diff --git a/bin/server.ml b/bin/server.ml
index da3e373d..db59cb3e 100644
--- a/bin/server.ml
+++ b/bin/server.ml
@@ -1,13 +1,13 @@
 open LibASL
 
 
-open Yojson 
+open Yojson
 open Cohttp
 open Cohttp_lwt
 open Cohttp_lwt_unix
 open String
 open List
-open Array 
+open Array
 open Asl_ast
 open Value
 open Eval
@@ -15,49 +15,54 @@ open Asl_utils
 open Lwt
 
 
-let persistent_env     = Option.get (aarch64_evaluation_environment ()) 
+let persistent_env     = Option.get (aarch64_evaluation_environment ())
 
-let eval_instr (opcode: string) : string = 
+let eval_instr (opcode: string) : string =
     let praw a : string = Utils.to_string (Asl_parser_pp.pp_raw_stmt a) |> String.trim  in
     let address = None                                     in
-    let res :Asl_ast.stmt list  = Dis.retrieveDisassembly ?address persistent_env (Dis.build_env persistent_env) opcode in 
+    let res :Asl_ast.stmt list  = Dis.retrieveDisassembly ?address persistent_env (Dis.build_env persistent_env) opcode in
     let ascii   = List.map praw res                                                 in
     let indiv (s: string) = List.init (String.length s) (String.get s) |> List.map (String.make 1)  in
     List.map indiv ascii |>  List.map (String.concat "") |> String.concat "\n"
 
 
-let get_reply (jsonin: string) : string = 
+let get_reply (jsonin: string) : Cohttp.Code.status_code * string =
   (*let json  = Yojson.Safe.from_string jsonin in *)
-   match (eval_instr jsonin) with 
-  | exception e ->  Yojson.Safe.to_string  (`Assoc [("instruction", `String jsonin); ("error", `String (Printexc.to_string e))])
-  | x -> Yojson.Safe.to_string (`Assoc [("instruction", `String jsonin); ("semantics", `String x)] )
-  | _ ->  Yojson.Safe.to_string  (`Assoc [("instruction", `String jsonin); ("error", `String "unknown")])
+  let make_reply code tail =
+    (code, Yojson.Safe.to_string (`Assoc [("instruction", `String jsonin); tail])) in
+  match (eval_instr jsonin) with
+  | exception e -> make_reply `Internal_server_error ("error", `String (Printexc.to_string e))
+  | x -> make_reply `OK ("semantics", `String x)
 
 
-let unsupp_method_resp : string = (Yojson.Safe.to_string  (`Assoc [("error", `String "unsupported method.")]))
-let missing_param : string = (Yojson.Safe.to_string  (`Assoc [("error", `String "missing opcode param.")]))
+let unsupp_method_resp : Cohttp.Code.status_code * string =
+  (`Method_not_allowed, Yojson.Safe.to_string (`Assoc [("error", `String "unsupported method.")]))
+
+let missing_param : Cohttp.Code.status_code * string =
+  (`Bad_request, Yojson.Safe.to_string (`Assoc [("error", `String "missing opcode param.")]))
 
 (*let () = ignore (List.map (fun (f: string) -> print_endline (eval_instr f)) (tl (to_list Sys.argv))) *)
 
 
-let get_resp (opcode: string) : string = 
+let get_resp (opcode: string) : Cohttp.Code.status_code * string =
     get_reply opcode
 
 let server addr port =
-  print_endline "Started server.";
+  Printf.printf "Started aslp-server at http://%s:%d\n" addr port;
+  flush stdout;
   let callback _conn req body =
     let uri = req |> Request.uri in
     let meth = req |> Request.meth |> Code.string_of_method in
     let headers = req |> Request.headers |> Header.to_string in
-    body |> Cohttp_lwt.Body.to_string >|= (fun body ->
-        match  (Request.meth req) with 
-            | `POST -> get_resp body
-            | `GET ->  (match  (Uri.get_query_param uri "opcode") with 
-                | Some s ->  get_resp s
-                | None -> missing_param)
-            |  _ ->  unsupp_method_resp
-        )
-    >>= fun body -> Server.respond_string ~status:`OK ~body ()
+    let body' = body |> Cohttp_lwt.Body.to_string in
+    let resp' =
+      match (Request.meth req, Uri.get_query_param uri "opcode") with
+      | `POST, _ -> body' >|= get_resp
+      | `GET, Some param -> Lwt.return (get_resp param)
+      | `GET, None -> Lwt.return missing_param
+      |  _ -> Lwt.return unsupp_method_resp
+    in
+    resp' >>= fun (code, body) -> Server.respond_string ~status:code ~body ()
   in
   Server.create ~mode:(`TCP (`Port port)) (Server.make ~callback ())
 
@@ -70,10 +75,9 @@ let speclist =
     ("--port", Arg.Set_int port_opt, "Server port (default 8000)");
   ]
 
-let () = Arg.parse speclist ignore "server --host localhost --port 8000"
-
+let () = Arg.parse speclist ignore "usage: aslp-server --host HOSTNAME --port PORT"
 
-let () = 
-     let address = Unix.(ADDR_INET (inet_addr_of_string !addr_opt, !port_opt)) in
-    ignore (Lwt_main.run (server !addr_opt !port_opt))
+let () =
+  let address = Unix.(ADDR_INET (inet_addr_of_string !addr_opt, !port_opt)) in
+  Lwt_main.run (server !addr_opt !port_opt)