diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,12 @@
 # Changelog for keid-core
 
+## 0.1.1.0
+
+- Added `--display NUM` to application arguments.
+- Allow `Timed` workers to skip output update.
+- `spawnTimed` and `spawnTimed_` changed args: active/dt moved to front.
+- `spawnTimed` now uses initialization function instead of deriving initial output from initial state.
+
 ## 0.1.0.1
 
 - Inter-stage resource release waits for render job to finish.
diff --git a/keid-core.cabal b/keid-core.cabal
--- a/keid-core.cabal
+++ b/keid-core.cabal
@@ -5,7 +5,7 @@
 -- see: https://github.com/sol/hpack
 
 name:           keid-core
-version:        0.1.0.1
+version:        0.1.1.0
 synopsis:       Core parts of Keid engine.
 category:       Game Engine
 author:         IC Rainbow
diff --git a/src/Engine/Setup.hs b/src/Engine/Setup.hs
--- a/src/Engine/Setup.hs
+++ b/src/Engine/Setup.hs
@@ -17,7 +17,7 @@
 import Engine.Setup.Device (allocatePhysical, allocateLogical)
 import Engine.Setup.Window qualified as Window
 import Engine.Types (GlobalHandles(..))
-import Engine.Types.Options (Options, optionsFullscreen)
+import Engine.Types.Options (Options(..))
 import Engine.Vulkan.Swapchain (SwapchainResources)
 import Engine.Vulkan.Types (PhysicalDeviceInfo(..))
 import Engine.Worker qualified as Worker
@@ -33,6 +33,7 @@
 
   (windowReqs, ghWindow) <- Window.allocate
     (optionsFullscreen opts)
+    (optionsDisplay opts)
     Window.pickLargest
     "Keid Engine"
 
diff --git a/src/Engine/Setup/Window.hs b/src/Engine/Setup/Window.hs
--- a/src/Engine/Setup/Window.hs
+++ b/src/Engine/Setup/Window.hs
@@ -49,15 +49,16 @@
      , MonadResource m
      )
   => Bool
+  -> Natural
   -> SizePicker
   -> Text
   -> m ([InstanceRequirement], GLFW.Window)
-allocate fullscreen sizePicker title = do
+allocate fullscreen displayNum sizePicker title = do
   UnliftIO unliftIO <- askUnliftIO
 
   let
     create = unliftIO $
-      createWindow fullscreen sizePicker title
+      createWindow fullscreen displayNum sizePicker title
 
     destroy (_exts, window) = unliftIO $
       destroyWindow window
@@ -66,24 +67,40 @@
 
 createWindow
   :: (MonadIO m, MonadReader env m, HasLogFunc env)
-  => Bool -> SizePicker -> Text -> m ([InstanceRequirement], GLFW.Window)
-createWindow fullScreen sizePicker title = do
+  => Bool
+  -> Natural
+  -> SizePicker
+  -> Text
+  -> m ([InstanceRequirement], GLFW.Window)
+createWindow fullScreen displayNum sizePicker title = do
   runGlfwIO_ InitError GLFW.init
   runGlfwIO_ VulkanError GLFW.vulkanSupported
 
   liftIO . GLFW.windowHint $ GLFW.WindowHint'ClientAPI GLFW.ClientAPI'NoAPI
 
-  sizes <- runGlfwIO MonitorError GLFW.getMonitors >>= \case
-    [] ->
-      liftIO . throwIO $ MonitorError GLFW.Error'PlatformError "No monitors returned"
-    some ->
-      fmap NonEmpty.fromList $
-        for some \monitor -> do
-          mode <- runGlfwIO VideoModeError $ GLFW.getVideoMode monitor
-          pure (monitor, mode)
+  monitors <- runGlfwIO MonitorError GLFW.getMonitors
+  when (null monitors) $
+    liftIO . throwIO $ MonitorError GLFW.Error'PlatformError "No monitors returned"
 
+  modes <- for (zip [1..] monitors) \(ix, monitor) -> do
+    mode <- runGlfwIO VideoModeError $ GLFW.getVideoMode monitor
+    logDebug $ mconcat
+      [ "[display ", displayShow ix, "] "
+      , displayShow mode
+      ]
+    if displayNum /= 0 && displayNum /= ix then
+      pure Nothing
+    else do
+      pure $ Just (monitor, mode)
+
+  (monitor, mode) <-
+    case catMaybes modes of
+      [] ->
+        liftIO . throwIO $ MonitorError GLFW.Error'PlatformError "Selected display number not available"
+      so : me ->
+        pure $ sizePicker (so :| me)
+
   let
-    (monitor, mode) = sizePicker sizes
     GLFW.VideoMode{videoModeWidth=width, videoModeHeight=height} = mode
     fsMonitor =
       if fullScreen then
diff --git a/src/Engine/Types/Options.hs b/src/Engine/Types/Options.hs
--- a/src/Engine/Types/Options.hs
+++ b/src/Engine/Types/Options.hs
@@ -12,8 +12,9 @@
 
 -- | Command line arguments
 data Options = Options
-  { optionsVerbose :: Bool
+  { optionsVerbose    :: Bool
   , optionsFullscreen :: Bool
+  , optionsDisplay    :: Natural
   }
   deriving (Show)
 
@@ -45,6 +46,12 @@
     [ Opt.long "fullscreen"
     , Opt.short 'f'
     , Opt.help "Run in fullscreen mode"
+    ]
+
+  optionsDisplay <- Opt.option Opt.auto $ mconcat
+    [ Opt.long "display"
+    , Opt.help "Select display number"
+    , Opt.value 0
     ]
 
   pure Options{..}
diff --git a/src/Engine/Worker.hs b/src/Engine/Worker.hs
--- a/src/Engine/Worker.hs
+++ b/src/Engine/Worker.hs
@@ -19,6 +19,8 @@
   , modifyConfigSTM
 
   , HasOutput(..)
+  , pushOutput
+  , pushOutputSTM
   , updateOutput
   , updateOutputSTM
   , getOutputData
@@ -333,32 +335,37 @@
 
 spawnTimed_
   :: MonadUnliftIO m
-  => Int
-  -> Bool
+  => Bool
+  -> Int
+  -> output
   -> m output
   -> m (Timed () output)
-spawnTimed_ dt startActive stepF =
+spawnTimed_ startActive dt initialOutput stepF =
   spawnTimed
-    (\_old () -> fmap (,()) stepF)
-    (Left dt)
     startActive
-    (error "assert: spawnTimed_ ignores old")
+    (Left dt)
+    (\() -> pure (initialOutput, ()))
+    (\_old () ->
+        stepF >>= \res ->
+          pure (Just res, ())
+    )
     ()
 
 spawnTimed
   :: MonadUnliftIO m
-  => (state -> config -> m (output, state))
+  => Bool
   -> Either Int (config -> Int)
-  -> Bool
-  -> state
+  -> (config -> m (output, state))
+  -> (state -> config -> m (Maybe output, state))
   -> config
   -> m (Timed config output)
-spawnTimed stepF dtF startActive initialState initialConfig = do
+spawnTimed startActive dtF initF stepF initialConfig = do
   tActive <- newTVarIO startActive
   tConfig <- newTVarIO initialConfig
-  (initialOutput, nextState) <- stepF initialState initialConfig
+  (initialOutput, initialState) <- initF initialConfig
   tOutput <- newVar initialOutput
-  tWorker <- forkIO $ step tActive tConfig tOutput nextState
+  tWorker <- forkIO $
+    step tActive tConfig tOutput initialState
   pure Timed{..}
   where
     step activeVar configVar output curState = do
@@ -373,7 +380,7 @@
       if active then do
         config <- readTVarIO configVar
         (nextOutput, nextState) <- stepF curState config
-        pushOutput output $ const nextOutput
+        updateOutput output $ const nextOutput
         step activeVar configVar output nextState
       else
         step activeVar configVar output curState