A full adder is a combinational circuit that performs the arithmetic sum of three bits: A, B and a carry in, C, from a previous addition

Q) What is the meaning of this bold text? (i.e from a previous addition)

I know what a carry bit is, but when it comes to Full Adder I get confused by this statement...

  • \$\begingroup\$ You can cascade several 1 bit adders to make a multi-bit adder. Except for serial adders, where a 1 bit adder is coupled with shift registers and additions are calculated over many cycles, the "normal" case is that the carry-in input is connected to another adder. \$\endgroup\$
    – Grabul
    Apr 16, 2017 at 7:32
  • \$\begingroup\$ @TEMLIB: I'm really new to this lesson and not familiar with the words either... can you explain this as simple as possible??? \$\endgroup\$
    – JaanuB
    Apr 16, 2017 at 8:19
  • \$\begingroup\$ This is exactly how it works when we learned to add in grade school. 9+6 = 5 carry the 1, then the next column you add the 1 and the other two operands A+B plus the carry in from the prior column. You already knew how this worked... \$\endgroup\$
    – old_timer
    Apr 16, 2017 at 12:57
  • \$\begingroup\$ @old_timer: Thank you very much... Now I can get it \$\endgroup\$
    – JaanuB
    Apr 16, 2017 at 15:02

1 Answer 1


For example if you want to add 2 numbers 2 bits each. $$AB+CD$$ First you preform B+D+0 (because you have no carry from a previous addition) and then you preform A+C+carry from a previous addition, i.e. from adding B+C. You do this the same way you do a long addition you did in school.

  • \$\begingroup\$ And same goes for the Half adder too right??? \$\endgroup\$
    – JaanuB
    Apr 16, 2017 at 8:18
  • \$\begingroup\$ Half adder has only 2 inputs (only the bits you want to add without the carry). You can use it for the first operation of B+D since the carry in is 0. You can just google the difference between half and full adder. There are many simple sources on this topic. \$\endgroup\$ Apr 16, 2017 at 8:33

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