diff --git a/MakefileVC b/MakefileVC
index 82fcf5c2b..d11f97a46 100644
--- a/MakefileVC
+++ b/MakefileVC
@@ -49,6 +49,9 @@ endif
 
 all: Diablo.exe
 
+debug: CFLAGS += /D "_DEBUG"
+debug: Diablo.exe
+
 # fix compilation order to match the VC6 workspace files
 DIABLO_SRC=$(sort $(filter-out Source/render.cpp, $(wildcard Source/*.cpp)))
 DIABLO_SRC += Source/render.cpp
diff --git a/Source/dx.cpp b/Source/dx.cpp
index d48e2d199..cf23fa27f 100644
--- a/Source/dx.cpp
+++ b/Source/dx.cpp
@@ -10,6 +10,9 @@ int sgdwLockCount;
 Screen *gpBuffer;
 IDirectDrawSurface *lpDDSBackBuf;
 IDirectDrawSurface *lpDDSPrimary;
+#ifdef _DEBUG
+int locktbl[256];
+#endif
 static CRITICAL_SECTION sgMemCrit;
 char gbBackBuf; // weak
 char gbEmulate; // weak
@@ -184,7 +187,10 @@ HRESULT __fastcall dx_DirectDrawCreate(GUID *guid, IDirectDraw **DD, void *unkno
 	return ((int (__stdcall *)(GUID *, IDirectDraw **, void *))v5)(v8, v3, unknown);
 }
 
-void __fastcall j_lock_buf_priv(char a1) {
+void __fastcall j_lock_buf_priv(BYTE idx) {
+#ifdef _DEBUG
+	++locktbl[idx];
+#endif
 	lock_buf_priv();
 }
 
@@ -219,7 +225,12 @@ LABEL_9:
 }
 // 69CF0C: using guessed type int gpBufEnd;
 
-void __fastcall j_unlock_buf_priv(char a1) {
+void __fastcall j_unlock_buf_priv(BYTE idx) {
+#ifdef _DEBUG
+	if ( !locktbl[idx] )
+		TermMsg("Draw lock underflow: 0x%x", idx);
+	--locktbl[idx];
+#endif
 	unlock_buf_priv();
 }
 
diff --git a/Source/dx.h b/Source/dx.h
index 8b94a1fde..7afb6a161 100644
--- a/Source/dx.h
+++ b/Source/dx.h
@@ -23,9 +23,9 @@ void __fastcall dx_init(HWND hWnd);
 void __cdecl dx_create_back_buffer();
 void __cdecl dx_create_primary_surface();
 HRESULT __fastcall dx_DirectDrawCreate(GUID *guid, IDirectDraw **DD, void *unknown);
-void __fastcall j_lock_buf_priv(char a1);
+void __fastcall j_lock_buf_priv(BYTE idx);
 void __cdecl lock_buf_priv();
-void __fastcall j_unlock_buf_priv(char a1);
+void __fastcall j_unlock_buf_priv(BYTE idx);
 void __cdecl unlock_buf_priv();
 void __cdecl dx_cleanup();
 void __cdecl dx_reinit();