Solution:

Four-Bit Parallel Adders:

• A group of four bits is called a nibble. A basic 4-bit parallel adder is implemented with four full-adder stages as shown in Figure (a).

• Again, the LSBs (A1 and B1) in each number being added go into the right-most full-adder; the higher-order bits are applied as shown to the successively higher-order adders, with the MSBs (A4 and B4) in each number being applied to the left-most full-adder.

  

• The carry output of each adder is connected to the carry input of the next higher-order adder as indicated. These are called internal carries.

  

• In keeping with most manufacturers’ data sheets, the input labeled C0 is the input carry to the least significant bit adder; C4 , in the case of four bits, is the output carry of the most significant bit adder; and ©1 (LSB) through ©4 (MSB) are the sum outputs. The logic symbol is shown in Figure (b).

• In terms of the method used to handle carries in a parallel adder, there are two types: the ripple carry adder and the carry look-ahead adder. These are discussed in Section (a).

Truth Table for a 4-Bit Parallel Adder:

• Table (b) is the truth table for a 4-bit adder. On some data sheets, truth tables may be called function tables or functional truth tables.

• The subscript n represents the adder bits and can be 1, 2, 3, or 4 for the 4-bit adder.

  

• Cn-1 is the carry from the previous adder.

• Carries C1 , C2 , and C3 are generated internally.

• C0 is an external carry input and C4 is an output. Example illustrates how to use Table (b).