diff --git a/package.yaml b/package.yaml
index 1f4bc04..f73e84a 100644
--- a/package.yaml
+++ b/package.yaml
@@ -20,6 +20,7 @@ dependencies:
   - bytestring
   - containers
   - filepath
+  - megaparsec
   - optparse-applicative
   - text
   - transformers
diff --git a/src/Mima/Asm/Phase1.hs b/src/Mima/Asm/Phase1.hs
new file mode 100644
index 0000000..8103f30
--- /dev/null
+++ b/src/Mima/Asm/Phase1.hs
@@ -0,0 +1,193 @@
+module Mima.Asm.Phase1
+  ( Onion(..)
+  -- * Types
+  , Name(..)
+  , Address(..)
+  , Location(..)
+  , SmallOpcode(..)
+  , LargeOpcode(..)
+  , MimaWord(..)
+  , SmallValue(..)
+  , Instruction(..)
+  , RegisterDirective(..)
+  , JsonValue(..)
+  , Directive(..)
+  -- * Parsers
+  , Phase1
+  , parsePhase1
+  ) where
+
+import qualified Data.Aeson          as A
+import           Data.Char
+import qualified Data.Text           as T
+import           Data.Void
+import           Text.Megaparsec
+
+import qualified Mima.Vm.Instruction as Vm
+import qualified Mima.Vm.Word        as Vm
+
+{-
+<value>       := <word> | <address>
+<address>     := <absolute address> | <relative address> | <label>
+<name>        := [a-z]([a-z0-9_]*)
+<label>       := <name>:
+<instruction> := as usual
+
+.reg ACC <value>
+.reg <other register> <address>
+
+.org (<absolute address> | <positive relative address> | <negative address>) ???
+.lit <value>
+.arr [<value>]
+
+.meta <name> <json value>
+.meta-start <name> [<json value>]
+.meta-stop <name>
+-}
+
+class Onion o where
+  peel :: o a -> a
+
+{- Types -}
+
+data Name a = Name a T.Text
+  deriving (Show)
+
+instance Onion Name where
+  peel (Name a _) = a
+
+data Address a
+  = AddressAbsolute a Vm.MimaAddress
+  | AddressRelative a Int
+  deriving (Show)
+
+instance Onion Address where
+  peel (AddressAbsolute a _) = a
+  peel (AddressRelative a _) = a
+
+data Location a
+  = LocationAddress (Address a)
+  | LocationLabel (Name a)
+  deriving (Show)
+
+instance Onion Location where
+  peel (LocationAddress a) = peel a
+  peel (LocationLabel a)   = peel a
+
+data SmallOpcode a = SmallOpcode a Vm.SmallOpcode
+  deriving (Show)
+
+instance Onion SmallOpcode where
+  peel (SmallOpcode a _) = a
+
+data LargeOpcode a = LargeOpcode a Vm.LargeOpcode
+  deriving (Show)
+
+instance Onion LargeOpcode where
+  peel (LargeOpcode a _) = a
+
+data MimaWord a
+  = WordRaw a Vm.MimaWord
+  | WordLocation (Location a)
+  deriving (Show)
+
+instance Onion MimaWord where
+  peel (WordRaw a _)    = a
+  peel (WordLocation a) = peel a
+
+data SmallValue a = SmallValue a Vm.SmallValue
+  deriving (Show)
+
+instance Onion SmallValue where
+  peel (SmallValue a _) = a
+
+data Instruction a
+  = SmallInstruction a (SmallOpcode a) (Location a)
+  | LargeInstruction a (LargeOpcode a) (SmallValue a)
+  deriving (Show)
+
+instance Onion Instruction where
+  peel (SmallInstruction a _ _) = a
+  peel (LargeInstruction a _ _) = a
+
+data RegisterDirective a
+  = RegIar a (Location a)
+  | RegAcc a (MimaWord a)
+  | RegRa a (Location a)
+  | RegSp a (Location a)
+  | RegFp a (Location a)
+  deriving (Show)
+
+instance Onion RegisterDirective where
+  peel (RegIar a _) = a
+  peel (RegAcc a _) = a
+  peel (RegRa  a _) = a
+  peel (RegSp  a _) = a
+  peel (RegFp  a _) = a
+
+data JsonValue a = JsonValue a A.Value
+  deriving (Show)
+
+instance Onion JsonValue where
+  peel (JsonValue a _) = a
+
+-- | The first @a@ parameter represents the span of the whole thing. The second
+-- @a@ parameter represents the span of the directive literal (e. g. @.org@).
+data Directive a
+  = Reg       a a (RegisterDirective a)
+  | Org       a a (Address a)
+  | Lit       a a (MimaWord a)
+  | Arr       a a [MimaWord a]
+  | Meta      a a (Name a) (JsonValue a)
+  | MetaStart a a (Name a) (JsonValue a)
+  | MetaStop  a a (Name a)
+  deriving (Show)
+
+instance Onion Directive where
+  peel (Reg       a _ _)   = a
+  peel (Org       a _ _)   = a
+  peel (Lit       a _ _)   = a
+  peel (Arr       a _ _)   = a
+  peel (Meta      a _ _ _) = a
+  peel (MetaStart a _ _ _) = a
+  peel (MetaStop  a _ _)   = a
+
+data AsmToken a
+  = TokenLabel (Name a)
+  | TokenInstruction (Instruction a)
+  | TokenDirective (Directive a)
+  | TokenComment a T.Text
+  deriving (Show)
+
+instance Onion AsmToken where
+  peel (TokenLabel a)       = peel a
+  peel (TokenInstruction a) = peel a
+  peel (TokenDirective a)   = peel a
+  peel (TokenComment a _)   = a
+
+{- Parsers -}
+
+type Parser = Parsec Void T.Text
+
+data Span = Span SourcePos SourcePos
+  deriving (Show)
+
+withSpan :: Parser a -> Parser (a, Span)
+withSpan f = do
+  start <- getSourcePos
+  result <- f
+  stop <- getSourcePos
+  pure (result, Span start stop)
+
+name :: Parser (Name Span)
+name = do
+  (a, s) <- withSpan $ do
+    firstChar <- satisfy isLower <?> "lowercase character"
+    otherChars <- takeWhileP (Just "alphanumeric character") isAlphaNum
+    pure $ T.pack [firstChar] <> otherChars
+  pure $ Name s a
+
+type Phase1 = [AsmToken Span]
+
+parsePhase1 :: Parser Phase1
+parsePhase1 = undefined name