Design an $4-$bit ALU. The circuit has a $3-$bit select bus $($Sel$),8-$bit input datapaths $($A$[3$:$0]$ and B$[3$:$0]),$ an $4-$bit output datapath $($y$[3$:$0]),$ and performs the operations:

Sel Operation Description

$000$ y $=4\text{'}$b$0$ Reset y to all $0\text{'}$s

$001$ y $=$ A & B Bitwise AND

$010$ y $=$ A $|$ B Bitwise OR

$011$ y $=$ A $^$ B Bitwise exclusive$-$OR

$100$ y $=$ ~A Bitwise complement

$101$ y $=$ A $$ B Subtract

$110$ y $=$ A $+$ B Add $($Unsigned A and B$)$

$111$ y $=4\text{'}$hF Set y to all $1\text{'}$s

Remember to write the testbench for simulating all circuit operations.