Confused as to how implement ALU using structured VHDL

Question

I am new to VHDL, working on assignment for my Computer architecture class:

implement 32 bit ALU using VHDL, that performs only certain operations: and, shift left, shift right, complementing one of the input, rest of operations will do simple addition. Trick is, code needs to be structured, since instructor provided bitshifter and adder.

I have googled for examples, but all of them use straightforward code(such as result <= a + b). What would be a good way to implement ALU using above requirements ???
Thanks !

Answer 1

If the bitshifter and adder are already provided for you, it seems all you have left to do is connect them together. Build a module that directs the proper inputs given a control signal. If you have six possible operations you'll need at least three bits for a control signal, right? You might want to choose the various operations to have a similar bit in common, and you can use that bit to mux the outputs from each component block to the output of the block you're building. I'm going to use some pseudo code because, first, this is your homework and, second, I haven't written VHDL in years.

  case control_signal is
    when 1 =>   -- Control=1 means add inputs
      adder_input_A <= top_level_input_A;  -- Move inputs of top lever to the adder
      adder_input_B <= top_level_input_B;
    when 2 =>   -- Control=2 means shift the bits left
      bit_shift_input <= top_level_input_A;  -- Move input to bitshift block
      bit_shift_value <= top_level_shift;    -- This one can more likely be always connected
    when 3 =>
      do another one, like shift right
    when others =>
      null;
  end case;

So, it's you're basically creating a mux that switches the inputs depending on the operation being performed. Obviously, I have no idea what your current blocks look like and how they're implemented, but I think the general concept should still apply.

Answer 2

You need to build up the various operations from addition and other lower level ones. For example in elementary school we learned how to do multiplication of multi-digit numbers from single digit numbers. In binary multiplication is either by 1 or 0, which is either take the input as-is or output a zero. Repeat this for the LSB of one multiplicand, then shift and repeat.

Answer 3

Send the inputs to the cores you have been provided. You may also need some additional control signals (your bit shifter may have a 2-s complement shift amount, or separate "number of bits" and direction signals for example).

Choose (ie use a mux to select) which of your cores to send to the output signal.