diff --git a/bin/main.ml b/bin/main.ml
index f234568..c81267d 100644
--- a/bin/main.ml
+++ b/bin/main.ml
@@ -13,5 +13,11 @@ let () =
     in
 
     let prog = Spice.Code.make_program ep in
-    Fmt.pr "%a" Spice.Code.pp_program prog
+    let ret = Spice.run prog in
+    Fmt.pr
+      "{\"program\":%a,\"output\":%a}"
+      Spice.Code.pp_program
+      prog
+      Spice.Value.pp
+      ret
   with Spice.Error msg -> Logs.err (fun m -> m "%s" msg)
diff --git a/lib/runtime/code.ml b/lib/runtime/code.ml
index a688c54..c9ebfd8 100644
--- a/lib/runtime/code.ml
+++ b/lib/runtime/code.ml
@@ -40,6 +40,36 @@ type program = { entrypoint : basic_block }
 
 let make_program entrypoint = { entrypoint }
 
+let frame_size prog =
+  let visited = ref [] in
+  let work_list = ref [ prog.entrypoint ] in
+  let enqueue bb = if not (List.memq bb !visited) then work_list := bb :: !work_list in
+  let reg acc i = max acc (i + 1) in
+  let op acc = function
+    | Reg i -> reg acc i
+    | _ -> acc
+  in
+  let ins acc = function
+    | MOV (l, r) | ADD (l, r) | SUB (l, r) | MUL (l, r) -> op (reg acc l) r
+    | BRT (v, b1, b2) ->
+        enqueue b1;
+        enqueue b2;
+        op acc v
+    | JMP b ->
+        enqueue b;
+        acc
+    | RET -> acc
+  in
+  let rec loop acc =
+    match !work_list with
+    | [] -> acc
+    | bb :: rest ->
+        visited := bb :: !visited;
+        work_list := rest;
+        List.fold_left ins acc (instructions bb) |> loop
+  in
+  loop 1
+
 (* pretty printing *)
 
 let pp_reg ppf r = Fmt.pf ppf "$%d" r
diff --git a/lib/runtime/interp.ml b/lib/runtime/interp.ml
new file mode 100644
index 0000000..454126d
--- /dev/null
+++ b/lib/runtime/interp.ml
@@ -0,0 +1,63 @@
+exception Runtime_error of string
+
+module Op = struct
+  let add v1 v2 =
+    match v1, v2 with
+    | Value.Int x, Value.Int y -> Value.Int (Int64.add x y)
+    | _, _ -> raise (Runtime_error "cannot add non integer values")
+
+  let sub v1 v2 =
+    match v1, v2 with
+    | Value.Int x, Value.Int y -> Value.Int (Int64.sub x y)
+    | _, _ -> raise (Runtime_error "cannot sub non integer values")
+
+  let mul v1 v2 =
+    match v1, v2 with
+    | Value.Int x, Value.Int y -> Value.Int (Int64.mul x y)
+    | _, _ -> raise (Runtime_error "cannot mul non integer values")
+
+  let is_true = function
+    | Value.True -> true
+    | _ -> false
+end
+
+type frame = {
+  regs : Value.t array;
+  mutable pc : Code.ins list;
+}
+
+let make_frame prog =
+  {
+    regs = Array.make (Code.frame_size prog) Value.Nil;
+    pc = Code.instructions Code.(prog.entrypoint);
+  }
+
+let return_value fr = fr.regs.(0)
+
+let eval fr = function
+  | Code.Cst_nil -> Value.Nil
+  | Code.Cst_true -> Value.Nil
+  | Code.Cst_false -> Value.Nil
+  | Code.Cst_int n -> Value.Int n
+  | Code.Reg i -> fr.regs.(i)
+
+let exec fr = function
+  | Code.MOV (l, r) -> fr.regs.(l) <- eval fr r
+  | Code.ADD (l, r) -> fr.regs.(l) <- Op.add fr.regs.(l) (eval fr r)
+  | Code.SUB (l, r) -> fr.regs.(l) <- Op.add fr.regs.(l) (eval fr r)
+  | Code.MUL (l, r) -> fr.regs.(l) <- Op.mul fr.regs.(l) (eval fr r)
+  | Code.RET -> fr.pc <- []
+  | Code.JMP l -> fr.pc <- Code.instructions l
+  | Code.BRT (v, l1, l2) ->
+      if Value.equal (eval fr v) True then fr.pc <- Code.instructions l1
+      else fr.pc <- Code.instructions l2
+
+let rec run fr =
+  match fr.pc with
+  | [] -> return_value fr
+  | is :: rest ->
+      fr.pc <- rest;
+      exec fr is;
+      run fr
+
+let run_program pr = run (make_frame pr)
diff --git a/lib/spice.ml b/lib/spice.ml
index 4d53073..e8af4af 100644
--- a/lib/spice.ml
+++ b/lib/spice.ml
@@ -16,3 +16,7 @@ let parse input =
 let compile syn =
   try Spice_lower.Normalize.to_anf syn
   with Failure msg -> failf "compilation error: %s" msg
+
+let run prog =
+  try Spice_runtime.Interp.run_program prog
+  with Spice_runtime.Interp.Runtime_error msg -> failf "runtime error: %s" msg