-cpu :: State [Word | 4] -> Word
- -> [(Index 6, Index 6) | 4]
- -> (State [Word | 4], Word)
-cpu (State regsOut) input addrs = (State regsIn, out)
+data Opcode = Shift | Xor | Equal
+
+multiop :: Opcode -> Word -> Word -> Word
+multiop Shift a b = shift a b
+multiop Xor a b = xor a b
+multiop Equal a b | a == b = 1
+ | otherwise = 0
+\end{code}
+
+The cpu function ties everything together. It applies the \hs{fu}
+function four times, to create a different function unit each time. The
+first application is interesting, because it does not just pass a
+function to \hs{fu}, but a partial application of \hs{multiop}. This
+shows how the first function unit effectively gets an extra input,
+compared to the others.
+
+The vector \hs{inputs} is the set of data sources, which is passed to
+each function unit for operand selection. The cpu also receives a vector
+of address pairs, which are used by each function unit to select their
+operand. The application of the function units to the \hs{inputs} and
+\hs{addrs} arguments seems quite repetive and could be rewritten to use
+a combination of the \hs{map} and \hs{zipwith} functions instead.
+However, the prototype does not currently support working with lists of
+functions, so the more explicit version of the code is given instead).
+
+\begin{code}
+type CpuState = State [Word | 4]
+
+cpu :: CpuState -> Word -> [(Index 6, Index 6) | 4]
+ -> Opcode -> (CpuState, Word)
+cpu (State s) input addrs opc = (State s', out)