1) The document discusses parallel adders and subtractors for n-bit binary numbers. It specifically examines a 4-bit parallel adder that uses full adders connected in cascade, with the carry output of one full adder connected to the next's carry input. 2) A 4-bit parallel subtractor is also examined, which takes the 2's complement of the number to be subtracted and adds it to the other number using a 4-bit parallel adder. 3) Carry propagation time is discussed, which is the time it takes the carry to ripple through all the full adders in the parallel adder from the least to most significant bit.

1. Name :- Smit Shah -140410109096
S.Y electrical 2 Sem 4
Subject:-Digital Electronics
Topic :- Parallel Adder and Subtractor….

2. The n-Bit Parallel Adder:
 The full adder is capable of adding only two single
digit binary numbers along with a carry input.
 But in practice we need to add binary numbers which
are much larger in size than just one bit. The Two
binary numbers to be added could be 4 bit, 8 bit, 16 bit
long. In general we assume that both the numbers are
n bit long.

3.  It uses a number of full adders which are connected in
cascade.
 The carry output of the previous full adder is
connected to the carry input of the next full adder.

4. A Four Bit Parallel Adder Using Full
Adder..
 The block diagram of a four bit parallel adder using
full adders is show in fig. Lets the two four bit word
that are to be added be A and B.
 A=A3 A2 A1 A0 B=B3 B2 B1 B0
 Hence Full Adder -0 is the lowest stage. Hence its cin
has been connected to 0 permanenty.

5.  The rest of connection are exactly same as those done
for the n-bit parallel adder.

6.  The 4-bit parallel adder is a very common logic circuit.
It is normally shown by a block diagram as shown in
fig.

7.  An n-bit parallel adder requires n full-adders
 It can be constructed from 4-bit, 2-bit and 1-bit full-
adders ICs by cascading several packages.
 The 4-bit binary parallel adder is a typical example of
an MSI function.
 It can be used in many applications involving arithmetic
operations.
 The application of this MSI function to the design of a
combinational circuit is demonstrated in the example of
BCD to excess-1 code converter.

8. CARRY PROPAGATION
The addition of two binary numbers in parallel implies
that all the bits of the augend and the addend are
available for computation at the same time.
As in any combinational circuit, the signal must
propagate through gates before the correct output sum is
available in output terminals.

9.  The total propagation time is equal to the propagation
delay of typical gate times the number of gate levels in
the circuit.
 The longest propagation delay time in a parallel adder
is the time it takes the carry to propagate through the
full-adders

10. N-Bit Parallel subtractor
 The subtractor can be carried out by taking 1s or 2s
complement of the number to be subtracted.
 For example we can perform the subtraction (A-B) by
adding either 1s complement or 2s complement of B to
A. that means we can use a binary adder to perform
the binary subtraction..

11. 4 bit parallel subtractor using 2s
complement
 A 4-bit parallel subtractor using 4 bit parallel adder is
shown in fig.
 The number to be subtracted B is first passed through
inverters to obtain its 1s complement. One inverter per
bit of word B is used so that all bits of B get inverted.

12.  Then 1 is added to 1s complement of B, by making
cin=1. Thus we obtain the 2s complement of B.

13.  The 4-bit adder then add a and 2s complement of B to
produce the subtraction at its sum output .
 If A>B then c out =0 and the result is in true binary
form but if A < B then c out=1 and the result is negative
and in the 2s complement form…