diff --git a/mandel.s b/mandel.s
index 8b63941..6977582 100644
--- a/mandel.s
+++ b/mandel.s
@@ -453,6 +453,8 @@ viewport_oy:
     sta dest + 1
 .endmacro
 
+; input: arg as u8
+; input/output: dest as u16
 ; clobbers a, x
 .macro sqr8_add16 dest, arg
     ldx arg