I have been giving a problem were I have to design 1-bit subtractor, which performs single bit binary subtraction. It has three inputs: the digits ai, bi (bi is subtracted from ai) and the borrow from the previous 1-bit subtractor Bi-1. The outputs are Di (the difference) and the borrow Bi from the next 1-bit subtractor. Present all four stages (truth tables of the both functions, sum of products, minimization and logical circuits). During minimization follow the same approach that we used in class for 1-bit adder design.

I have search around, but I still cannot figure out how to approach this problem. If anyone could shed some light it would be greatly appreciated.

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    \$\begingroup\$ Write the truth table then Karnaugh-map for minimization? \$\endgroup\$
    – dext0rb
    Commented Oct 15, 2012 at 17:36
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    \$\begingroup\$ You seriously expect us to have a crystal ball and use it to see what approach was used in your class for the 1-bit adder?? \$\endgroup\$ Commented Oct 15, 2012 at 17:46
  • \$\begingroup\$ @WoutervanOoijen I do not expect you to use a "crystal ball" I was asking if anyone could "shed light" on how to approach this problem. The professor does not teach. He gives you problems and yells "Do!" This is new to me and I just want to get a hold of it. \$\endgroup\$
    – Leo
    Commented Oct 15, 2012 at 19:49
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    \$\begingroup\$ @Leo but you question contains the phrase "follow the same approach we used in class". If that is not supposed to mean anything why did you include it? \$\endgroup\$ Commented Oct 15, 2012 at 20:03

1 Answer 1


I'm not going to tell you too much; you show too little of your own efforts. Just a few basic tips to get you started.

You'll have to start with the truth table. With 3 inputs you have only 8 combinations. Start without the borrow bit (the half of the table for which it's zero). Then subtracting shouldn't be too hard: what's the result if you subtract 0 from 1? Or 1 from 1? If you want to introduce the borrow bit it could be useful to do the subtraction of two decimal numbers, that should be familiar since elementary school. Observe how the result is different with or without borrow. Keep it simple: 25 - 13 is without borrow, 25-17 is with borrow. That should get you started.

  • \$\begingroup\$ I understand and that helps. Thanks, I'll give it a shot. \$\endgroup\$
    – Leo
    Commented Oct 15, 2012 at 19:52

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