Add support for D register, more opcodes

examples/test2.cpp

@@ -0,0 +1,49 @@
+#include <cstdint>
+
+enum class Colors : uint8_t
+{
+  WHITE=0x01,
+  BLACK=0x00
+};
+
+volatile uint8_t &memory_loc(const uint16_t loc)
+{
+  return *reinterpret_cast<volatile uint8_t *>(loc);
+}
+
+void decrement_border_color()
+{
+  --memory_loc(0xd020);
+}
+
+void increment_border_color()
+{
+  ++memory_loc(0xd020);
+}
+
+bool joystick_down()
+{
+  uint8_t joystick_state = memory_loc(0xDC00);
+  return (joystick_state & 0x2) == 0;
+}
+
+int main()
+{
+  const auto background_color = [](Colors col) {
+    memory_loc(0xd021) = static_cast<uint8_t>(col);
+  };
+
+  const auto border_color = [](Colors col) {
+    memory_loc(0xd020) = static_cast<uint8_t>(col);
+  };
+
+  background_color(Colors::WHITE);
+
+  while(true) {
+    if (joystick_down()) {
+      border_color(Colors::WHITE);
+    } else {
+      border_color(Colors::BLACK);
+    }
+  }
+}