diff --git a/Makefile b/Makefile
index 2a9b77f..78ba118 100644
--- a/Makefile
+++ b/Makefile
@@ -48,6 +48,10 @@ ASMFILES := $(wildcard $(SRCDIR)/*.asm)
 %.pb16: %
 	src/tools/pb16.py $< $@
 
+# RGBGFX generates tilemaps with sequential tile IDs, which works fine for $8000 mode but not $8800 mode; `bit7ify.py` takes care to flip bit 7 so maps become $8800-compliant
+%.bit7.tilemap: src/tools/bit7ify.py %.tilemap
+	$^ $@
+
 
 CLEANTARGETS := $(BINDIR) $(DEPSDIR) $(OBJDIR) dummy # The list of things that must be cleared; expanded by the resource Makefiles
 INITTARGETS :=
diff --git a/src/tools/bit7ify.py b/src/tools/bit7ify.py
new file mode 100644
index 0000000..f830383
--- /dev/null
+++ b/src/tools/bit7ify.py
@@ -0,0 +1,8 @@
+#!/usr/bin/env python3
+
+from sys import argv
+
+
+with open(argv[1], "rb") as input:
+    with open(argv[2], "wb") as output:    
+        output.write(bytes((byte ^ 0x80  for byte in input.read())))