diff --git a/mrbgems/mruby-pack/README.md b/mrbgems/mruby-pack/README.md
index 10a538095..2871fb4b3 100644
--- a/mrbgems/mruby-pack/README.md
+++ b/mrbgems/mruby-pack/README.md
@@ -45,7 +45,9 @@ There is no dependency on other mrbgems.
 - V : 32-bit unsigned, VAX (little-endian) byte order
 - v : 16-bit unsigned, VAX (little-endian) byte order
 - x : null byte
-- Z : same as "a", except that null is added with *
+- X : back up bytes
+- Z : same as "a", except that null is added with \*
+- @ : absolute position
 
 ## License