Rough outline

app/src/main/java/hatelace/Addi.java

@@ -0,0 +1,28 @@
+package hatelace;
+
+/** Add immediate to address, not register */
+public class Addi extends Instruction {
+    private int dest_addr;
+    private int src1_addr;
+    private int imm;
+
+    public Addi(int dest_addr, int src1_addr, int imm) {
+        this.dest_addr = dest_addr;
+        this.src1_addr = src1_addr;
+        this.imm = imm;
+    }
+
+    public void execute(Memory memory, ProgramCounter PC) {
+        int src1 = memory.read(this.src1_addr);
+        memory.write(this.dest_addr, src1 + this.imm);
+        PC.incPC();
+    }
+
+    protected String opcode() {
+        return "addi";
+    }
+
+    protected Object[] operands() {
+        return new Object[] {this.dest_addr, this.src1_addr, this.imm};
+    }
+}

app/src/main/java/hatelace/App.java

@@ -1,14 +1,16 @@
-/*
- * This Java source file was generated by the Gradle 'init' task.
- */
 package hatelace;
 
 public class App {
-    public String getGreeting() {
-        return "Hello World!";
-    }
-
     public static void main(String[] args) {
-        System.out.println(new App().getGreeting());
+        System.out.println("Starting...");
+
+        Memory memory = new Memory();
+        Computer computer = new Computer(memory);
+        Program program = new Program(new int[] { 1, 2, 3, 4, 5 });
+        computer.load(program);
+        computer.run();
+        memory.dump();
+
+        System.out.println("Completed");
     }
 }

app/src/main/java/hatelace/Computer.java

@@ -0,0 +1,23 @@
+package hatelace;
+
+public class Computer {
+    private Memory memory;
+
+    public Computer(Memory Memory) {
+        this.memory = Memory;
+    }
+
+    public void load(Program program) {
+        int[] instructions = program.getInstructions();
+        for (int i = 0; i < instructions.length; i++) {
+            this.memory.write(i, instructions[i]);
+        }
+    }
+
+    public void run() {
+        for(ProgramCounter PC = new ProgramCounter(); PC.getPC() < this.memory.size(); PC.incPC()) {
+            int instruction = this.memory.read(PC.getPC());
+            System.out.println(instruction);
+        }
+    }
+}

app/src/main/java/hatelace/Instruction.java

@@ -0,0 +1,23 @@
+package hatelace;
+
+public abstract class Instruction {
+    public abstract void execute(Memory memory, ProgramCounter pc);
+
+    protected abstract String opcode();
+
+    protected abstract Object[] operands();
+
+    @Override
+    public String toString() {
+        // Buffer to hold our formatted instruction
+        StringBuilder sb = new StringBuilder();
+        sb.append(this.opcode());
+
+        for (Object operand : this.operands()) {
+            sb.append(" ");
+            sb.append(operand);
+        }
+
+        return sb.toString();
+    }
+}

app/src/main/java/hatelace/Memory.java

@@ -0,0 +1,35 @@
+package hatelace;
+
+public class Memory {
+    protected int[] memory;
+
+    public Memory() {
+        this.memory = new int[1024];
+    }
+
+    public int read(int address) {
+        return this.memory[address];
+    }
+
+    public int size() {
+        return this.memory.length;
+    }
+
+    public void write(int address, int data) {
+        if (address < 0 || address >= this.memory.length) {
+            throw new IllegalArgumentException("Invalid memory address");
+        }
+        this.memory[address] = data;
+    }
+
+    /** Dump as IHEX-like format (see https://en.wikipedia.org/wiki/Intel_HEX) */
+    public void dump() {
+        for(int i = 0; i < this.memory.length; i+= 16) {
+            System.out.printf(":%02X%04X00 ", 16, i);
+            for(int j = 0; j < 16; j++) {
+                System.out.printf("%02X ", this.memory[i + j]);
+            }
+            System.out.println();
+        }
+    }
+}

app/src/main/java/hatelace/Program.java

@@ -0,0 +1,13 @@
+package hatelace;
+
+public class Program {
+    private int[] instructions;
+
+    public Program(int[] instructions) {
+        this.instructions = instructions;
+    }
+
+    public int[] getInstructions() {
+        return this.instructions;
+    }
+}

app/src/main/java/hatelace/ProgramCounter.java

@@ -0,0 +1,22 @@
+package hatelace;
+
+public class ProgramCounter {
+    private int PC;
+
+    public ProgramCounter() {
+        this.PC = 0;
+    }
+
+    public ProgramCounter(int PC) {
+        this.PC = PC;
+    }
+
+    public int getPC() {
+        return this.PC;
+    }
+
+    public int incPC() {
+        this.PC = this.PC + 1 <= 0 ? 0 : this.PC + 1;
+        return this.PC;
+    }
+}

app/src/test/java/hatelace/AppTest.java

@@ -1,6 +1,3 @@
-/*
- * This Java source file was generated by the Gradle 'init' task.
- */
 package hatelace;
 
 import org.junit.jupiter.api.Test;
@@ -8,7 +5,6 @@ import static org.junit.jupiter.api.Assertions.*;
 
 class AppTest {
     @Test void appHasAGreeting() {
-        App classUnderTest = new App();
+        assertNotNull(1);
-        assertNotNull(classUnderTest.getGreeting(), "app should have a greeting");
     }
 }