Component Instantiations in VHDL

Hi All,

I am relatively new to VHDL and I'm trying to implement the following actions for the various ctrl strings stated:

--ADD when ctrl="000" else
--SUB when ctrl="001" else
ALUOUT <= ABUS and BBUS when ctrl="010" else
ALUOUT <= ABUS or BBUS when ctrl="011" else
ALUOUT <= ABUS xor BBUS when ctrl="100" else
ALUOUT <= not ABUS when ctrl="101" else
ALUOUT <= ABUS when ctrl="110"

My problem is that I already have an adder designed in a verilog module which I am calling using component instantiation and the principle
"A-B=A+(not B) + 1"
for the Subtraction operation.

The two component instantiations are as below:
u1: NBitAdder port map(Aalu=>ABUS, Balu=>BBUS, c=>'0', sumA=>ALUOUT, carryA=>cout);
u2: NBitAdder port map(Aalu=>ABUS, Balu=>not BBUS, c=>'0', sumA=>ALUOUT, carryA=>cout);

But how can I implement only 1 of these depending on the ctrl inputs?? As, from what I understand an instantiation cannot be used within the WHEN process.

Any help would be much appreciated.

Thanks in advance.