Please help me for this programming code.. Thank you,

LAB one: Use VHDL to implement a 4-bit binary adder/subtractor by the following steps: 1. Create your half adder firstly; 2. Create your full adder by the established half adder; 3. Obtain a 4-bit binary adder from the full adder; 4. Modify the 4-bit binary adder to perform as an adder/subtractor 5. Write your test bench program to run your VHDL and show out the timing diagram