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eWASM Contract Interface (ECI) Specification (Revision 0)

The eWASM Contract Interface (ECI) specifies the structure of a contract module.

Wire format

Every contract must be stored in the WebAssembly Binary Encoding format (in short, WASM bytecode).

The WASM bytecode must be prefixed with an invalid EVM1 opcode (0xEF).

Imports

A contract can only import symbols specified in the Ethereum Environment Interface.

Exports

A contract must have exactly two exported symbols:

  • memory: the shared memory space available for the EEI to write into.
  • main: a function with no parameters and no result value.

Entry point

The method exported as main will be executed by the VM.

On successful execution, the code should return via a normal code path.

If it needs to abort due to a failure, an unreachable instruction should be executed.

Debug-mode

Debug-mode is a special VM option, where an additional set of debugging interfaces are available to contracts. On a live VM, any bytecode trying to import these symbols should be rejected.

The imports are available under the debug namespace:

  • print(i64): print a number
  • printMem(i32 offset, i32 length): print a string as pointed by offset
  • printMemHex(i32 offset, i32 length): print a hex representation of the memory pointed to by offset