon987/mar/server/game/objects/HardwareHost.java
@@ -12,5 +12,4 @@ public interface HardwareHost {
     boolean hardwareInterrupt(int address, Status status);
 
     int hardwareQuery(int address);
-
 }
diff --git a/src/test/java/net/simon987/mar/server/FakeHardwareHost.java b/src/test/java/net/simon987/mar/server/FakeHardwareHost.java
new file mode 100644
index 0000000..e456aa0
--- /dev/null
+++ b/src/test/java/net/simon987/mar/server/FakeHardwareHost.java
@@ -0,0 +1,53 @@
+package net.simon987.mar.server;
+
+import net.simon987.mar.server.assembly.CPU;
+import net.simon987.mar.server.assembly.CpuState;
+import net.simon987.mar.server.assembly.HardwareModule;
+import net.simon987.mar.server.assembly.Status;
+import net.simon987.mar.server.game.objects.HardwareHost;
+
+import java.util.ArrayList;
+import java.util.List;
+
+
+public class FakeHardwareHost implements HardwareHost {
+
+    public final List<HwiCall> callHistory = new ArrayList<>();
+    private final CPU cpu;
+
+    public FakeHardwareHost(CPU cpu) {
+        this.cpu = cpu;
+    }
+
+    @Override
+    public void attachHardware(HardwareModule hardware, int address) {
+        // noop
+    }
+
+    @Override
+    public void detachHardware(int address) {
+        // noop
+    }
+
+    @Override
+    public boolean hardwareInterrupt(int address, Status status) {
+        callHistory.add(new HwiCall(address, cpu.getState()));
+        return true;
+    }
+
+    @Override
+    public int hardwareQuery(int address) {
+        return -1;
+    }
+
+    public static class HwiCall {
+        public HwiCall(int address, CpuState state) {
+            this.address = address;
+            this.state = state;
+        }
+
+        public int address;
+        public CpuState state;
+    }
+}
+
diff --git a/src/test/java/net/simon987/mar/server/TestExecutionResult.java b/src/test/java/net/simon987/mar/server/TestExecutionResult.java
new file mode 100644
index 0000000..198f4e4
--- /dev/null
+++ b/src/test/java/net/simon987/mar/server/TestExecutionResult.java
@@ -0,0 +1,35 @@
+package net.simon987.mar.server;
+
+import net.simon987.mar.server.assembly.AssemblyResult;
+import net.simon987.mar.server.assembly.CpuState;
+
+import java.util.List;
+
+public class TestExecutionResult {
+
+    public CpuState state;
+
+    public List<FakeHardwareHost.HwiCall> hwiHistory;
+
+    private AssemblyResult ar;
+
+    public TestExecutionResult(CpuState state, List<FakeHardwareHost.HwiCall> hwiHistory, AssemblyResult ar) {
+        this.state = state;
+        this.hwiHistory = hwiHistory;
+        this.ar = ar;
+    }
+
+    public int regValue(String register) {
+        return state.registers.getRegister(register).getValue();
+    }
+
+    public int labelOffset(String label) {
+        return ar.labels.get(label);
+    }
+
+    public int memValue(int offset) {
+        return state.memory.get(offset);
+    }
+
+    // Add more utility methods here
+}
diff --git a/src/test/java/net/simon987/mar/server/assembly/RegisterSetTest.java b/src/test/java/net/simon987/mar/server/assembly/RegisterSetTest.java
index d69cd1a..fc80665 100644
--- a/src/test/java/net/simon987/mar/server/assembly/RegisterSetTest.java
+++ b/src/test/java/net/simon987/mar/server/assembly/RegisterSetTest.java
@@ -74,5 +74,4 @@ public class RegisterSetTest {
         //Test unknown indexes
         registerSet.set(3, 10);
     }
-
 }
diff --git a/src/test/java/net/simon987/mar/server/assembly/TestHelper.java b/src/test/java/net/simon987/mar/server/assembly/TestHelper.java
index 570d590..bb5ab28 100644
--- a/src/test/java/net/simon987/mar/server/assembly/TestHelper.java
+++ b/src/test/java/net/simon987/mar/server/assembly/TestHelper.java
@@ -1,18 +1,57 @@
 package net.simon987.mar.server.assembly;
 
 import net.simon987.mar.server.FakeConfiguration;
+import net.simon987.mar.server.FakeHardwareHost;
 import net.simon987.mar.server.IServerConfiguration;
+import net.simon987.mar.server.TestExecutionResult;
+import net.simon987.mar.server.assembly.exception.CancelledException;
 
-class TestHelper {
+public class TestHelper {
 
-    static Assembler getTestAsm() {
+    private static final int TIMEOUT = 100;
 
-        IServerConfiguration configuration = new FakeConfiguration();
-
-        configuration.setInt("memory_size", 1000);
-        configuration.setInt("org_offset", 400);
-
-        return new Assembler(new DefaultInstructionSet(), new DefaultRegisterSet(), configuration);
+    public static Assembler getTestAsm() {
+        IServerConfiguration config = getTestConfig();
+        CPU cpu = getTestCpu();
+        return new Assembler(cpu.getInstructionSet(), cpu.getRegisterSet(), config);
     }
 
+    public static IServerConfiguration getTestConfig() {
+        IServerConfiguration configuration = new FakeConfiguration();
+
+        configuration.setInt("memory_size", 65536);
+        configuration.setInt("org_offset", 400);
+        return configuration;
+    }
+
+    public static CPU getTestCpu() {
+
+        CPU cpu = null;
+        try {
+            cpu = new CPU(getTestConfig());
+        } catch (CancelledException e) {
+            e.printStackTrace();
+        }
+        return cpu;
+    }
+
+    public static TestExecutionResult executeCode(String code) {
+        AssemblyResult ar = getTestAsm().parse(code);
+        CPU cpu = TestHelper.getTestCpu();
+
+        FakeHardwareHost host = new FakeHardwareHost(cpu);
+        cpu.setHardwareHost(host);
+
+        cpu.getMemory().clear();
+
+        char[] assembledCode = ar.getWords();
+
+        cpu.getMemory().write((char) ar.origin, assembledCode, 0, assembledCode.length);
+        cpu.setCodeSectionOffset(ar.getCodeSectionOffset());
+
+        cpu.reset();
+        cpu.execute(TIMEOUT);
+
+        return new TestExecutionResult(cpu.getState(), host.callHistory, ar);
+    }
 }
diff --git a/src/test/java/net/simon987/mar/server/assembly/instruction/CallInstructionTest.java b/src/test/java/net/simon987/mar/server/assembly/instruction/CallInstructionTest.java
index 9fd19cf..e5f286e 100644
--- a/src/test/java/net/simon987/mar/server/assembly/instruction/CallInstructionTest.java
+++ b/src/test/java/net/simon987/mar/server/assembly/instruction/CallInstructionTest.java
@@ -1,6 +1,30 @@
 package net.simon987.mar.server.assembly.instruction;
 
+import net.simon987.mar.server.FakeConfiguration;
+import net.simon987.mar.server.IServerConfiguration;
+import net.simon987.mar.server.TestExecutionResult;
+import net.simon987.mar.server.assembly.*;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
 public class CallInstructionTest {
 
+    @Test
+    public void callSimple1() {
+        String code = "" +
+                "my_routine:            \n" +
+                "   MOV X, 0x1234       \n" +
+                "   RET                 \n" +
+                ".text                  \n" +
+                "CALL my_routine        \n" +
+                "MOV Y, 0x4567          \n" +
+                "brk                    \n";
 
+        TestExecutionResult res = TestHelper.executeCode(code);
+
+        assertEquals(0x1234, res.regValue("X"));
+        assertEquals(0x4567, res.regValue("Y"));
+    }
 }
diff --git a/src/test/java/net/simon987/mar/server/assembly/instruction/HwiInstructionTest.java b/src/test/java/net/simon987/mar/server/assembly/instruction/HwiInstructionTest.java
new file mode 100644
index 0000000..1ab7b22
--- /dev/null
+++ b/src/test/java/net/simon987/mar/server/assembly/instruction/HwiInstructionTest.java
@@ -0,0 +1,26 @@
+package net.simon987.mar.server.assembly.instruction;
+
+import net.simon987.mar.server.FakeHardwareHost;
+import net.simon987.mar.server.TestExecutionResult;
+import net.simon987.mar.server.assembly.TestHelper;
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class HwiInstructionTest {
+
+    @Test
+    public void hwiSimple1() {
+        String code = "" +
+                "MOV A, 0x123           \n" +
+                "HWI 0x4567             \n" +
+                "MOV A, 0x789           \n" +
+                "brk                    \n";
+
+        TestExecutionResult res = TestHelper.executeCode(code);
+
+        FakeHardwareHost.HwiCall call = res.hwiHistory.get(0);
+        assertEquals(0x4567, call.address);
+        assertEquals(0x123, call.state.registers.getRegister("A").getValue());
+    }
+}