diff --git a/src/emu.hpp b/src/emu.hpp
index bae413a..53c7cfd 100644
--- a/src/emu.hpp
+++ b/src/emu.hpp
@@ -20,7 +20,8 @@ struct EmulatorState {
     
     uint16_t stack[stack_size] = {};
     int stack_pointer = 0;
-    
+
+    // this is technically 12-bits wide, but of course we dont have access to that :-)
     uint16_t I = 0;
     
     uint8_t v[16] = {};
diff --git a/tests/test.cpp b/tests/test.cpp
index 97875aa..0b8c37b 100644
--- a/tests/test.cpp
+++ b/tests/test.cpp
@@ -4,6 +4,23 @@
 
 #include "emu.hpp"
 
+TEST_CASE("Test 0x0") {
+    state.reset();
+
+    state.pixels[0] = 1;
+
+    // Display clear
+    process_opcode(0x00E0);
+    CHECK(state.pixels[0] == 0);
+
+    state.stack_pointer = 1;
+
+    // Subroutine return
+    process_opcode(0x00EE);
+    CHECK(state.stack_pointer == 0);
+}
+
+
 TEST_CASE("Test 0x1") {
     state.reset();