Merge pull request #114 from jakelang/startfn-notes

Add an extra note about EEI imports and disallowing a start function

contract_interface.md

@@ -8,7 +8,10 @@ Every contract must be stored in the [WebAssembly Binary Encoding](https://githu
 
 ### Imports
 
-A contract can only import symbols specified in the [Ethereum Environment Interface](./eth_interface.md).
+A contract can only import symbols specified in the [Ethereum Environment Interface or EEI](./eth_interface.md).
+
+In practice, this means that all imports specified by an eWASM module must be from the `ethereum` namespace,
+and having a function signature and name directly correspondent to a function specified in the EEI.
 
 As mentioned below, there is a `debug` namespace as well, but that is disallowed in production systems.
 
@@ -38,3 +41,13 @@ 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.
+
+### Start function
+
+The use of a [start function](https://webassembly.github.io/spec/core/syntax/modules.html#start-function) is disallowed.
+
+The reason for this is that an eWASM VM would need to have access to the memory space of a contract and that must be acquired prior to executing it.
+In the [WebAssembly Javascript API](https://webassembly.org/docs/js/) however the start function is executed right during instantiation, which
+leaves no time for the client to acquire the memory area.
+
+*Note:* This decision was made on WebAssembly version 0xb (pre version 1) and should be revisited.