Multi-byte opcodes

Reserve 0xEB and 0xEC for usage as extended opcode space.

It would be convenient to introduce new opcodes that are likely to be infrequently used, whilst also being able to have greater than 256 opcodes in total. As a single byte opcode is half the size of a double byte opcode, the greatest efficiency in code sizes will be one where frequently used opcodes are single bytes. Two prefix bytes are used to accommodate up to 510 double byte opcodes.

For example, a new arithmetic opcode may be allocated to 0xED 01(ADD), and a novel opcode may be introduced at 0xEB F4(DELEGATECALL).

Triple byte opcodes may be doubly-prefixed by 0xEB EB, 0xEC EC, 0xEB EC and 0xEC EB. It is possible to allocate experimental opcodes to this triple-byte space initially, and if they prove safe and useful, they could later be allocated a location in double-byte or single-byte space.

Only 0xEB EB, 0xEC EC, 0xEC EC, and 0xEB EC may be interpreted as further extensions of the opcode space. 0xEB and 0xEC do not themselves affect the stack or memory, however opcodes specified by further bytes may. If a multi-byte opcode is yet to be defined, it is to be treated as INVALID rather than as a NOP, as per usual for undefined opcodes.

Both 0xEB and 0xEC were chosen to be part of the E-series of opcodes. For example, the0xEF byte is reserved for contracts conforming to the Ethereum Object Format. By having unassigned opcodes for extending the opcode space, there will be a lower risk of breaking the functionalities of deployed contracts compared to choosing assigned opcodes.

Backwards Compatibility
Previous usage of 0xEB and 0xEC may result in unexpected behavior and broken code.

Copyright and related rights waived via CC0.

I think your EIP is under-specified in a scenario like 600456eb5b. If the 0xeb were replaced with 0x60, the JUMPDEST would be considered invalid. In your EIP, it is currently considered valid.

1 Like

What is the reason for reserving two opcodes rather than a single extension opcode? It seems that one would suffice just as well as two, and we gain essentially nothing from reserving 2.