One of the most fundamental operations performed by computers, aside from the logical operations that we've already discussed, is the arithmetic operation of addition.
Let's consider what's required to add two, single-bit binary integers. We'll need one bit to represent the sum of the integers, and another to handle the carry. Representing this in the form of a truth table yields:
This is formally termed a half-adder, a logic circuit capable of adding two bits.
- What truth table do you recognize that produces the output of the Carry column?
- What truth table do you recognize that produces the output of the Sum column?
In order to add two single-bit binary integers PLUS a carry, we need an adder capable of adding three single-bit binary numbers. Again, we'll need one bit to represent the sum of the integers, and another to handle the carry. Representing this in the form of a truth table yields:
This is formally termed a full-adder, a logic circuit capable of adding three bits.
- What do you notice about the relationship between the first-half (top four rows) of the full-adder as compared to all of the rows of the half-adder?
- Why is this true?
Ripple Carry Adder
We've learned that a half-adder can add two bits and full-adder can add three bits. How can we add a multi-bit number such as a 16-bit word? By cascading four adders such that the carry output of the prior adder feeds the carry input of the subsequent adder we can add two four-bit numbers. This concept can be easily extended to an arbitrary number of bits.
- Why does the least significant bit position use a half-adder rather than a full-adder?
- Assume that proper inputs are applied for all bits in numbers A and B. Will the correct output from S be available instantaneously? If not, why not?
- Assume that we have a standard (non-scientific calculator) capable of adding two 16-bit words. Two numbers, A and B, are added together. After the addition, it is noted that is high. What can we infer? What is this state commonly called?
- A half-adder is a logic circuit capable of adding two bits and output a carry bit and a sum bit.
- A full-adder is a logic circuit capable of adding three bits and output a carry bit and a sum bit.
- A ripple-carry-adder is a logic circuit constructed of adders, cascaded in such a manner that the carry output of each adder feeds the carry input of the subsequent adder. Using this method we are able to add an arbitrary number of bits.
- J1017 Create a journal and answer all questions in this experience. Be sure to:
- edit your journal using emacs within your ~/Journals directory
- properly name your journal as J1017.txt
- include all sections of the journal, properly formatted
- push your changes to GitHub
- properly tag your journal as J1017.Final
- push your tag to GitHub
- Construct your work using Falstad's Editor
- All circuits must be on a single page
- Label the page (using Text) with:
- Your name
- The date
- Begin each circuit with a Blank Circuit
- Label each circuit diagram (using Text) with:
- The name of the logic gate (e.g. "NOT")
- Each output (e.g. "sum", "carry out")
- Save the document using the
Save As...option from the File submenu and then click on the link presented
- The file contains your work for the exercise. Create a new subdirectory, J1017, in your Journals directory. Upload the file to the J1017 directory via SFTP. Be sure to push the files to your GitHub repository.
- You may use any of the following logic gates in your implementation: AND, OR, NOT, XOR, NAND, and NOR
- Construct a half-adder
- Construct a full-adder
- M1017-31 Complete Merlin Mission Manager Mission M1017-31.
- Adder (Wikipedia)
- Schocken, Simon and Nisan, Noam. The Elements of Computing Systems. MIT Press, 2005.
|Knowledge and skills||§10.331|
|Topic areas||Boolean algebra|
|Classroom time||30 minutes|
|Study time||1 hour60 minutes <br />|
|Acquired knowledge||understand the construction and use of binary adders|
|Acquired skill||demonstrate proficiency in constructing and using binary adders|