Add section about EXTCODEHASH

content/20.zksync-protocol/70.differences/10.evm-instructions.md

@@ -263,6 +263,15 @@ Contract bytecode cannot be accessed on zkEVM architecture. Only its size is acc
 
 `EXTCODECOPY` always produces a compile-time error with the zkEVM compiler.
 
+## `EXTCODEHASH`
+
+Retrieving the code hash of a smart contract in the zkEVM protocol differs slightly from EVM. In EVM hash of non-existing account is always `0`, and hash of existing account is `keccak("")`. Nonexisting contracts have no code, and their nonce and balance are both zero.
+
+In zkEVM `EXTCODEHASH` operation has the following differences:
+- It doesn't check the balance of account.
+- All contracts in precompiles space (plus `0x00`) are treated as existing empty accounts.
+
+
 ## `DATASIZE`, `DATAOFFSET`, `DATACOPY`
 
 Contract deployment is handled by two parts of the zkEVM protocol: the compiler front end and the system contract called `ContractDeployer`.