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Starting Point [TOC]
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For this worksheet, we need a 4-bit adder that can also perform subtraction.
The adder has the following characteristics:

- Inputs `a` and `b` are 4 bits wide.
- The input `add/sub` (1 bit) selects addition with 0 and subtraction with 1.
- The 4-bit output `s` shows the result of the operation.
- The `Cout` output indicates overflow and is directly connected to the `Cout`
  of the 4-bit adder from CircuitVerse. In subtraction, the value is not
  negated.

---- IMAGE (width=600) -----------
session04/s04_1.png
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