diff --git a/src/Mima/Format/Common.hs b/src/Mima/Format/Common.hs
new file mode 100644
index 0000000..4e39a18
--- /dev/null
+++ b/src/Mima/Format/Common.hs
@@ -0,0 +1,19 @@
+module Mima.Format.Common
+  ( toHex
+  , fixedWidthHex
+  , fixedWidthHexAddress
+  ) where
+
+import qualified Data.Text as T
+import           Numeric
+
+import           Mima.Word
+
+toHex :: (Integral a, Show a) => a -> T.Text
+toHex a = T.pack $ showHex a ""
+
+fixedWidthHex :: (Integral a, Show a) => Int -> a -> T.Text
+fixedWidthHex n = T.justifyRight n '0' . toHex
+
+fixedWidthHexAddress :: MimaAddress -> T.Text
+fixedWidthHexAddress = fixedWidthHex 5
diff --git a/src/Mima/Format/FlagFile.hs b/src/Mima/Format/FlagFile.hs
new file mode 100644
index 0000000..c5556ab
--- /dev/null
+++ b/src/Mima/Format/FlagFile.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Mima.Format.FlagFile
+  ( formatFlagFile
+  ) where
+
+import qualified Data.Map as Map
+import qualified Data.Set as Set
+import qualified Data.Text as T
+
+import           Mima.Flag
+import           Mima.Format.Common
+
+fFlagSet :: Set.Set Char -> T.Text
+fFlagSet = T.pack . Set.toAscList
+
+fRange :: AddressRange -> T.Text
+fRange r
+  | lower == upper = fixedWidthHexAddress lower
+  | otherwise      = fixedWidthHexAddress lower <> "-" <> fixedWidthHexAddress upper
+  where
+    lower = lowerAddress r
+    upper = upperAddress r
+
+fLine :: (AddressRange, Set.Set Char) -> T.Text
+fLine (r, s) = fRange r <> ": " <> fFlagSet s <> "\n"
+
+formatFlagFile :: AllFlags -> T.Text
+formatFlagFile = mconcat . map fLine . Map.assocs