diff --git a/src/GBC/CPU.hs b/src/GBC/CPU.hs
index 5c87366..ba144b3 100644
--- a/src/GBC/CPU.hs
+++ b/src/GBC/CPU.hs
@@ -11,6 +11,12 @@ import Data.Bits
 import Data.Int
 import qualified Data.Vector.Sized as V
 
+splitWord :: Word16 -> (Word8, Word8)
+splitWord w = (fromIntegral $ w .>>. 8, fromIntegral $ w .&. 255)
+
+combineWords :: Word8 -> Word8 -> Word16
+combineWords msb lsb = (fromIntegral msb .<<. 8) + (fromIntegral lsb)
+
 class Convert a b where
   convert :: a -> b
 
@@ -64,15 +70,15 @@ makeLenses ''Registers
 
 rbc :: Lens' Registers Word16
 rbc = lens (\r -> (( .<<. 8) . fromIntegral $ r ^. rb) + (fromIntegral $ r ^. rc))
-          (\r w -> r & rb %~ (const $ fromIntegral $ w .>>. 8) & rc %~ (const $ fromIntegral $ w .&. 255))
+          (\r w -> let (msb, lsb) = splitWord w in r & rb .~ msb & rc .~ lsb)
 
 rde :: Lens' Registers Word16
 rde = lens (\r -> (( .<<. 8) . fromIntegral $ r ^. rd) + (fromIntegral $ r ^. re))
-          (\r w -> r & rd %~ (const $ fromIntegral $ w .>>. 8) & re %~ (const $ fromIntegral $ w .&. 255))
+          (\r w -> let (msb, lsb) = splitWord w in r & rd .~ msb & re .~ lsb)
 
 rhl :: Lens' Registers Word16
 rhl = lens (\r -> ((.<<. 8) . fromIntegral $ r ^. rh) + (fromIntegral $ r ^. rl))
-          (\r w -> r & rh %~ (const $ fromIntegral $ w .>>. 8) & rl %~ (const $ fromIntegral $ w .&. 255))
+          (\r w -> let (msb, lsb) = splitWord w in r & rh .~ msb & rl .~ lsb)
 
 data CPU = CPU { _registers :: Registers
                , _ir :: Word8
@@ -107,6 +113,7 @@ negative  = lens (^. flags . fNegative)  (\cpu _f -> cpu & flags . fNegative .~
 halfCarry = lens (^. flags . fHalfCarry) (\cpu _f -> cpu & flags . fHalfCarry .~ _f)
 carry     = lens (^. flags . fCarry)     (\cpu _f -> cpu & flags . fCarry .~ _f)
 
+
 type ByteTarget = Lens' CPU Word8
 type WordTarget = Lens' CPU Word16
 type FlagTarget = Lens' CPU Bool
@@ -419,14 +426,13 @@ execute cpu = \case
   LDNNSP v -> write cpu v . fetch cpu $ cpu ^. sp
   LDSPHL -> cpu & sp .~ cpu ^. hl
   PUSHRR t -> let cpu' = cpu & sp %~ subtract 1
-                  msb = fromIntegral $ (cpu ^. t) .>>. 4
-                  lsb = fromIntegral $ (cpu ^. t) .&. 255
+                  (msb, lsb) = splitWord $ cpu ^. t
                   cpu'' = write cpu' (cpu' ^. sp) msb & sp %~ subtract 1 in
                  write cpu'' (cpu'' ^. sp) lsb
   POPRR t -> let lsb = fetch cpu (cpu ^. sp)
                  cpu' = cpu & sp %~ (+1)
                  msb = fetch cpu' (cpu' ^. sp)
-                 value = (fromIntegral msb .<<. 8) + (fromIntegral lsb)
+                 value = combineWords msb lsb
              in
                cpu & t .~ value
                    & sp %~ (+1)
@@ -450,14 +456,27 @@ execute cpu = \case
                      cpu
   CALLNN v -> undefined
   CALLCCNN v _f op -> undefined
-  RET -> undefined
+  RET -> let lsb = fetch cpu (cpu ^. sp)
+             cpu' = cpu & sp %~ (+1)
+             msb = fetch cpu' (cpu' ^. sp)
+             value = combineWords msb lsb in
+           cpu & pc .~ value
+               & ie .~ 1
   RETCC _f op -> undefined
-  RETI -> undefined
-  RSTN v -> undefined
+  RETI -> let lsb = fetch cpu (cpu ^. sp)
+              cpu' = cpu & sp %~ (+1)
+              msb = fetch cpu' (cpu' ^. sp)
+              value = combineWords msb lsb in
+            cpu & pc .~ value
+                & ie .~ 1
+  RSTN v -> let cpu' = cpu & sp %~ subtract 1
+                (msb,lsb) = splitWord $ cpu ^. pc
+                cpu'' = write cpu' (cpu' ^. sp) msb & sp %~ subtract 1 in
+              write cpu'' (cpu'' ^. sp) lsb & pc .~ v
   HALT -> undefined
   STOP -> undefined
-  DI -> undefined
-  EI -> undefined
+  DI -> cpu & ie .~ 0
+  EI -> cpu & ie .~ 1
   NOP -> cpu
   where
     add :: Word8 -> Word8 -> Bool -> (Word8, FlagRegister)
@@ -484,7 +503,7 @@ execute cpu = \case
                  (new, FlagRegister {..})
 
     fetch :: CPU -> Word16 -> Word8
-    fetch _cpu addr = (`V.index` (fromIntegral addr)) $ _cpu ^. bus
+    fetch _cpu addr = _cpu ^. bus . V.ix (fromIntegral addr)
 
     write :: CPU -> Word16 -> Word8 -> CPU
-    write _cpu target value = _cpu & bus %~  (`V.update` V.singleton (fromIntegral target, value))
+    write _cpu target value = _cpu & bus . V.ix (fromIntegral target) .~ value