vmcompiler: fix FFunction compile

src/Nand2Tetris/VM.hs

@@ -310,11 +310,18 @@ compileInstruction instr = do
     compileInstruction' (VMFunction (FFunction name nvars)) = do
       modify' (\s -> s { ceCurrentFunction = Just name })
       return ([ ALabel name,
-                Code (A (Label "SP")) ]<>
+                Code (A (Label "SP")),
+                Code (C [RegA] (SReg RegM) JNone)
+                ] <>
                (concat . replicate nvars $
                  [ Code (C [RegM] S0 JNone),
                    Code (C [RegA] (SRegPlus1 RegA) JNone)
-                 ]))
+                 ]) <>
+             [ Code (C [RegD] (SReg RegA) JNone),
+               Code (A (Label "SP")),
+               Code (C [RegM] (SReg RegD) JNone)
+             ]
+             )
 
     compileInstruction' (VMFunction (FCall name nargs)) = do
       lab <- nextLabelId