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Cadenti
logic family
medium scale integration
metal oxide semiconductor
ac register
accumulator logic
adder circuit
basic computer design
branch unconditionally
flowchart
input and output communication
input output instruction
input register
interrupt cycle
logic adder circuits
logic gates
output register
register and memory
binary code
binary number
clock pulse
data types
decimal numbers
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Adder-subtractor
Binary Adder-Subtractor
The subtraction A - B can be done by taking the 2's complement of B and adding it to A. The 2's complement can be obtained by taking the 1's complement and adding one to the least significant pair of bits. The 1's complement can be implemented with inverters and a one can be added to the sum through the input carry.
The addition and subtraction operations can be combined into one common circuit by including an exclusive-OR gate with each full-adder. A 4-bit adder-subtractor circuit is shown in Fig. 4-7. The mode input M controls the operation. When M = 0 the circuit is an adder and when M = 1 the circuit becomes a subtractor. Each exclusive-OR gate receives input M and one of the inputs of B. When M = 0, we have B® 0 = B. The full-adders receive the value of B, the input carry is 0, and the circuit performs A plus B. When M = 1, we have B® 1 = B' and Co = 1. The B inputs are all complemented and a 1 is added through the input carry. The circuit performs the operation A plus the 2's complement of B. For unsigned numbers, this gives A - B if A? B or the 2's complement of (B - A) if A< B. For signed numbers, the result is A - B provided that there is no overflow.