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WebAssembly Smart Contracts

Number: ella-2018-0002
Title: WebAssembly Smart Contracts
Author: "ellaismer" <>
Status: Proposed
Type: Standards Track
Layer: Consensus
Created: 2018-04-30


This lies out a basic specification documentation for the WebAssembly scripting implementation in Parity Kovan network and possibly Ellaism network in the future.

Encoding and Parsing

The contract is encoded as specified in WebAssembly Binary Encoding. To distinguish WASM contracts and EVM contracts, WASM modules always start with "\0asm" (0x6d736100) magic number. In a contract creation transaction, data are appended after the WASM module encoding.


WASM are instantiated as normal. The exported function call will be called without arguments.

Host Functions

Index Name Arguments Return Value
0 storage_write (key_ptr: u32, value_ptr: u32) void
10 storage_read (key_ptr: u32, value_ptr: u32) void
20 ret (ptr: u32, len: u32) (Stop execution)
30 gas (amount: u32) void
40 fetch_input (ptr: u32) void
50 input_length () i32
60 ccall (gas: u64, address_ptr: u32, val_ptr: u32, input_ptr: u32, input_len: u32, result_ptr: u32, result_len: u32) i32
70 scall (gas: u64, address_ptr: u32, input_ptr: u32, input_len: u32, result_ptr: u32, result_len: u32) i32
80 dcall (gas: u64, address_ptr: u32, input_ptr: u32, input_len: u32, result_ptr: u32, result_len: u32) i32
90 value (ptr: u32) void
100 create (endowment_ptr: u32, code_ptr: u32, code_len: u32, result_ptr: u32) i32
110 suicide (refund_ptr: u32) (Stop execution)
120 blockhash (index: u64, hash_ptr: u32) void
130 blocknumber () u64
140 coinbase (dest_ptr: u32) void
150 difficulty (dest_ptr: u32) void
160 gaslimit (dest_ptr: u32) void
170 timestamp () u64
180 address (dest_ptr: u32) void
190 sender (dest_ptr: u32) void
200 origin (dest_ptr: u32) void
210 elog (topic_ptr: u32, topic_count: u32, data_ptr: u32, data_len: u32) void
1000 panic (ptr: u32, len: u32) (Stop execution)
1010 debug (ptr: u32, len: u32) void

Module env can be imported with the above names.

Gas Costs, Stack Limits and Opcodes

Load and Store type instructions have fixed cost of 2. This includes:

I32Load, I64Load, I32Load8S, I32Load8U, I32Load16S, I32Load16U, I64Load8S, I64Load8U, I64Load16S, I64Load16U, I64Load32S, I64Load32U, I32Store, I64Store, I32Store8, I32Store16, I64Store8, I64Store16, I64Store32

Div type instructions have fixed cost of 16. This includes:

I32DivS, I32DivU, I32RemS, I32RemU, I64DivS, I64DivU, I64RemS, I64RemU

Mul type instructions have fixed cost of 4. This includes:

I32Mul, I64Mul

All floating type instructions are disabled. For other instructions, the cost is 1.

The initial memory region has a cost of 4096 per page, and growing memories have costs of 8192 per page.

For external host functions, gas costs as in EVM applies, in addition:

  • Copying values into the memory has costs 1 per u64.
  • Reading an unsigned 256-bit integer from memory has cost 64.
  • Reading an address from memory has cost 40.

Gas passed to WASM are also adjusted by 8/3 compared with EVM. Stack size limit for WASM is 64KB.