Full Adder and Subtractor SOLVED

W, X, Y, Z are one bit numbers, using full/half adder and subtractor design the following operation : W+X-Y-Z.

I seriously do not know what should i do to solve this and i couldn't understand it. Anyone can help please?

by MarinaGoal
May 13, 2020

1 Answer

Answer by blasphemydog

parenthesis () represent compliments.

A half adder is used to add together two bits. The output will be two bits, a carry and the sum. Using a truth table and Kanaugh maps, the sum of a half adder is the two bits exclusive OR (XOR) together. The borrow is the two bits AND together.

Use a half adder to add W and X. The outputs will be: sum = W(X) + (W)X which is W XOR X (exclusive OR), carry = WX

As you can see, our output is two bits (sum, carry).

Remember that a full adder/subractor is used for three bits. We have found the ouputs (2 bits) for W + X, and now must use a full subtractor in order to subtract Y from the previous result (a 3 bit operation).

Using truth tables and Kanaugh maps, if the inputs to a full subtractor are A,B,C, then the outputs will be as follows: diff = A XOR B XOR C, borrow = (A)B + BC + (A)C Using our result for W + X above, Let A = sum Let B = carry Let C = Y Then the outputs from the full subtractor will be: diff = sum XOR carry XOR Y borrow = (sum)carry + carry.Y + (sum)Y

Now, use a full subtractor again in order to subtract Z. Inputs are: diff, borrow, Z Outputs are: answer = diff XOR borrow XOR Z final borrow = (diff)borrow + borrow.Z + (diff)Z

Review: half adder: inputs A,B then sum = A XOR B, carry = AB full subractor: inputs A,B,C then diff = A XOR B XOR C, borrow = (A)B + BC + (A)C

Use a half adder on W,X. Output will be sum and carry. Use a full subtractor on sum,carry,Y. The outputs will be diff and borrow. Use a full subtractor on diff, borrow, Z. The final output will be answer and final borrow.

Hope that helps.

ACCEPTED +1 vote
by blasphemydog
May 14, 2020

Your Answer

You must log in or create an account (free!) to answer a question.

Log in Create an account

Go Ad-Free. Activate your CircuitLab membership. No more ads. Save unlimited circuits. Run unlimited simulations.

Search Questions & Answers

Ask a Question

Anyone can ask a question.

Did you already search (see above) to see if a similar question has already been answered? If you can't find the answer, you may ask a question.

About This Site

CircuitLab's Q&A site is a FREE questions and answers forum for electronics and electrical engineering students, hobbyists, and professionals.

We encourage you to use our built-in schematic & simulation software to add more detail to your questions and answers.

Acceptable Questions:

  • Concept or theory questions
  • Practical engineering questions
  • “Homework” questions
  • Software/hardware intersection
  • Best practices
  • Design choices & component selection
  • Troubleshooting

Unacceptable Questions:

  • Non-English language content
  • Non-question discussion
  • Non-electronics questions
  • Vendor-specific topics
  • Pure software questions
  • CircuitLab software support

Please respect that there are both seasoned experts and total newbies here: please be nice, be constructive, and be specific!

About CircuitLab

CircuitLab is an in-browser schematic capture and circuit simulation software tool to help you rapidly design and analyze analog and digital electronics systems.