4-bit Full Adder

Description:

• 
• Made with 1-bit Full Adder
• When $k$ is 0, it adds 2 numbers

Subtraction:

• When we wish to subtract A to B, we use A + (flip bits of B) + 1
  • flip + 1 is Two’s Complement
• Conveniently, when K is 1, C_in can also act as 1
• Then we use multiplexer to choose B or not B (flipped) which is B_i XOR K