Adders

ALU - Is a subsystem that carries out operations on operands.

Half Adder

All operations are based upon addition.

Truth table

Logic

Full Adder

Truth table

equivalence equation

Logic

Alternative logic

4-bit addition with ripple adder cascade methods

Carry look ahead adder

Note: ripple adder cascades are bad because the carry bit must ripple though each adder

This can be solved with a carry look ahead adder

Logic

Why???

• They are only any use in smaller adder cascades

Implementation

To get the negative of a twos compliment binary number you must:

1. Get the ones compliment (bit-wise inversion)

2. Add one

Bit-wise inversion

Adder/Subtraction

Note: when negative is being entered carry in is set to 1, thus adding 1

Bringing it all together (Accumulator circuit)