diff --git a/src/app/asm/script.cc b/src/app/asm/script.cc
index e7bc30b0..1972e958 100644
--- a/src/app/asm/script.cc
+++ b/src/app/asm/script.cc
@@ -56,6 +56,10 @@ std::string GenerateBytePool(char mosaic_tiles[core::kNumOverworldMaps]) {
 }
 
 absl::Status Script::ApplyPatchToROM(ROM &rom) {
+  if (patch_contents_.empty() || patch_filename_.empty()) {
+    return absl::InvalidArgumentError("No patch loaded!");
+  }
+
   char *data = (char *)rom.data();
   int size = rom.GetSize();
   int count = 0;
@@ -68,7 +72,8 @@ absl::Status Script::ApplyPatchToROM(ROM &rom) {
 }
 
 absl::Status Script::GenerateMosaicChangeAssembly(
-    ROM &rom, char mosaic_tiles[core::kNumOverworldMaps], int routine_offset) {
+    ROM &rom, char mosaic_tiles[core::kNumOverworldMaps], int routine_offset,
+    int hook_offset) {
   std::fstream file("assets/asm/mosaic_change.asm",
                     std::ios::out | std::ios::in);
   if (!file.is_open()) {
@@ -81,6 +86,7 @@ absl::Status Script::GenerateMosaicChangeAssembly(
   file.close();
 
   auto assembly_string = assembly.str();
+
   if (!core::StringReplace(assembly_string, "<HOOK>", kMosaicChangeOffset)) {
     return absl::InternalError(
         "Mosaic template did not have proper `<HOOK>` to replace.");
@@ -94,6 +100,7 @@ absl::Status Script::GenerateMosaicChangeAssembly(
   }
 
   assembly_string += GenerateBytePool(mosaic_tiles);
+  patch_contents_ = assembly_string;
   patch_filename_ = "assets/asm/mosaic_change_generated.asm";
   std::ofstream new_file(patch_filename_, std::ios::out);
   if (new_file.is_open()) {
diff --git a/src/app/asm/script.h b/src/app/asm/script.h
index e5edcfe9..4b8e4a7c 100644
--- a/src/app/asm/script.h
+++ b/src/app/asm/script.h
@@ -23,16 +23,24 @@ namespace snes_asm {
 const std::string kMosaicChangeOffset = "$02AADB";
 constexpr int kSNESToPCOffset = 0x138000;
 
-class Script {
+class ScriptTemplate {
+ public:
+  virtual absl::Status ApplyPatchToROM(ROM& rom) = 0;
+  virtual absl::Status GenerateMosaicChangeAssembly(
+      ROM& rom, char mosaic_tiles[core::kNumOverworldMaps], int routine_offset,
+      int hook_offset = 0) = 0;
+};
+
+class Script : public ScriptTemplate {
  public:
   Script() { asar_init_with_dll_path("assets/libasar.dll"); }
 
+  absl::Status ApplyPatchToROM(ROM& rom) override;
   absl::Status GenerateMosaicChangeAssembly(
-      ROM& rom, char mosaic_tiles[core::kNumOverworldMaps], int routine_offset);
+      ROM& rom, char mosaic_tiles[core::kNumOverworldMaps], int routine_offset,
+      int hook_offset = 0) override;
 
  private:
-  absl::Status ApplyPatchToROM(ROM& rom);
-
   int64_t patch_size_;
   std::string patch_filename_;
   std::string patch_contents_;
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 0b83417e..125c4c0d 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -14,6 +14,7 @@ add_executable(
   yaze_test
   yaze_test.cc
   rom_test.cc
+  asm_test.cc
   ../src/app/rom.cc
   ../src/app/asm/script.cc
   ../src/app/gfx/bitmap.cc
diff --git a/test/asm_test.cc b/test/asm_test.cc
new file mode 100644
index 00000000..6ce8cfcc
--- /dev/null
+++ b/test/asm_test.cc
@@ -0,0 +1,66 @@
+#include <asardll.h>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <array>
+#include <cstdint>
+#include <fstream>
+#include <sstream>
+#include <string>
+
+#include "absl/status/status.h"
+#include "absl/status/statusor.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "app/asm/script.h"
+#include "app/core/constants.h"
+#include "app/rom.h"
+
+namespace yaze_test {
+namespace asm_test {
+
+using yaze::app::ROM;
+using yaze::app::snes_asm::Script;
+
+using ::testing::_;
+using ::testing::ElementsAreArray;
+using ::testing::Eq;
+using ::testing::Return;
+
+class MockScript : public Script {
+ public:
+  MOCK_METHOD(absl::Status, ApplyPatchToROM, (ROM & rom));
+  MOCK_METHOD(absl::Status, GenerateMosaicChangeAssembly,
+              (ROM & rom, char mosaic_tiles[yaze::app::core::kNumOverworldMaps],
+               int routine_offset, int hook_offset));
+};
+
+TEST(ASMTest, ApplyMosaicChangePatchOk) {
+  ROM rom;
+  MockScript script;
+  char mosaic_tiles[yaze::app::core::kNumOverworldMaps];
+
+  EXPECT_CALL(script, GenerateMosaicChangeAssembly(_, Eq(mosaic_tiles),
+                                                   Eq(0x1301D0 + 0x138000), 0))
+      .WillOnce(Return(absl::OkStatus()));
+
+  EXPECT_CALL(script, ApplyPatchToROM(_)).WillOnce(Return(absl::OkStatus()));
+
+  EXPECT_THAT(script.GenerateMosaicChangeAssembly(rom, mosaic_tiles,
+                                                  0x1301D0 + 0x138000, 0),
+              absl::OkStatus());
+  EXPECT_THAT(script.ApplyPatchToROM(rom), absl::OkStatus());
+}
+
+TEST(ASMTest, NoPatchLoadedError) {
+  ROM rom;
+  MockScript script;
+  EXPECT_CALL(script, ApplyPatchToROM(_))
+      .WillOnce(Return(absl::InvalidArgumentError("No patch loaded!")));
+
+  EXPECT_THAT(script.ApplyPatchToROM(rom),
+              absl::InvalidArgumentError("No patch loaded!"));
+}
+
+}  // namespace asm_test
+}  // namespace yaze_test
\ No newline at end of file