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diff --git a/fig-emulator-gb/src/Fig/Emulator/GB/Test/Instr.hs b/fig-emulator-gb/src/Fig/Emulator/GB/Test/Instr.hs
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+++ b/fig-emulator-gb/src/Fig/Emulator/GB/Test/Instr.hs
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+{-# Language TemplateHaskell, ApplicativeDo #-}
+module Fig.Emulator.GB.Test.Instr where
+
+import Control.Lens.TH (makeLenses)
+
+import Control.Lens ((^.))
+import Control.Monad.State (StateT(..))
+
+import Data.Word (Word8, Word16)
+import qualified Data.Aeson as Aeson
+
+import Fig.Prelude
+import Fig.Emulator.GB.Utils
+import Fig.Emulator.GB.CPU
+import Fig.Emulator.GB.CPU.Instruction
+import Fig.Emulator.GB.Bus (Bus(..), Addr(..))
+import qualified Fig.Emulator.GB.Bus as Bus
+import Fig.Emulator.GB.Component.RAM
+
+newtype InstrTestError = InstrTestError Text
+  deriving Show
+instance Exception InstrTestError
+instance Pretty InstrTestError where
+  pretty (InstrTestError b) = mconcat
+    [ "Instruction test error: "
+    , b
+    ]
+
+data TestVals = TestVals
+  { _testvalsA :: !Word8
+  , _testvalsB :: !Word8
+  , _testvalsC :: !Word8
+  , _testvalsD :: !Word8
+  , _testvalsE :: !Word8
+  , _testvalsF :: !Word8
+  , _testvalsH :: !Word8
+  , _testvalsL :: !Word8
+  , _testvalsPC :: !Word16
+  , _testvalsSP :: !Word16
+  , _testvalsRAM :: ![(Word16, Word8)]
+  }
+makeLenses 'TestVals
+instance Aeson.FromJSON TestVals where
+  parseJSON = Aeson.withObject "TestVals" $ \v -> do
+    _testvalsA <- v Aeson..: "a"
+    _testvalsB <- v Aeson..: "b"
+    _testvalsC <- v Aeson..: "c"
+    _testvalsD <- v Aeson..: "d"
+    _testvalsE <- v Aeson..: "e"
+    _testvalsF <- v Aeson..: "f"
+    _testvalsH <- v Aeson..: "h"
+    _testvalsL <- v Aeson..: "l"
+    _testvalsPC <- v Aeson..: "pc"
+    _testvalsSP <- v Aeson..: "sp"
+    _testvalsRAM <- v Aeson..: "ram"
+    pure TestVals{..}
+
+data Testcase = Testcase
+  { _testcaseName :: !Text
+  , _testcaseInitial :: !TestVals
+  , _testcaseFinal :: !TestVals
+  }
+makeLenses 'Testcase
+
+instance Aeson.FromJSON Testcase where
+  parseJSON = Aeson.withObject "Testcase" $ \v -> do
+    _testcaseName <- v Aeson..: "name"
+    _testcaseInitial <- v Aeson..: "initial"
+    _testcaseFinal <- v Aeson..: "final"
+    pure Testcase {..}
+
+readTestcases :: (MonadIO m, MonadThrow m) => FilePath -> m [Testcase]
+readTestcases p = liftIO (Aeson.decodeFileStrict p) >>= \case
+  Just ts -> pure ts
+  Nothing -> throwM . InstrTestError $ "failed to read testcases at " <> pack p
+
+cpuInstrTest :: (MonadIO m, MonadThrow m) => TestVals -> m (CPU m)
+cpuInstrTest vs = do
+  let
+    (z, n, h, c) = w8flags $ vs ^. testvalsF
+    initialBus = Bus [compWRAM 0x0000 $ 64 * 1024]
+  finalBus <- foldM (\b (addr, v) -> Bus.write b (Addr addr) v) initialBus $ vs ^. testvalsRAM
+  pure CPU
+    { _lastPC = 0x0
+    , _lastIns = Nop
+    , _running = True
+    , _regs = initialRegs
+      { _regA = vs ^. testvalsA
+      , _regB = vs ^. testvalsB
+      , _regC = vs ^. testvalsC
+      , _regD = vs ^. testvalsD
+      , _regE = vs ^. testvalsE
+      , _regH = vs ^. testvalsH
+      , _regL = vs ^. testvalsL
+      , _regPC = vs ^. testvalsPC - 1
+      , _regSP = vs ^. testvalsSP
+      , _regFlagZ = z
+      , _regFlagN = n
+      , _regFlagH = h
+      , _regFlagC = c
+      }
+    , _bus = finalBus
+    }
+
+checkCPU :: forall m. (MonadIO m, MonadThrow m) => Text -> TestVals -> CPU m -> m ()
+checkCPU tnm vs c = do
+  let
+    check :: (Eq a, Show a) => Text -> a -> a -> m ()
+    check nm eval aval = if eval == aval
+      then pure ()
+      else throwM . InstrTestError $ mconcat
+      [ "while running test ", tnm, ":\n"
+      , nm <> " mismatch: expected "
+      , tshow eval,  ", got ", tshow aval
+      ]
+  check "register A" (vs ^. testvalsA) (c ^. regs . regA)
+  check "register B" (vs ^. testvalsB) (c ^. regs . regB)
+  check "register C" (vs ^. testvalsC) (c ^. regs . regC)
+  check "register D" (vs ^. testvalsD) (c ^. regs . regD)
+  check "register E" (vs ^. testvalsE) (c ^. regs . regE)
+  check "register H" (vs ^. testvalsH) (c ^. regs . regH)
+  check "register L" (vs ^. testvalsL) (c ^. regs . regL)
+  check "PC" (vs ^. testvalsPC - 1) (c ^. regs . regPC)
+  check "SP" (vs ^. testvalsSP) (c ^. regs . regSP)
+  let (fz, fn, fh, fc) = w8flags $ vs ^. testvalsF
+  check "flag Z" fz (c ^. regs . regFlagZ)
+  check "flag N" fn (c ^. regs . regFlagN)
+  check "flag H" fh (c ^. regs . regFlagH)
+  check "flag C" fc (c ^. regs . regFlagC)
+  forM_ (vs ^. testvalsRAM) \(Addr -> addr, eval) -> do
+    Bus.read (c ^. bus) addr >>= \case
+      Nothing -> throwM . InstrTestError $ "failed to read expected address: " <> pretty addr
+      Just aval -> check ("memory address " <> pretty addr) eval aval
+
+runTestcase :: (MonadIO m, MonadThrow m) => Testcase -> m ()
+runTestcase tc = liftIO do
+  initial <- cpuInstrTest $ tc ^. testcaseInitial
+  let
+    body :: forall m'. Emulating m' => m' Instruction
+    body = do
+      ins <- decode
+      step ins
+      pure ins
+  (ins, final) <- runStateT body initial
+  checkCPU (tc ^. testcaseName <> " (" <> tshow ins <> ")") (tc ^. testcaseFinal) final
+  log $ "Passed: " <> tc ^. testcaseName