Adders
General-purpose processors usually support the addition operation for performing calculations on data values and, separately, to manage the instruction pointer. Following the execution of each instruction, the instruction pointer increments to the next instruction location.
When the processor supports multi-word instructions, the updated instruction pointer must be set to its current value plus the number of words in the just-completed instruction.
A simple adder circuit adds two data bits plus an incoming carry and produces a 1-bit sum and a carry output. This circuit, shown in Figure 2.15, is called a full adder because it includes the incoming carry in the calculation. A half adder adds only the two data bits without an incoming carry:
Figure 2.15: Full adder circuit
The full adder uses logic gates to produce its output as follows. The sum bit S is 1 only if the total number of bits with a value of 1 in the collection A, B, Cin is an odd number. Otherwise...
