diff --git a/src/z80io.py b/src/z80io.py
index cc8dc4d..af0562a 100644
--- a/src/z80io.py
+++ b/src/z80io.py
@@ -5,7 +5,8 @@
 #
 
 def isprintable(c):
-    return c >= 0x20 and c <= 0x7D
+    """True if character is printable ASCII"""
+    return 0x20 <= c <= 0x7D
 
 
 class IO: