diff --git a/shell/README.md b/shell/README.md
index 654ff8d6..3a9cdf3c 100644
--- a/shell/README.md
+++ b/shell/README.md
@@ -18,7 +18,7 @@ Currently runs a tiny dialect of Lisp. Steps to run it from the top-level:
 2. Create a data disk with a library of functions.
 
   ```sh
-  dd if=/dev/zero of=data.img count=20160
+  dd if=/dev/zero of=data.img count=20160         # approximately 10MB
   dd if=shell/data.limg of=data.img conv=notrunc
   ```