Add documentation.

1 files changed, 10 insertions, 0 deletions

diff --git a/docs/assembly.rst b/docs/assembly.rst
index 705cd1b8..978e71e3 100644
--- a/docs/assembly.rst
+++ b/docs/assembly.rst
@@ -405,6 +405,16 @@ changes during the call, and thus references to local variables will be wrong.
         }
     }
 
+.. note::
+    If you access variables of a type that spans less than 256 bits
+    (for example ``uint64``, ``address``, ``bytes16`` or ``byte``),
+    you cannot make any assumptions about bits not part of the
+    encoding of the type. Especially, do not assume them to be zero.
+    To be safe, always clear the data properly before you use it
+    in a context where this is important:
+    ``uint32 x = f(); assembly { x := and(x, 0xffffffff) /* now use x */ }``
+    To clean signed types, you can use the ``signextend`` opcode.
+
 Labels
 ------