diff --git a/fst.equ b/fst.equ
index 4133e71..c288b4a 100644
--- a/fst.equ
+++ b/fst.equ
@@ -53,6 +53,9 @@ dirent_offset		equ mark+2 ; current directory offset
 
 dirent_entry		ds.w 1 ; current directory entry.
 
+;data_zone		ds.w 1 ; current zone * 2
+;data_block		ds.w 1 ; 0 if invalid.
+;data			ds.b 1024 ; ?
 
 __sizeof		equ *
 			endr
diff --git a/fst.macros b/fst.macros
index 7c10aa7..07e253a 100644
--- a/fst.macros
+++ b/fst.macros
@@ -262,3 +262,13 @@
 @ok
 	mend
 
+	; reverse subtract.
+	macro
+&l	rsb &target
+	eor #$ffff
+	sec
+	adc &target
+	mend
+
+
+