ALU: 1-bit

How about subtraction?

We can use the adder to add the negated form of the operand

a - b = a + (-b)

Add an inverter to the circuit to negate b

This gives 1's complement

How do we get 2C value?

Use cin = 1 for least significant bit

a + ~b + 1 = a + (~b + 1) = a + (-b) = a - b

Another MUX with control input Binvert

can select b or -b

1-bit ALU with subtraction

(Fig. 4.16)