diff --git a/hs/src/Aoc/Y2019.hs b/hs/src/Aoc/Y2019.hs
index 8eac36c..b57d087 100644
--- a/hs/src/Aoc/Y2019.hs
+++ b/hs/src/Aoc/Y2019.hs
@@ -7,6 +7,7 @@ import qualified Aoc.Y2019.D01 as D01
 import qualified Aoc.Y2019.D02 as D02
 import qualified Aoc.Y2019.D03 as D03
 import qualified Aoc.Y2019.D04 as D04
+import qualified Aoc.Y2019.D05 as D05
 
 year :: Year
 year = Year 2019
@@ -14,4 +15,5 @@ year = Year 2019
   , ( 2, D02.day)
   , ( 3, D03.day)
   , ( 4, D04.day)
+  , ( 5, D05.day)
   ]
diff --git a/hs/src/Aoc/Y2019/A05.hs b/hs/src/Aoc/Y2019/A05.hs
deleted file mode 100644
index f530647..0000000
--- a/hs/src/Aoc/Y2019/A05.hs
+++ /dev/null
@@ -1,117 +0,0 @@
-{-# LANGUAGE GeneralizedNewtypeDeriving #-}
-
-module Aoc.Y2019.A05
-  ( solve201905
-  ) where
-
-import           Control.Monad
-import           Data.Bifunctor
-import           Data.Foldable
-import qualified Data.Map.Strict as M
-import           Data.Maybe
-import qualified Data.Set as S
-import qualified Data.Text as T
-import qualified Data.Text.IO as T
-
--- Basic types
-
--- Typesafe addresses and values so we don't confuse the two unless we want to.
-
-newtype Addr = Addr Integer
-  deriving (Show, Eq, Ord, Enum, Num, Real, Integral)
-
-newtype Value = Value Integer
-  deriving (Show, Eq, Ord, Enum, Num, Real, Integral)
-
--- Memory
-
-newtype Memory = Memory { unmemory :: M.Map Addr Value }
-
-instance Show Memory where
-  show mem = "Memory " <> show (memToList mem)
-
-newMem :: [Integer] -> Memory
-newMem = Memory . M.fromList . map (bimap Addr Value) . zip [0..]
-
-memToList :: Memory -> [Integer]
-memToList (Memory m) =
-  let maxAddr = fromMaybe 0 $ S.lookupMax $ M.keysSet m
-  in  map (toInteger . fromMaybe 0 . (m M.!?)) [0..maxAddr]
-
-readMem :: Addr -> Memory -> Maybe Value
-readMem addr (Memory mem) = mem M.!? addr
-
-writeMem :: Addr -> Value -> Memory -> Memory
-writeMem addr val = Memory . M.insert addr val . unmemory
-
--- State
-
-data State = State
-  { stateMem :: Memory
-  , stateIdx :: Addr
-  , stateInput :: [Integer]
-  } deriving (Show)
-
-newState :: Memory -> State
-newState mem = State mem 0 []
-
-data StepError
-  = Halted
-  | CouldNotRead Addr
-  | UnknownOpcode Integer
-  deriving (Show)
-
-readAt :: State -> Addr -> Either StepError Value
-readAt s i = case readMem i $ stateMem s of
-  Nothing -> Left $ CouldNotRead i
-  Just v -> Right v
-
-writeAt :: Addr -> Value -> State -> State
-writeAt addr val s = s{stateMem = writeMem addr val $ stateMem s}
-
--- Opcode
-
-data ParamMode = PositionMode | ImmediateMode
-  deriving (Show, Eq)
-
-digits :: Integer -> [Integer]
-digits i = (i `mod` 10) : digits (i `div` 10)
-
-pmFromDigit :: Integer -> ParamMode
-pmFromDigit 0 = PositionMode
-pmFromDigit 1 = ImmediateMode
-pmFromDigit _ = undefined
-
-paramModes :: Integer -> [ParamMode]
-paramModes i = map pmFromDigit $ drop 2 $ (digits i ++ repeat 0)
-
-data Operand = Direct Value | Indirect Addr
-  deriving (Show)
-
-data Opcode
-  = OpAdd Operand Operand Addr
-  | OpMul Operand Operand Addr
-  | OpInput Addr
-  | OpOutput Operand Addr
-  | OpHalt
-  deriving (Show)
-
-opWidth :: Opcode -> Addr
-opWidth (OpAdd    _ _ _) = 4
-opWidth (OpMul    _ _ _) = 4
-opWidth (OpInput  _)     = 2
-opWidth (OpOutput _ _)   = 3
-opWidth  OpHalt          = 1
-
-parseOpcode :: State -> Either StepError Opcode
-parseOpcode s = do
-  let idx = stateIdx s
-  undefined
-
-solve201905 :: FilePath -> IO ()
-solve201905 f = do
-  stuff <- readFile f
-
-  putStrLn ">> Part 1"
-
-  putStrLn ">> Part 2"
diff --git a/hs/src/Aoc/Y2019/D05.hs b/hs/src/Aoc/Y2019/D05.hs
new file mode 100644
index 0000000..0a9b7a8
--- /dev/null
+++ b/hs/src/Aoc/Y2019/D05.hs
@@ -0,0 +1,211 @@
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE OverloadedStrings          #-}
+
+module Aoc.Y2019.D05
+  ( day
+  ) where
+
+import           Data.Bifunctor
+import           Data.Maybe
+import           Text.Read       (readMaybe)
+
+import qualified Data.Map.Strict as M
+import qualified Data.Set        as S
+import qualified Data.Text       as T
+import qualified Data.Text.IO    as T
+
+import           Aoc.Day
+import           Aoc.Parse
+
+-------------------
+-- General types --
+-------------------
+
+newtype Addr = Addr Integer
+  deriving (Show, Eq, Ord, Enum, Num, Real, Integral)
+
+newtype Value = Value Integer
+  deriving (Show, Eq, Ord, Enum, Num, Real, Integral)
+
+data StepError
+  = Halted State
+  | CouldNotRead Addr
+  | UnknownOpcode Addr Integer
+  | InvalidInput T.Text
+  deriving (Show)
+
+displayError :: StepError -> T.Text
+displayError (Halted s)          = "Halted at " <> T.pack (show $ stateIdx s)
+displayError (CouldNotRead a)    = "Could not read value at " <> T.pack (show a)
+displayError (UnknownOpcode a i) = "Unknown opcode " <> T.pack (show i) <> " at " <> T.pack (show a)
+displayError (InvalidInput t)    = "Invalid input: " <> t
+
+type StepM = Either StepError
+
+------------
+-- Memory --
+------------
+
+newtype Memory = Memory { unmemory :: M.Map Addr Value }
+
+instance Show Memory where
+  show mem = "Memory " <> show (memToList mem)
+
+newMem :: [Integer] -> Memory
+newMem = Memory . M.fromList . map (bimap Addr Value) . zip [0..]
+
+memToList :: Memory -> [Integer]
+memToList (Memory m) =
+  let maxAddr = fromMaybe 0 $ S.lookupMax $ M.keysSet m
+  in  map (toInteger . fromMaybe 0 . (m M.!?)) [0..maxAddr]
+
+readMem :: Addr -> Memory -> Maybe Value
+readMem addr (Memory mem) = mem M.!? addr
+
+writeMem :: Addr -> Value -> Memory -> Memory
+writeMem addr val = Memory . M.insert addr val . unmemory
+
+-----------
+-- State --
+-----------
+
+data State = State
+  { stateMem :: Memory
+  , stateIdx :: Addr
+  } deriving (Show)
+
+newState :: Memory -> State
+newState mem = State mem 0
+
+readAt :: State -> Addr -> StepM Value
+readAt s i = case readMem i $ stateMem s of
+  Nothing -> Left $ CouldNotRead i
+  Just v  -> Right v
+
+writeAt :: State -> Addr -> Value -> State
+writeAt s addr val = s{stateMem = writeMem addr val $ stateMem s}
+
+-------------
+-- Opcodes --
+-------------
+
+data ParamMode = PositionMode | ImmediateMode
+  deriving (Show, Eq)
+
+-- | Infinite list of param modes based on the digits of a number. The default
+-- mode is 'PositionMode'.
+paramModes :: Integer -> [ParamMode]
+paramModes = map pmFromDigit . digits
+  where
+    digits i = (i `mod` 10) : digits (i `div` 10)
+    pmFromDigit 0 = PositionMode
+    pmFromDigit _ = ImmediateMode
+
+data Operand = Direct Value | Indirect Addr
+  deriving (Show)
+
+pmToOp :: ParamMode -> Value -> Operand
+pmToOp PositionMode  = Indirect . fromIntegral
+pmToOp ImmediateMode = Direct
+
+data Opcode
+  = OpAdd Operand Operand Addr
+  | OpMul Operand Operand Addr
+  | OpInput Addr
+  | OpOutput Operand
+  | OpHalt
+  deriving (Show)
+
+--------------
+-- Stepping --
+--------------
+
+-- Parsing opcodes
+
+getOp :: State -> Addr -> [ParamMode] -> Int -> StepM Operand
+getOp s a pms i = do
+  let pm = pms !! i
+  value <- readAt s $ a + 1 + fromIntegral i
+  pure $ pmToOp pm value
+
+getAddr :: State -> Addr -> Int -> StepM Addr
+getAddr s a i = fromIntegral <$> readAt s (a + 1 + fromIntegral i)
+
+parseOpcode :: State -> StepM Opcode
+parseOpcode s = do
+  let a = stateIdx s
+  value <- toInteger <$> readAt s a
+  let opcode = value `mod` 100
+      pms = paramModes $ value `div` 100
+  case opcode of
+     1  -> OpAdd <$> getOp s a pms 0 <*> getOp s a pms 1 <*> getAddr s a 2
+     2  -> OpMul <$> getOp s a pms 0 <*> getOp s a pms 1 <*> getAddr s a 2
+     3  -> OpInput <$> getAddr s a 0
+     4  -> OpOutput <$> getOp s a pms 0
+     99 -> pure OpHalt
+     _  -> Left $ UnknownOpcode a opcode
+
+-- Executing opcodes
+
+data StepResult
+  = NormalStep State
+  | InputStep (Integer -> State)
+  | OutputStep State Integer
+
+readOp :: State -> Operand -> StepM Value
+readOp _ (Direct v)   = pure v
+readOp s (Indirect a) = readAt s a
+
+incIdx :: Int -> State -> State
+incIdx i s = s{stateIdx = stateIdx s + fromIntegral i}
+
+execOpcode :: State -> Opcode -> StepM StepResult
+execOpcode s (OpAdd x y r) = do
+  vx <- readOp s x
+  vy <- readOp s y
+  pure $ NormalStep $ incIdx 4 $ writeAt s r $ vx + vy
+execOpcode s (OpMul x y r) = do
+  vx <- readOp s x
+  vy <- readOp s y
+  pure $ NormalStep $ incIdx 4 $ writeAt s r $ vx * vy
+execOpcode s (OpInput r) = pure $ InputStep $ incIdx 2 . writeAt s r . fromInteger
+execOpcode s (OpOutput x) = do
+  vx <- readOp s x
+  pure $ OutputStep (incIdx 2 s) $ toInteger vx
+execOpcode s OpHalt = Left $ Halted s
+
+step :: State -> Either StepError StepResult
+step s = execOpcode s =<< parseOpcode s
+
+run :: State -> IO StepError
+run s = case step s of
+  Left e -> pure e
+  Right (NormalStep s') -> run s'
+  Right (InputStep f) -> do
+    putStr "?> "
+    t <- T.getLine
+    case readMaybe $ T.unpack t of
+      Nothing -> pure $ InvalidInput t
+      Just i  -> run $ f i
+  Right (OutputStep s' o) -> do
+    putStrLn $ "-> " ++ show o
+    run s'
+
+runAndPrintResult :: State -> IO ()
+runAndPrintResult s = do
+  e <- run s
+  T.putStrLn $ displayError e
+
+parser :: Parser Memory
+parser = newMem <$> (signed (pure ()) decimal `sepBy` char ',') <* newline
+
+solver :: Memory -> IO ()
+solver mem = do
+  putStrLn ">> Part 1"
+  runAndPrintResult $ newState mem
+
+  putStrLn ""
+  putStrLn ">> Part 2"
+
+day :: Day
+day = dayParse parser solver