diff --git a/games/keen/mars.s b/games/keen/mars.s
index fca1f888..46f66b81 100644
--- a/games/keen/mars.s
+++ b/games/keen/mars.s
@@ -365,7 +365,7 @@ done_parts:
 	; snapshot tile map
 	;===============================
 	;===============================
-	;	copy currnet tilemap graphics
+	;	copy current tilemap graphics
 	;	to $c00 for fade-in/fade-out
 
 snapshot_tilemap:
@@ -579,8 +579,9 @@ recenter_map:
 
 	lda	MARS_TILEX
 	cmp	#10
-	bcc	mars_tilex_fine
-	sec
+	bcc	mars_tilex_fine		; if MARS_TILEX < 10, leave tilemap 0
+
+	sec			; otherwise tilemap=mars_tilex-10
 	sbc	#10
 	sta	TILEMAP_X
 
@@ -588,8 +589,9 @@ mars_tilex_fine:
 
 	lda	MARS_TILEY
 	cmp	#6
-	bcc	mars_tiley_fine
-	sec
+	bcc	mars_tiley_fine	; if MARS_TILEY < 6 leave tilemap 0
+
+	sec			; otherwise tilemap_y=mars_tiley-6
 	sbc	#6
 	sta	TILEMAP_Y
 
@@ -597,7 +599,6 @@ mars_tiley_fine:
 
 	rts
 
-
 location_x:
 	.byte 19,22, 9,22,13,16,25,38
 	.byte 36,43,52,36,44,60,38,29
diff --git a/games/keen/mars_keyboard.s b/games/keen/mars_keyboard.s
index 83a6bd64..e8ae18b9 100644
--- a/games/keen/mars_keyboard.s
+++ b/games/keen/mars_keyboard.s
@@ -286,9 +286,8 @@ up_pressed:
 	beq	move_keen_up
 
 	sec
-	lda	TILEMAP_Y
-	sbc	MARS_TILEY
-
+	lda	MARS_TILEY
+	sbc	TILEMAP_Y
 	cmp	#4
 	bcc	scroll_keen_up
 
diff --git a/games/keen/tilemap_lookup.s b/games/keen/tilemap_lookup.s
index f8549fc7..c0cd9a3b 100644
--- a/games/keen/tilemap_lookup.s
+++ b/games/keen/tilemap_lookup.s
@@ -13,7 +13,7 @@ tilemap_lookup_high:
 .byte	(>big_tilemap)+ 7,(>big_tilemap)+ 7 	; 14,15
 .byte	(>big_tilemap)+ 8,(>big_tilemap)+ 8	; 16,17
 .byte	(>big_tilemap)+ 9,(>big_tilemap)+ 9	; 18,19
-.byte	(>big_tilemap)+10,(>big_tilemap)+ 0	; 20,21
+.byte	(>big_tilemap)+10,(>big_tilemap)+10	; 20,21
 .byte	(>big_tilemap)+11,(>big_tilemap)+11	; 22,23
 .byte	(>big_tilemap)+12,(>big_tilemap)+12	; 24,25
 .byte	(>big_tilemap)+13,(>big_tilemap)+13	; 26,27