Merge pull request #116 from ewasm/debug-namespace

 Explain debug mode (and debug namespace) a bit better

Author: axic

contract_interface.md

@@ -10,6 +10,21 @@ Every contract must be stored in the [WebAssembly Binary Encoding](https://githu
 
 A contract can only import symbols specified in the [Ethereum Environment Interface](./eth_interface.md).
 
+As mentioned below, there is a `debug` namespace as well, but that is disallowed in production systems.
+
+#### 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:
+- `print32(value: i32)` - print value
+- `print64(value: i64)` - print value
+- `printMem(offset: i32, len: i32)` - print memory segment as printable characters
+- `printMemHex(offset: i32, len: i32)` - print memory segment as hex
+- `printStorage(pathOffset: i32)` - print storage value as printable characters
+- `printStorageHex(pathOffset: i32)` - print storage value as hex
+
 ### Exports
 
 A contract must have exactly two exported symbols:
@@ -23,13 +38,3 @@ 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`