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+---
+eip: <to be assigned>
+title: Disallow Deployment of Unused Opcodes
+author: Wei Tang (@sorpaas)
+discussions-to: <URL>
+status: Draft
+type: Standards Track
+category: Core
+created: 2019-01-17
+---
+
+## Simple Summary
+
+Hard fork can change existing contract behavior. One of the point can
+be raised about adding new opcodes is that it modifies existing unused
+opcodes from throwing out of gas to another behavior. While we can
+mostly argue that deploying unused opcode is not of much use so no
+sane developers would do that, it would be better to just disallow
+deployment of unused opcodes.
+
+## Specification
+
+After `HARD_FORK` number, before executing a contract creation
+transaction, or adding contract code to the state, do the following
+check:
+
+* Iterate over the code bytes one by one.
+  * If the code byte is a PUSH(n) opcode, skip next n bytes.
+  * If the code byte is a valid opcode or designated invalid
+    instruction (`0xfe`), continue.
+  * Otherwise, throw out-of-gas.
+
+Note that this check is similar to jump destination checks.
+
+## Copyright
+
+Copyright and related rights waived via
+[CC0](https://creativecommons.org/publicdomain/zero/1.0/).